TY - JOUR
AU - El, Enshasy, Hesham
AB - Abstract Lactic acid bacteria constitute a diverse group of industrially significant, safe microorganisms that are primarily used as starter cultures and probiotics, and are also being developed as production systems in industrial biotechnology for biocatalysis and transformation of renewable feedstocks to commodity- and high-value chemicals, and health products. Development of strains, which was initially based mainly on natural approaches, is also achieved by metabolic engineering that has been facilitated by the availability of genome sequences and genetic tools for transformation of some of the bacterial strains. The aim of this paper is to provide a brief overview of the potential of lactic acid bacteria as biological catalysts for production of different organic compounds for food and non-food sectors based on their diversity, metabolic- and stress tolerance features, as well as the use of genetic/metabolic engineering tools for enhancing their capabilities. lactic acid bacteria, metabolic engineering, rerouting metabolism, biological catalysts, biobased chemicals INTRODUCTION Lactic acid bacteria (LAB) have been associated since time immemorial with fermentation of foods and their preservation, and today they are clearly the most important group of industrial microorganisms with a market in the range of multibillion dollars. LAB are used as starter cultures for fermentation of milk, vegetables, meat, fish and cereals, and also animal feed in the form of silage. The fermented dairy products are economically the most important with an estimated value of over 80 billion Euros (in 2011) (de Vos 2011). The well-known features of LAB that are utilized to formulate functional starter cultures are production of exopolysaccharides (EPS), organic acids, polyols, aromatic compounds, bacteriocins, among others, which are released into the food matrix giving improved characteristics in terms of texture, aroma, flavor, health effects and shelf life (Leroy and De Vuyst 2004). The application that has experienced growing global market is the use of LAB as probiotics—estimated at 20 billion Euros, a market that was predicted to grow 10% per year (de Vos 2011). Other important applications include their use as delivery vehicles for preventive and therapeutic drugs including proteins and DNA vaccines (Michon et al.2016), and as biological catalysts for production of value added products for both food and non-food sectors from renewable feedstocks in a biobased economy. The efforts in the latter area have gathered momentum with the growing awareness of the environmental impact of the fossil based production in terms of greenhouse gas emissions, climate change, etc. Industrial biotechnology, considered to be a key enabling technology for transition from fossil to biobased economy, is dependent to a large part on the feasibility of development of efficient and economical microbial production systems and processes. So far only a few products are produced at industrial scale using a limited number of microorganisms, among them lactic acid produced by LAB has become an important product with a continuously growing demand. The features favoring widespread industrial applications include the documented GRAS (Generally Regarded As Safe) status of the majority of LAB, their tolerance to various stress environments, their simple metabolism and ability to metabolize variety of carbon sources (Mazzoli et al.2014). Furthermore, development of LAB as bio-factories benefits from the past and ongoing efforts inspired by their immense significance for foods and human health, which have led to increased understanding of genomics, metabolism and physiology, and tools for high throughput screening of microorganisms and development of improved strains (de Vos 2011; Derkx et al.2014). Both fundamental and applied aspects of LAB are extensively covered in the literature, reference is hereby provided to publications in recent years (Gaspar et al.2013; Zhang and Cai 2014; Bosma, Forster and Nielsen 2017; Sauer et al.2017; Wu, Huang and Zhou 2017). This paper provides a concise review of this important group of microorganisms, including their diversity, metabolism and the strain development options along with examples of their use as biological catalysts utilizing a metabolic pathway or a single enzyme for food and non-food uses. LAB GENERAL FEATURES LAB comprise a genetically and ecologically diverse group of non-motile, microaerophilic Gram-positive bacteria including several genera (Enterococcus, Lactobacillus, Pediococcus, Leuconostoc, Oenococcus, Lactococcus, Streptococcus, Weissella, etc. within the order Lactobacillales) belonging to the phylum Firmicutes, and anaerobic Bifidobacterium genus under the phylum Actinobacteria. The most commercially formulated starter cultures are represented by Lactobacillus, Lactococcus, Streptococcus species, while the most commonly reported probiotics belong to the genera Lactobacillus and Bifidobacterium (Corona-Hernandez et al.2013), especially the former because of their known antimicrobial effects and other health benefits (Guandalini 2011). LAB genomes are characterized by small size ranging from 1.23 Mb (Lactobacillus sanfranciscensis) to 4.91 Mb (Lactobacillus parakefiri). Since the early 21st century, genomic data from more than 200 LAB strains has been collected in different public databases (Douillard and de Vos 2014; Sun et al.2015). Comparative genomic analysis has revealed vast diversity among the LAB, which is attributed to their varying interactions with the environmental niches involving gene loss and -gain through horizontal gene transfer (Wu, Huang and Zhou 2017). This diversity is reflected in the large phenotypic variability observed among the species and even among strains. Noteworthy is the phenomenon of reductive evolution of the genomes involving loss of several metabolic genes and related biosynthetic limitations, presence of pseudogenes, and also fewer higher-level genetic control systems as compared to many other microbes, which is attributed to their adaptation to nutrient rich niches (Schroeter and Klaenhammer 2009; Wu, Huang and Zhou 2017). The sequenced genomes have revealed the presence of several families of glycoside hydrolases in the CAZy database, many of which remain uncharacterized (Sun et al.2015). Several LAB have retained transporter genes for enabling the microbes to acquire nutrients from their environment, and also genes that allow them to tolerate environmental stresses like temperature, pH, salts, etc. and inhibit pathogens (Zhang and Cai 2014). With respect to their safety features, genome analysis of different LAB has shown the absence of virulence-related and toxin encoding genes (Wu, Huang and Zhou 2017). LAB possess a rich ensemble of genetic elements like plasmids, conjugative transposons and bacteriophages (de Vos 2011). Megaplasmids with sizes in the range of 110–490 kb have been found in several species of LAB (Zhang and Cai 2014; Sun et al.2015). The plasmids and conjugative transposons encode variety of functions like lactose and citrate metabolism, bacteriophage resistance, bacteriocin production, proteolysis, etc. (Schroeter and Klaenhammer 2009). Also, CRISPRs and associated cas genes are widespread in the genomes of a number of LAB (Barrangou et al.2007; Sun et al.2015), which provide adaptive immunity against potentially harmful invading foreign DNA, for example, from bacteriophages and plasmids present in the complex environments they inhabit (Schroeter and Klaenhammer 2009; Sun et al.2015). The LAB features of growth, nutritional requirements, metabolism and production of antimicrobial compounds are crucial for inhibiting the growth of pathogens and spoilage organisms and hence in the production of fermented foods (Wu, Huang and Zhou 2017). These bacteria used various strategies for countering environmental stresses in both industrial milieu and gastrointestinal tract, such as maintaining cell membrane functionality, regulating cellular metabolism, EPS production and expression of stress response proteins (Wu, Huang and Zhou 2012, 2014; Papadimitriou et al.2016). Several LAB exhibit tolerance to stresses like low pH, high temperature, ethanol, high salt concentration, inhibitors generated on pretreatment of lignocellulosic biomass, etc. (Fiocco et al.2007; Abdel-Rahman and Sonomoto 2016; Bosma, Forster and Nielsen 2017). STRAIN DEVELOPMENT Strain development of LAB has been a continuous process in the food industry to yield products with improved features such as texture, taste, reduction of additives, calorie content, modulating the compositions of acids produced and even eliminating a certain undesirable property such as antibiotic resistance. Due to strict food legislations and consumer acceptance, strain improvements have been limited to the use of natural strategies such as random mutagenesis, adaptive evolution, dominant selection and even natural transduction and conjugation systems (Derkx et al.2014). Extensive characterization of the inherent genetic elements in LAB has led to the development of a number of gene cloning, expression and secretion systems, which are being applied for metabolic engineering of the bacteria for improving the production titers, modifying substrate utilization, making novel compounds, etc. in biotechnological applications (Gaspar et al.2013; Bosma, Forster and Nielsen 2017). The genetic and metabolic studies were focused traditionally on Lactococcus lactis and Lactobacillus plantarum because of their importance as starter cultures and probiotics, respectively, and which still continue to be used because of the available genetic tools and high transformation efficiencies (de Vos 2011). Several other LAB strains have since been found to be accessible to genetic modification, however with efficiencies lower than the paradigm strains of L. lactis and Lb. plantarum (de Vos 2011; Bosma, Forster and Nielsen 2017). The currently available genetic tools developed for LAB are listed in Table 1. NICE (nisin controlled gene expression), based on the production of the antimicrobial peptide nisin has become a popular system for efficient, controlled gene expression in L. lactis for diverse applications like controlled lysis for accelerated cheese ripening, production of alanine, and other compunds (Hols et al.1999; Mierau and Kleerebezem 2005). Later on, a prototype inducible gene expression system in Lb. sakei, nowadays also adapted to other Lactobacillus spp., was constructed based upon the sakacin A regulatory system (Axelsson, Lindstad and Naterstad 2003; Sørvig et al.2003). Recently, potential of this so called pSIP system for efficient, tightly controlled expression of enzymes in Lb. plantarum WCSF1 was also shown, for eample, β-galactosidase production amounted to 55% of the total intracellular protein in the host cells (Halbmayr et al.2008; Nguyen et al.2015). Table 1. Genetic toolbox for gene modifications in LAB Tools Purpose Species studied Characteristics Reference Expression System Nisin system Inducible gene expression L. lactis, Lb. helveticus, Lb. plantarum (a) Based on quorum-sensing regulatory mechanism (b) Under control of PnisA (c) Adapted for gene expression in lactobacilli and other Gram-positive bacteria (d) Food-grade markers available Kuipers et al.1995 Hols et al.1999 Gaspar et al.2011, Neves et al.2006 Douillard et al.2011 Sakacin system Inducible gene expression Lb. sakei, Lb. plantarum, Lb. casei (a) A variant of NICE system, but with tighter control and lower background expression (b) High production of target protein Axelsson et al.2003 Mathiesen et al.2008 Nguyen et al.2011 Promoters development Controllable gene expression L. lactis, Lb. plantarum (a) Constitutive gene expression (b) Provides a flexible genetic tool Jensen and Hammer 1998 Heiss et al.2016 Reporter genes development Measuring and tracking gene expression L. lactis, Lb. plantarum, Lb. reuteri (a) Established reported: gusA, GFP, CBRluc, mCherry (b) Aid studies on mechanisms of probiosis, localization and interaction with the host Rud et al.2006 Guo et al.2013 Frese et al.2011 Karimi et al.2016 Chromosomal modification pORI system Gene deletion and insertion (markerless) L. lactis (a) Based on conditional replication of vector pORI19 (b) High stability of mutants and integration plasmid can be readily recovered (c) The stability of the insertional mutations after single-crossover HR requires maintenance of antibiotic selection (d) Same selection marker can not introduce multiple mutations Law et al.1995 Leenhouts et al.1996 pTRK system Gene deletion and insertion (markerless) Lb. acidophilus, Lb. gasseri, Lb. casei, L. lactis (a) Independent of transformation efficiency (b) Allows growth of mutant strains at preferred growth temperatures and efficient recovery of stable integrants (c) Markerless gene replacement system that involves the use of upp as counter-selectable marker for positive system selection of double recombinants Russell and Klaenhammer 2001 Goh et al.2009 Song et al.2014 Cre-loxP system Gene deletion and insertion L. lactis, Lb. plantarum (a) Based on double-crossover integration of nonreplicating vectors (b) Enables selectable marker removal upon marker selection of deletion variants (c) Introduction of multiple loxP sites recognizable by Cre might lead to genome instability Campo et al.2002 Lambert et al.2007 Single stranded recombineering Targeted mutations on chromosome Lb. reuteri, L. lactis, Lb. plantarum, Lb. gasseri (a) Mutations were incorporated in the chromosome of L. reuteri and L. lactis without selection at efficiency of 0.4–19% b) High ssDNA concentration, optimal electroporation and optimized oligonucleotide design were required Van Pijkeren and Britton 2012 CRISPR-Cas9 Targeted mutations on chromosome Lb. reuteri, S. thermophilus, Lb. gasseri (a) Counter-selection against wild-type cells after genome modification (b) Combined with single-stranded DNA recombineering for the generation of subtle mutations Van Pijkeren and Britton 2014 Hidalgo-Cantabrana et al.2017 Tools Purpose Species studied Characteristics Reference Expression System Nisin system Inducible gene expression L. lactis, Lb. helveticus, Lb. plantarum (a) Based on quorum-sensing regulatory mechanism (b) Under control of PnisA (c) Adapted for gene expression in lactobacilli and other Gram-positive bacteria (d) Food-grade markers available Kuipers et al.1995 Hols et al.1999 Gaspar et al.2011, Neves et al.2006 Douillard et al.2011 Sakacin system Inducible gene expression Lb. sakei, Lb. plantarum, Lb. casei (a) A variant of NICE system, but with tighter control and lower background expression (b) High production of target protein Axelsson et al.2003 Mathiesen et al.2008 Nguyen et al.2011 Promoters development Controllable gene expression L. lactis, Lb. plantarum (a) Constitutive gene expression (b) Provides a flexible genetic tool Jensen and Hammer 1998 Heiss et al.2016 Reporter genes development Measuring and tracking gene expression L. lactis, Lb. plantarum, Lb. reuteri (a) Established reported: gusA, GFP, CBRluc, mCherry (b) Aid studies on mechanisms of probiosis, localization and interaction with the host Rud et al.2006 Guo et al.2013 Frese et al.2011 Karimi et al.2016 Chromosomal modification pORI system Gene deletion and insertion (markerless) L. lactis (a) Based on conditional replication of vector pORI19 (b) High stability of mutants and integration plasmid can be readily recovered (c) The stability of the insertional mutations after single-crossover HR requires maintenance of antibiotic selection (d) Same selection marker can not introduce multiple mutations Law et al.1995 Leenhouts et al.1996 pTRK system Gene deletion and insertion (markerless) Lb. acidophilus, Lb. gasseri, Lb. casei, L. lactis (a) Independent of transformation efficiency (b) Allows growth of mutant strains at preferred growth temperatures and efficient recovery of stable integrants (c) Markerless gene replacement system that involves the use of upp as counter-selectable marker for positive system selection of double recombinants Russell and Klaenhammer 2001 Goh et al.2009 Song et al.2014 Cre-loxP system Gene deletion and insertion L. lactis, Lb. plantarum (a) Based on double-crossover integration of nonreplicating vectors (b) Enables selectable marker removal upon marker selection of deletion variants (c) Introduction of multiple loxP sites recognizable by Cre might lead to genome instability Campo et al.2002 Lambert et al.2007 Single stranded recombineering Targeted mutations on chromosome Lb. reuteri, L. lactis, Lb. plantarum, Lb. gasseri (a) Mutations were incorporated in the chromosome of L. reuteri and L. lactis without selection at efficiency of 0.4–19% b) High ssDNA concentration, optimal electroporation and optimized oligonucleotide design were required Van Pijkeren and Britton 2012 CRISPR-Cas9 Targeted mutations on chromosome Lb. reuteri, S. thermophilus, Lb. gasseri (a) Counter-selection against wild-type cells after genome modification (b) Combined with single-stranded DNA recombineering for the generation of subtle mutations Van Pijkeren and Britton 2014 Hidalgo-Cantabrana et al.2017 View Large Table 1. Genetic toolbox for gene modifications in LAB Tools Purpose Species studied Characteristics Reference Expression System Nisin system Inducible gene expression L. lactis, Lb. helveticus, Lb. plantarum (a) Based on quorum-sensing regulatory mechanism (b) Under control of PnisA (c) Adapted for gene expression in lactobacilli and other Gram-positive bacteria (d) Food-grade markers available Kuipers et al.1995 Hols et al.1999 Gaspar et al.2011, Neves et al.2006 Douillard et al.2011 Sakacin system Inducible gene expression Lb. sakei, Lb. plantarum, Lb. casei (a) A variant of NICE system, but with tighter control and lower background expression (b) High production of target protein Axelsson et al.2003 Mathiesen et al.2008 Nguyen et al.2011 Promoters development Controllable gene expression L. lactis, Lb. plantarum (a) Constitutive gene expression (b) Provides a flexible genetic tool Jensen and Hammer 1998 Heiss et al.2016 Reporter genes development Measuring and tracking gene expression L. lactis, Lb. plantarum, Lb. reuteri (a) Established reported: gusA, GFP, CBRluc, mCherry (b) Aid studies on mechanisms of probiosis, localization and interaction with the host Rud et al.2006 Guo et al.2013 Frese et al.2011 Karimi et al.2016 Chromosomal modification pORI system Gene deletion and insertion (markerless) L. lactis (a) Based on conditional replication of vector pORI19 (b) High stability of mutants and integration plasmid can be readily recovered (c) The stability of the insertional mutations after single-crossover HR requires maintenance of antibiotic selection (d) Same selection marker can not introduce multiple mutations Law et al.1995 Leenhouts et al.1996 pTRK system Gene deletion and insertion (markerless) Lb. acidophilus, Lb. gasseri, Lb. casei, L. lactis (a) Independent of transformation efficiency (b) Allows growth of mutant strains at preferred growth temperatures and efficient recovery of stable integrants (c) Markerless gene replacement system that involves the use of upp as counter-selectable marker for positive system selection of double recombinants Russell and Klaenhammer 2001 Goh et al.2009 Song et al.2014 Cre-loxP system Gene deletion and insertion L. lactis, Lb. plantarum (a) Based on double-crossover integration of nonreplicating vectors (b) Enables selectable marker removal upon marker selection of deletion variants (c) Introduction of multiple loxP sites recognizable by Cre might lead to genome instability Campo et al.2002 Lambert et al.2007 Single stranded recombineering Targeted mutations on chromosome Lb. reuteri, L. lactis, Lb. plantarum, Lb. gasseri (a) Mutations were incorporated in the chromosome of L. reuteri and L. lactis without selection at efficiency of 0.4–19% b) High ssDNA concentration, optimal electroporation and optimized oligonucleotide design were required Van Pijkeren and Britton 2012 CRISPR-Cas9 Targeted mutations on chromosome Lb. reuteri, S. thermophilus, Lb. gasseri (a) Counter-selection against wild-type cells after genome modification (b) Combined with single-stranded DNA recombineering for the generation of subtle mutations Van Pijkeren and Britton 2014 Hidalgo-Cantabrana et al.2017 Tools Purpose Species studied Characteristics Reference Expression System Nisin system Inducible gene expression L. lactis, Lb. helveticus, Lb. plantarum (a) Based on quorum-sensing regulatory mechanism (b) Under control of PnisA (c) Adapted for gene expression in lactobacilli and other Gram-positive bacteria (d) Food-grade markers available Kuipers et al.1995 Hols et al.1999 Gaspar et al.2011, Neves et al.2006 Douillard et al.2011 Sakacin system Inducible gene expression Lb. sakei, Lb. plantarum, Lb. casei (a) A variant of NICE system, but with tighter control and lower background expression (b) High production of target protein Axelsson et al.2003 Mathiesen et al.2008 Nguyen et al.2011 Promoters development Controllable gene expression L. lactis, Lb. plantarum (a) Constitutive gene expression (b) Provides a flexible genetic tool Jensen and Hammer 1998 Heiss et al.2016 Reporter genes development Measuring and tracking gene expression L. lactis, Lb. plantarum, Lb. reuteri (a) Established reported: gusA, GFP, CBRluc, mCherry (b) Aid studies on mechanisms of probiosis, localization and interaction with the host Rud et al.2006 Guo et al.2013 Frese et al.2011 Karimi et al.2016 Chromosomal modification pORI system Gene deletion and insertion (markerless) L. lactis (a) Based on conditional replication of vector pORI19 (b) High stability of mutants and integration plasmid can be readily recovered (c) The stability of the insertional mutations after single-crossover HR requires maintenance of antibiotic selection (d) Same selection marker can not introduce multiple mutations Law et al.1995 Leenhouts et al.1996 pTRK system Gene deletion and insertion (markerless) Lb. acidophilus, Lb. gasseri, Lb. casei, L. lactis (a) Independent of transformation efficiency (b) Allows growth of mutant strains at preferred growth temperatures and efficient recovery of stable integrants (c) Markerless gene replacement system that involves the use of upp as counter-selectable marker for positive system selection of double recombinants Russell and Klaenhammer 2001 Goh et al.2009 Song et al.2014 Cre-loxP system Gene deletion and insertion L. lactis, Lb. plantarum (a) Based on double-crossover integration of nonreplicating vectors (b) Enables selectable marker removal upon marker selection of deletion variants (c) Introduction of multiple loxP sites recognizable by Cre might lead to genome instability Campo et al.2002 Lambert et al.2007 Single stranded recombineering Targeted mutations on chromosome Lb. reuteri, L. lactis, Lb. plantarum, Lb. gasseri (a) Mutations were incorporated in the chromosome of L. reuteri and L. lactis without selection at efficiency of 0.4–19% b) High ssDNA concentration, optimal electroporation and optimized oligonucleotide design were required Van Pijkeren and Britton 2012 CRISPR-Cas9 Targeted mutations on chromosome Lb. reuteri, S. thermophilus, Lb. gasseri (a) Counter-selection against wild-type cells after genome modification (b) Combined with single-stranded DNA recombineering for the generation of subtle mutations Van Pijkeren and Britton 2014 Hidalgo-Cantabrana et al.2017 View Large Besides the inducible gene expression systems, the development of specific gene regulatory elements in LAB, such as synthetic promoter libraries (Jensen and Hammer 1998; Solem and Jensen 2002; Rud et al.2006), inducible promoters (e.g. PlacA, PlacSynth and PxylA), as well as an orthologous expression system for controlled gene expression in L. plantarum and reporter gene mCherry expression have been developed and characterized (Heiss et al.2016). In the past decades, considerable effort has focused on the development of LAB genome modification in a markerless way. These include a system for generating chromosomal insertions based on conditional replication of Ori+ RepA- vector pORI19 and a temperature-sensitive helper plasmid pVE6007 in L. lactis (Law et al.1995), a Cre-lox-based system for multiple gene deletions in Lb. plantarum (Lambert, Bongers and Kleerebezem 2007), and a gene replacement system involving the use of upp- encoded uracil phosphoribosyltransferase (UPRTase) of Lb. acidophilus NCFM as a counter-selection marker (Goh et al.2009). More recent applications of LAB genome engineering involving single stranded oligonucleotide- mediated recombineering rather than double-stranded DNA, have been reported (van Pijkeren and Britton 2012). Some of the limitations of this approach are the need for high amount of oligonucleotide and the low recombineering efficiency. The advent of CRISPR technologies has now broadened the avenues for generating mutations, deletions, insertions, etc., and the LAB CRISPR-Cas systems provide great opportunities for improving starter culture functional characteristics and probiotic features (Barrangou and van Pijkeren 2016; Hidalgo-Cantabrana, O´Flaherty and Barrangou 2017) as well as for production of native or non-native metabolites. Targeted mutagenesis in Lb. reuteri chromosome through CRISPR-Cas single stranded DNA recombineering has been demonstrated with very high efficiencies (Oh and van Pijkeren 2014). CENTRAL CARBON METABOLISM AND LACTIC ACID PRODUCTION Besides the extensive application in the food industry, the main industrial application of LAB is in the fermentative production of lactic acid (a 3-carbon hydroxylacid), the primary product of carbohydrate metabolism. Lactic acid is among the most important industrial product to date, with a long history of use in food and nonfood sectors such as cosmetics, pharmaceuticals, chemicals and agriculture. But the demand for lactic acid during the past decades has been driven by the growing market for polylactic acid (PLA), a biodegradable and biocompatible thermoplastic polyester finding diverse applications in packaging materials, films, fibers, nonwoven fabrics, medical implants, drug delivery scaffolds, etc. (Castro-Aguirre et al.2016). The global market size for lactic acid and PLA was estimated at USD 2.08 billion and 1.29 billion, respectively, in 2016, and is expected to grow in the coming years to meet the demand for packaging, personal care and textiles (http://www.grandviewresearch.com/industry-analysis/lactic-acid-and-poly-lactic-acid-market). Natureworks LLC, Corbion, Galactic, Archer Daniel and Pyramid Bioplastics are currently among the major producers of lactic acid and PLA. The possibility to make optically pure lactic acid required for PLA production has made microbial fermentation the most desired route for lactic acid production from sugars in contrast to the petrochemical route. LAB metabolize sugars by fermentation under microaerophilic conditions and generate ATP by substrate level phosphorylation. Lactic acid is the main product from glucose in homofermentative LAB (Streptococcus, Lactococcus, Enterococcus, Pediococcus, and some Lactobacillus species) formed via the Embden–Meyerhof (EM) pathway that yields 2 mol pyruvate/mol glucose, which acts as an electron acceptor and is reduced to 2 mol lactic acid in a reaction catalyzed by lactate dehydrogenase (LDH) (Fig. 1A). Phosphofructokinase enzyme was shown to have an important role in the EM pathway flux, decreased or increased activity resulting in proportionally lowered or improved flux and lactate formation (Andersen et al.2001). The homofermentative bacteria have been found to shift to mixed acid fermentation under conditions of carbon limitation, low growth rates and change in oxygen concentration. In the presence of oxygen, increase in NADH oxidase (Nox) activity leads to competition for the available NADH and hence shift in metabolism to yield a mixture of products (de Felipe, Starrenburg and Hugenholtz 1997). The heterofermentative strains (Leuconostoc, Weissella genera and certain Lactobacillus species) produce also ethanol/acetate and CO2 (1 mol/mol sugar) besides lactate (1 mol) via the phosphoketolase (PK) pathway (Fig. 1B) (Bosma, Forster and Nielsen 2017). Figure 1. View largeDownload slide Overview of the central carbon metabolic pathways of glucose, maltose, sucrose, fructose, and glycerol in lactic acid bacteria. (A) Embden–Meyerhof (glycolytic) pathway showing the metabolism under standard fermentation conditions. Blue arrows indicate the different reactions of homolactic fermentation through glycolytic pathway, while purple arrows indicate the different reaction of heterolactic fermentation through the glycolytic pathway. (B) Phosphoketolase (PK) pathway showing the metabolism under standard fermentation conditions. Red arrows indicate the different reactions of heterolactic acid fermentation through the PK pathway. Substrates, major products, and important intermediates are marked in bold. Enzyme abbreviations (Blue capitals): ScrP: sucrose phosphorylase, HK: hexokinase, MP: maltose phosphorylase, PGM: phosphoglucomutase, PGI: phosphoglucose isomerase, PFK: phosphofructokinase, GK: glycerol kinase, GPDH: glycerol-3-phosphate dehydrogenase, ALDO: fructose bisphosphate aldolase, TPI: triose-phosphate isomerase, LDH: lactate dehydrogenase, PDC: pyruvate dehydrogenase complex, PTA: phosphotransacetylase, ACK: acetate kinase, ADHE: bifunctional aldehyde and alcohol dehydrogenase, G6PDH: glucose-6-P dehydrogenase, PGL: phosphoglucolactonase, 6PGDH: 6-P-gluconate dehydrogenase, RPE: ribulose epimerase, PK: phosphoketolase. Intermediates abbreviations: G-6-P: glucose-6-phosphate, G-1-P: glucose-1-phosphate, F-6-P: fructose-6-phosphate, FBP: fructose-1,6-biphosphate, GAP: glyceraldehyde-3-phosphate, DHAP: dihydroxyacetone phosphate, G3P: glycerol-3-phosphate, 6-PG: 6-phophogluconate, R-5-P: ribulose-5-phosphate, and X-5-P: xylulose-5-phosphate. Figure 1. View largeDownload slide Overview of the central carbon metabolic pathways of glucose, maltose, sucrose, fructose, and glycerol in lactic acid bacteria. (A) Embden–Meyerhof (glycolytic) pathway showing the metabolism under standard fermentation conditions. Blue arrows indicate the different reactions of homolactic fermentation through glycolytic pathway, while purple arrows indicate the different reaction of heterolactic fermentation through the glycolytic pathway. (B) Phosphoketolase (PK) pathway showing the metabolism under standard fermentation conditions. Red arrows indicate the different reactions of heterolactic acid fermentation through the PK pathway. Substrates, major products, and important intermediates are marked in bold. Enzyme abbreviations (Blue capitals): ScrP: sucrose phosphorylase, HK: hexokinase, MP: maltose phosphorylase, PGM: phosphoglucomutase, PGI: phosphoglucose isomerase, PFK: phosphofructokinase, GK: glycerol kinase, GPDH: glycerol-3-phosphate dehydrogenase, ALDO: fructose bisphosphate aldolase, TPI: triose-phosphate isomerase, LDH: lactate dehydrogenase, PDC: pyruvate dehydrogenase complex, PTA: phosphotransacetylase, ACK: acetate kinase, ADHE: bifunctional aldehyde and alcohol dehydrogenase, G6PDH: glucose-6-P dehydrogenase, PGL: phosphoglucolactonase, 6PGDH: 6-P-gluconate dehydrogenase, RPE: ribulose epimerase, PK: phosphoketolase. Intermediates abbreviations: G-6-P: glucose-6-phosphate, G-1-P: glucose-1-phosphate, F-6-P: fructose-6-phosphate, FBP: fructose-1,6-biphosphate, GAP: glyceraldehyde-3-phosphate, DHAP: dihydroxyacetone phosphate, G3P: glycerol-3-phosphate, 6-PG: 6-phophogluconate, R-5-P: ribulose-5-phosphate, and X-5-P: xylulose-5-phosphate. Production of L-(+)-/D-(-)-lactic acid The homofermentative bacteria, mainly Lactobacillus species, are used for industrial lactic acid production. Lactic acid production from a variety of raw materials by different LAB strains has been reviewed earlier (Mazzoli et al.2014). The optical purity of the lactic acid produced depends on the type of LDH present, L- or D-LDH or both and also the presence of lactate racemases that interconvert the two isomers. Lb. casei, Lb. paracasei and Lb. rhamnosus produce L-lactic acid, Lb. delbrueckii, Lb. coryniformis, Lb. jensenii and Lb. vitulinus produce D-lactic acid, while Lb. pentosus, Lb. plantarum, Lb. brevis, Lb. sake and Lb. acidophilus produce DL-lactic acid (Mack 2004). L-Lactic acid is the main product available commercially; in recent years, stereocomplex PLA, a highly thermostable polymer composed of both L- and D-lactic acid monomers has led to interest even for the production of D-lactic acid (Okano et al.2009a,b,c; 2010; 2017). Several studies have targeted increased production of optically pure lactic acid by different strains of LAB by disruption/deletion of the ldh gene encoding for the enzyme catalyzing the production of the unwanted isomer or by genome shuffling (Table 2). The application of genome shuffling to enhance glucose tolerance as well as L-lactic acid production by Lb. rhamnosus resulted in a strain producing 184 g/L of L-lactic acid from 200 g/L glucose in batch fermentation (Yu et al.2008). L-Lactic acid overproduction was also achieved in L. lactis strain subjected to UV-mutagenesis for enhancement of glucose and lactate tolerance; the mutant was found to possess reduced Nox activity and increased glucose uptake rate (Bai et al.2004a). Inactivation of the ldhD gene has resulted in the formation of pure L-lactic acid in several LAB including Lb. helveticus, Lb. paracasei and Pediococcus acidilactici (Bhowmik and Steele 1994; Kylä-Nikkilä et al.2000; Kuo et al.2015; Yi et al.2016), while selective production of D-LDH has been achieved by disruption of ldhL gene e.g. in P. acidilactici and Lb. plantarum (Yi et al.2016; Okano et al.2009a,b,c, 2017). Table 2. Examples of the metabolic engineering strategies used for production of optically pure lactic acid by different LAB species Lactic acid isomer LAB species Engineering strategy Comments Reference L-(+ )- Lactobacillus helveticus CNRZ32 ldhD-negative strains constructed by deletion of promoter region of the ldhD gene (GRL86) or by replacing the structural gene of ldhD with an additional copy of the structural gene of ldhL from the same species (GRL89) Maximal productivity of pure L-lactic acid was 3.34 and 3.21 g/L.h for GRL86 and GRL89, respectively, in modified whey medium. Product yield of 76.2% for GRL86 and 91.6% for GRL89. At low pH when the growth and production were uncoupled, GRL89 produced 20% more lactic acid than GRL86 Kylä-Nikkilä et al.2000 L-(+ )- Lactobacillus lactis BME5–18 UV mutagenesis to enhance tolerance to glucose and lactate repression 98.6 g/L L-lactic acid from 100 g/l glucose, volumetric productivity of 1.76 g/L.h Bai et al.2004a L-(+ )- Lactobacillus rhamnosus ATCC11443 Genome shuffling by generating the starting population by UV and nitrosoguanidine mutagenesis, followed by recursive protoplast fusion 184 g/L of L-lactic acid produced at 99% enantiomeric purity from 200 g/l glucose in batch fermentation. Yu et al.2008 L-(+ )- Lactobacillus paracasei 7BL Interruption of the ldhD gene 215 g/L of L-lactic acid produced during fed-batch fermentation on glucose. 99 g/L with 0.96 g/g yield and productivity of 2.25–3.23 g/L.h using non-detoxified wood hydrolysate. Productivity of 5.27 g/L.h using rice straw hydrolysate without detoxification Kuo et al.2015 L-(+ )- Pediococcus acidilactici TY112 ldhD gene disruption in the wild-type P. acidilactici DQ2 strain 77.66 g/L of L-lactic acid obtained at 25% (w/w) solids content of dry dilute acid pretreated and biodetoxified corn stover feedstock Yi et al.2016 D-(-)- P. acidilactici ZP26 ldhL gene disruption in the wild-type P. acidilactici DQ2 strain 76.76 g/L of D-lactic acid obtained at 25% (w/w) solids content of dry dilute acid pretreated and biodetoxified corn stover feedstock Yi et al.2016 D-(-)- Lactobacillus plantarum NCIMB8826 (i) L-LDH gene (ldhL1) deleted and a plasmid encoding Streptococcus bovis 148 α-amylase (AmyA) introduced (ii) Replacing the ldhL1 gene with an amyA-secreting expression cassette 73.2 g/L of lactic acid produced from raw corn starch with yield of 0.85 g/g of consumed sugar, and an optical purity of 99.6%. Okano et al.2009c D-(-)- Lactobacillus plantarum NCIMB8826 Disruption of L-LDH gene and adaptation of the cells to low pH condition during the early stage of simultaneous saccharification and fermentation 118–129.8 g/L of D-lactic acid obtained from brown rice at a yield of 0.93 g/g and optical purity of 99.6% and volumetric productivity of 2.18 g/L.h Okano et al.2017 Lactic acid isomer LAB species Engineering strategy Comments Reference L-(+ )- Lactobacillus helveticus CNRZ32 ldhD-negative strains constructed by deletion of promoter region of the ldhD gene (GRL86) or by replacing the structural gene of ldhD with an additional copy of the structural gene of ldhL from the same species (GRL89) Maximal productivity of pure L-lactic acid was 3.34 and 3.21 g/L.h for GRL86 and GRL89, respectively, in modified whey medium. Product yield of 76.2% for GRL86 and 91.6% for GRL89. At low pH when the growth and production were uncoupled, GRL89 produced 20% more lactic acid than GRL86 Kylä-Nikkilä et al.2000 L-(+ )- Lactobacillus lactis BME5–18 UV mutagenesis to enhance tolerance to glucose and lactate repression 98.6 g/L L-lactic acid from 100 g/l glucose, volumetric productivity of 1.76 g/L.h Bai et al.2004a L-(+ )- Lactobacillus rhamnosus ATCC11443 Genome shuffling by generating the starting population by UV and nitrosoguanidine mutagenesis, followed by recursive protoplast fusion 184 g/L of L-lactic acid produced at 99% enantiomeric purity from 200 g/l glucose in batch fermentation. Yu et al.2008 L-(+ )- Lactobacillus paracasei 7BL Interruption of the ldhD gene 215 g/L of L-lactic acid produced during fed-batch fermentation on glucose. 99 g/L with 0.96 g/g yield and productivity of 2.25–3.23 g/L.h using non-detoxified wood hydrolysate. Productivity of 5.27 g/L.h using rice straw hydrolysate without detoxification Kuo et al.2015 L-(+ )- Pediococcus acidilactici TY112 ldhD gene disruption in the wild-type P. acidilactici DQ2 strain 77.66 g/L of L-lactic acid obtained at 25% (w/w) solids content of dry dilute acid pretreated and biodetoxified corn stover feedstock Yi et al.2016 D-(-)- P. acidilactici ZP26 ldhL gene disruption in the wild-type P. acidilactici DQ2 strain 76.76 g/L of D-lactic acid obtained at 25% (w/w) solids content of dry dilute acid pretreated and biodetoxified corn stover feedstock Yi et al.2016 D-(-)- Lactobacillus plantarum NCIMB8826 (i) L-LDH gene (ldhL1) deleted and a plasmid encoding Streptococcus bovis 148 α-amylase (AmyA) introduced (ii) Replacing the ldhL1 gene with an amyA-secreting expression cassette 73.2 g/L of lactic acid produced from raw corn starch with yield of 0.85 g/g of consumed sugar, and an optical purity of 99.6%. Okano et al.2009c D-(-)- Lactobacillus plantarum NCIMB8826 Disruption of L-LDH gene and adaptation of the cells to low pH condition during the early stage of simultaneous saccharification and fermentation 118–129.8 g/L of D-lactic acid obtained from brown rice at a yield of 0.93 g/g and optical purity of 99.6% and volumetric productivity of 2.18 g/L.h Okano et al.2017 View Large Table 2. Examples of the metabolic engineering strategies used for production of optically pure lactic acid by different LAB species Lactic acid isomer LAB species Engineering strategy Comments Reference L-(+ )- Lactobacillus helveticus CNRZ32 ldhD-negative strains constructed by deletion of promoter region of the ldhD gene (GRL86) or by replacing the structural gene of ldhD with an additional copy of the structural gene of ldhL from the same species (GRL89) Maximal productivity of pure L-lactic acid was 3.34 and 3.21 g/L.h for GRL86 and GRL89, respectively, in modified whey medium. Product yield of 76.2% for GRL86 and 91.6% for GRL89. At low pH when the growth and production were uncoupled, GRL89 produced 20% more lactic acid than GRL86 Kylä-Nikkilä et al.2000 L-(+ )- Lactobacillus lactis BME5–18 UV mutagenesis to enhance tolerance to glucose and lactate repression 98.6 g/L L-lactic acid from 100 g/l glucose, volumetric productivity of 1.76 g/L.h Bai et al.2004a L-(+ )- Lactobacillus rhamnosus ATCC11443 Genome shuffling by generating the starting population by UV and nitrosoguanidine mutagenesis, followed by recursive protoplast fusion 184 g/L of L-lactic acid produced at 99% enantiomeric purity from 200 g/l glucose in batch fermentation. Yu et al.2008 L-(+ )- Lactobacillus paracasei 7BL Interruption of the ldhD gene 215 g/L of L-lactic acid produced during fed-batch fermentation on glucose. 99 g/L with 0.96 g/g yield and productivity of 2.25–3.23 g/L.h using non-detoxified wood hydrolysate. Productivity of 5.27 g/L.h using rice straw hydrolysate without detoxification Kuo et al.2015 L-(+ )- Pediococcus acidilactici TY112 ldhD gene disruption in the wild-type P. acidilactici DQ2 strain 77.66 g/L of L-lactic acid obtained at 25% (w/w) solids content of dry dilute acid pretreated and biodetoxified corn stover feedstock Yi et al.2016 D-(-)- P. acidilactici ZP26 ldhL gene disruption in the wild-type P. acidilactici DQ2 strain 76.76 g/L of D-lactic acid obtained at 25% (w/w) solids content of dry dilute acid pretreated and biodetoxified corn stover feedstock Yi et al.2016 D-(-)- Lactobacillus plantarum NCIMB8826 (i) L-LDH gene (ldhL1) deleted and a plasmid encoding Streptococcus bovis 148 α-amylase (AmyA) introduced (ii) Replacing the ldhL1 gene with an amyA-secreting expression cassette 73.2 g/L of lactic acid produced from raw corn starch with yield of 0.85 g/g of consumed sugar, and an optical purity of 99.6%. Okano et al.2009c D-(-)- Lactobacillus plantarum NCIMB8826 Disruption of L-LDH gene and adaptation of the cells to low pH condition during the early stage of simultaneous saccharification and fermentation 118–129.8 g/L of D-lactic acid obtained from brown rice at a yield of 0.93 g/g and optical purity of 99.6% and volumetric productivity of 2.18 g/L.h Okano et al.2017 Lactic acid isomer LAB species Engineering strategy Comments Reference L-(+ )- Lactobacillus helveticus CNRZ32 ldhD-negative strains constructed by deletion of promoter region of the ldhD gene (GRL86) or by replacing the structural gene of ldhD with an additional copy of the structural gene of ldhL from the same species (GRL89) Maximal productivity of pure L-lactic acid was 3.34 and 3.21 g/L.h for GRL86 and GRL89, respectively, in modified whey medium. Product yield of 76.2% for GRL86 and 91.6% for GRL89. At low pH when the growth and production were uncoupled, GRL89 produced 20% more lactic acid than GRL86 Kylä-Nikkilä et al.2000 L-(+ )- Lactobacillus lactis BME5–18 UV mutagenesis to enhance tolerance to glucose and lactate repression 98.6 g/L L-lactic acid from 100 g/l glucose, volumetric productivity of 1.76 g/L.h Bai et al.2004a L-(+ )- Lactobacillus rhamnosus ATCC11443 Genome shuffling by generating the starting population by UV and nitrosoguanidine mutagenesis, followed by recursive protoplast fusion 184 g/L of L-lactic acid produced at 99% enantiomeric purity from 200 g/l glucose in batch fermentation. Yu et al.2008 L-(+ )- Lactobacillus paracasei 7BL Interruption of the ldhD gene 215 g/L of L-lactic acid produced during fed-batch fermentation on glucose. 99 g/L with 0.96 g/g yield and productivity of 2.25–3.23 g/L.h using non-detoxified wood hydrolysate. Productivity of 5.27 g/L.h using rice straw hydrolysate without detoxification Kuo et al.2015 L-(+ )- Pediococcus acidilactici TY112 ldhD gene disruption in the wild-type P. acidilactici DQ2 strain 77.66 g/L of L-lactic acid obtained at 25% (w/w) solids content of dry dilute acid pretreated and biodetoxified corn stover feedstock Yi et al.2016 D-(-)- P. acidilactici ZP26 ldhL gene disruption in the wild-type P. acidilactici DQ2 strain 76.76 g/L of D-lactic acid obtained at 25% (w/w) solids content of dry dilute acid pretreated and biodetoxified corn stover feedstock Yi et al.2016 D-(-)- Lactobacillus plantarum NCIMB8826 (i) L-LDH gene (ldhL1) deleted and a plasmid encoding Streptococcus bovis 148 α-amylase (AmyA) introduced (ii) Replacing the ldhL1 gene with an amyA-secreting expression cassette 73.2 g/L of lactic acid produced from raw corn starch with yield of 0.85 g/g of consumed sugar, and an optical purity of 99.6%. Okano et al.2009c D-(-)- Lactobacillus plantarum NCIMB8826 Disruption of L-LDH gene and adaptation of the cells to low pH condition during the early stage of simultaneous saccharification and fermentation 118–129.8 g/L of D-lactic acid obtained from brown rice at a yield of 0.93 g/g and optical purity of 99.6% and volumetric productivity of 2.18 g/L.h Okano et al.2017 View Large Substrate utilization Lactic acid production at industrial scale uses primarily first generation feedstocks i.e. corn starch and cane sugar. Since LAB are able to metabolize a range of sugars as carbon sources (depending on the species/strains) they are suitable organisms for utilizing a wide range of biobased feedstocks including pretreated/hydrolysed lignocellulosic agricultural and forestry residues, algal biomass and municipal solid wastes (Abdel-Rahman, Tashiro and Sonomoto 2011a; Mazzoli et al.2014; Bai et al.2016; Overbeck, Steele and Broadbent 2016; Wang et al.2017; Tarraran and Mazzoli 2018). Except for some bacteria that secrete amylases and have the ability to utilize starch directly (Reddy et al.2008), LAB, in general, do not accept polysaccharides as substrates, thus a hydrolysis step normally precedes fermentation or can be performed simultaneously (SSF, simultaneous saccharification and fermentation) using externally added enzymes. Utilization of disaccharides including sucrose and lactose is common among LAB, while a few bacteria are able to metabolize also cellobiose and other cellodextrins (Gandini et al.2017) or oligosaccharides (with up to 4–6 units) derived from hemicellulose hydrolysis (Ohara, Owaki and Sonomoto 2006; Lawley, Sims and Tannock 2013). Mixed LAB cultures have been used to maximize yield and productivity from mixtures of pentose and hexose sugars (Taniguchi et al.2004; Cui, Li and Wan 2011); however, a few LAB strains, for example, Lb. brevis, Lb. buchneri, Lb. plantarum and Enterococcus mundtii, have been reported to consume C5 and C6 sugars simultaneously without carbon catabolite repression, (Guo et al.2010; Kim, Block and Mills 2010; Abdel-Rahman et al.2011b). Two different pathways are proposed for metabolism of pentoses, the PK pathway used by majority of pentose utilizing LAB yields 1 mol lactic acid/mol sugar, whereas the pentose phosphate (PP) pathway/glycolytic pathway provides a theoretical lactic acid yield of 1.67 mol/mol (Tanaka et al.2002; Oshiro et al.2009) (Fig. 2). Among wild type LAB, E. mundtii QU25 (Abdel-Rahman et al.2011b), Streptococcus sp., Lb. thermophilus (Fukui et al.1957) and Lactobacillus strain MONT4 (Barre 1978) were shown to display homofermentative metabolism of pentose sugars. An interesting observation made in case of Lactococcus lactis IO-1 was the effect of increase in xylose concentration from <5 g/L to >50 g/L on shift in xylulose 5-phosphate metabolism from the PK pathway to PP/glycolytic pathway, and of pyruvate metabolism from cleavage to acetyl-CoA and formic acid to the reduction to L-lactate by LDH (Tanaka et al.2002). Depleting the PK gene and overexpressing a heterologous transketolase gene in Lb. plantarum also shifted the carbon metabolism from PK to PP pathway with arabinose and glucose as substrates to give high molar yields of lactic acid (Okano et al.2009a,b). In another study, two copies of xylAB operon of Lb. pentosus have been introduced in the genome of Lb. plantarum, which converted a mixture of 25 g/L xylose and 75 g/L glucose without carbon catabolite repression effects, to produce D-lactic acid with a yield of 0.78 g/g of consumed sugar (Yoshida et al.2011). Figure 2. View largeDownload slide Overview of the central carbon metabolic pathways of pentoses in lactic acid bacteria under standard fermentative conditions. Golden arrows indicate the different steps of pentose phosphate (PP) pathway, Red arrows indicate the different reactions of heterolactic acid fermentation through the phosphoketolase (PK) pathway, Blue arrows indicate the different reactions of homolactic fermentation through glycolytic pathway, and Purple arrows indicate the different reactions of heterolactic fermentation through the EM pathway. Substrates, major products and important intermediates are marked in bold. Enzyme abbreviations (Blue capitals): PFK: phosphofructokinase, ALDO: fructose bisphosphate aldolase, TPI: triose-phosphate isomerase, LDH: lactate dehydrogenase, PDC: pyruvate dehydrogenase complex, PTA: phosphotransacetylase, ACK: acetate kinase, ADHE: bifunctional aldehyde and alcohol dehydrogenase, RPE: ribulose epimerase, XI: xylose isomerase, AI: arabinose isomerase, XK: xylulose kinase, RK: ribulose kinase, RPI: ribose-5-phosphate isomerase, TK: transketolase, TA: transaldolase, PK: phosphoketolase. Intermediates abbreviations: F-6-P: fructose-6-phosphate, FBP: fructose-1,6-biphosphate, GAP: glyceraldehyde-3-phosphate, DHAP: dihydroxyacetone phosphate, E-4-P: erythrose-4-phosphate, S-7-P: Sedoheptulose-7-phosphate, R-5-P: ribulose-5-phosphate, and X-5-P: xylulose-5-phosphate. Figure 2. View largeDownload slide Overview of the central carbon metabolic pathways of pentoses in lactic acid bacteria under standard fermentative conditions. Golden arrows indicate the different steps of pentose phosphate (PP) pathway, Red arrows indicate the different reactions of heterolactic acid fermentation through the phosphoketolase (PK) pathway, Blue arrows indicate the different reactions of homolactic fermentation through glycolytic pathway, and Purple arrows indicate the different reactions of heterolactic fermentation through the EM pathway. Substrates, major products and important intermediates are marked in bold. Enzyme abbreviations (Blue capitals): PFK: phosphofructokinase, ALDO: fructose bisphosphate aldolase, TPI: triose-phosphate isomerase, LDH: lactate dehydrogenase, PDC: pyruvate dehydrogenase complex, PTA: phosphotransacetylase, ACK: acetate kinase, ADHE: bifunctional aldehyde and alcohol dehydrogenase, RPE: ribulose epimerase, XI: xylose isomerase, AI: arabinose isomerase, XK: xylulose kinase, RK: ribulose kinase, RPI: ribose-5-phosphate isomerase, TK: transketolase, TA: transaldolase, PK: phosphoketolase. Intermediates abbreviations: F-6-P: fructose-6-phosphate, FBP: fructose-1,6-biphosphate, GAP: glyceraldehyde-3-phosphate, DHAP: dihydroxyacetone phosphate, E-4-P: erythrose-4-phosphate, S-7-P: Sedoheptulose-7-phosphate, R-5-P: ribulose-5-phosphate, and X-5-P: xylulose-5-phosphate. Alternative strategies are being investigated to develop systems wherein the lignocellulosic material can be used directly as the feedstock and consequently lower the dependence on pretreatment and enzymatic hydrolysis and the associated costs. These strategies include the use of consortia of cellulolytic microorganism and LAB, or engineering cellulolytic activities in LAB (Tarraran and Mazzoli 2018). A stable consortium of Trichoderma reesei, a well-known cellulolytic fungus and Lb. pentosus was used for consolidated bioprocessing of pretreated beech wood slurry with a lactic acid yield of 85.2% of the theoretical maximum (Shahab et al.2018). For engineering LAB, the cellulolytic enzymes from aerobic fungi and bacteria as well as cellulosome complexes present in anaerobic microorganisms have been used. Although several studies have reported the expression of a single enzyme often using inducible promoters (Tarraran and Mazzoli 2018), increasing number of reports are emerging on developing strains expressing more than one polysaccharide degrading enzymes, which allow the recombinant strains to grow on the feedstocks being treated (Nguyen et al.2016; Gandini et al.2017). Since expression of multiple enzymes puts a burden on individual cells, the genes for the different enzymes have been introduced in different cells to maximize their ability to grow and produce each enzyme. Recombinant Lb. plantarum strains developed by expression of Thermobifida fusca cellulase Cel6A and xylanase Xyn11A-encoding genes have shown a synergistic effect on the release of soluble sugars from hypochlorite-pretreated wheat straw when used as a two-strain consortium secreting the enzymes (Morais et al.2013). In another study, Lb. plantarum consortium secreting the enzymes was much more efficient than the one with anchored enzymes (Morais et al.2014). On the other hand, the consortium containing an additional strain expressing a scaffoldin on the cell surface for capturing the secreted dockerin-containing cellulase and xylanase to make up a functional cellulosome had only slightly reduced activity than the secreting consortium but provided higher stability of the enzymes (Morais et al.2014). The same research group has further developed adaptor scaffoldins with divergent cohesins for selectively binding enzymes containing different dockerins—an approach that can potentially allow the display of several enzymes by the cell consortia (Stern et al.2018). Besides the sugar containing feedstocks, glycerol generated in large amounts as a by-product from biodiesel, ethanol and soap manufacture has made it a renewable raw material of choice for a variety of chemicals. Glycerol is used as a carbon source for growth by some LAB, in which it enters the EM pathway at dihydroxyacetone phosphate (Fig. 1A). Although glycerol was predicted as a likely carbon source for Lb. plantarum according to the flux balance analysis model based on its genome sequence, the organism showed only limited growth on it. Adaptive evolution under growth conditions using oxygen as the electron acceptor resulted in a Lb. plantarum strain converting glycerol completely to lactic acid, the evolved strain revealing promoter mutations that relieved the catabolite repression of the glycerol operon (Teusink et al.2009). Overcoming lactic acid process limitations LAB acquire their nutrition from the environment and metabolize the substrate at a very high rate giving high yields and productivity of lactic acid at low cell biomass concentrations, which is interesting from an industrial perspective as much of the carbon source is used for product formation. However, the need for complex nutritional media for growth increases the production cost. Hence, besides using surplus/residual biomass as the feedstock inexpensive nutritional options, for example, cheese whey, soybean meal hydrolysate, corn steep liquor, vinasse, residual protein hydrolysates from poultry processing and other wastes have been of interest for LAB processes (Kwon et al.2000; Kim et al. 2006; Vázquez and Murado 2008; Salgado et al.2009; Lazzi et al.2013). Lactic acid fermentation is subject to substrate inhibition, which has been overcome by performing fermentations in fed-batch mode with resultant high product yield and concentration (Bai et al.2004b; Ding and Tan 2006). Lactic acid fermentations are invariably controlled at pH5.0–5.5. Traditionally, the pH control has been achieved using calcium hydroxide but lactic acid recovery by acidification of the calcium salt results in the generation of large amounts of gypsum, which becomes an environmental burden. pH control can also be achieved using NH4OH or NaOH but requires alternative separation techniques such as electrodialysis or chromatography for the recovery of pure lactic acid, which are cleaner but more expensive to run. The most desirable option is to run the fermentations at low pH, i.e. close to pH 3.8, the pK value of lactic acid, which besides lowering the chemical cost, would allow the product to be recovered in the acid form by, for example, extraction. Different non-specific approaches such as genome shuffling, adaptive evolution, and error-prone whole genome amplification have been used to obtain strains that are able to grow at substantially lower pH than the wild type strains (Patnaik et al.2002; Zhang et al.2012; Ye, Zhao and Wu 2013). Studies with acid resistant L. lactis mutants showed that the organism possesses several means for survival at low pH, one of which results in multiple stress resistance (Rallu et al.2000). In particular, levels of intracellular phosphate and guanine nucleotides determined the extent of the stress response. Amino acid regulation inside the cells is considered to be one of the main mechanisms applied by LAB to counter acid stress. Acid resistant Lb. casei strain obtained by adaptive evolution was shown to have higher intracellular pH, NH4+ concentration and lower inner membrane permeability besides higher amounts of arginine and aspartate levels under acid stress (Zhang et al.2012). Ammonia production and consequent intracellular pH control was attributed to arginine metabolism via arginine deiminase pathway that is active at low pH; even aspartate is likely to be converted to arginine. Amino acid decarboxylation pathway, identified in different LAB like Lb. buchneri and Lb. brevis, is proposed to provide resistance by way of releasing CO2 that results in consumption of protons, hence counteracting cytosol acidification and generating a proton motive force (Wolken et al.2006). Engineering of such a pathway (histidine decarboxylation) in L. lactis was shown to result in enhanced tolerance to acid stress (Trip, Mulder and Lolkema 2012). Overexpressing glutathione biosynthetic genes from Escherichia coli into L. lactis also showed improved resistance to acid stress, which was ascribed to increased intracellular pH and/or stabilization of glyceraldehyde-3-phosphate dehydrogenase activity (Zhang et al.2007). On the other hand, overexpression of endogenous genes in trehalose catabolic pathway, trePP and pgmB encoding trehalose 6-phosphate phosphorylase and β-phosphoglucomutase, respectively, together with Propionibacterium freudenrichii otsB gene encoding trehalose 6-phosphate phosphatase in L. lactis showed improved tolerance to acid as well as to cold and heat shock (Carvalho et al.2011). REROUTING CARBON METABOLISM TO FOOD INGREDIENTS, PLATFORM CHEMICALS AND BIOFUELS Engineering at the pyruvate node Pyruvate is a key metabolic intermediate in the central carbon metabolism in all organisms. The significance of pyruvate as an electron acceptor and of LDH catalyzed reduction to lactate in LAB have motivated many studies to understand the effect of inactivating LDH on the cells and also to reroute the carbon flow from pyruvate to another product through metabolic engineering of an alternative cofactor regeneration pathway. Deletion of the ldh gene has often turned out to be a difficult task as the mutants generated were unstable because of the activation of an alternative ldh gene via a genetic insertion event (Gaspar et al.2007). The alternative LDH enzymes compete with other native dehydrogenases in the regeneration of reduced cofactor and as a result diverting pyruvate to form a mixture of other products (Viana et al.2005; Gaspar et al.2013). Deletion of at least three of the four ldh genes was required to obtain a genetically stable strain for production of alternative reduced compounds (Gaspar et al.2011). Several LAB have the potential to establish an aerobic respiratory chain, which reduces O2 to H2O in the presence of H+ when grown in an aerobic environment in the presence of exogenous heme (and menaquinones for some bacteria) (Lechardeur et al.2011). This respiratory metabolism improves significantly the growth and survival of the bacteria and finds industrial applications in the production of dairy starter cultures (Garrigues et al.2006). Under aerobic conditions, L. lactis was found to consume lactate in the stationary phase with concomitant accumulation of diacetyl and acetoin, the reaction being brought about by reversal of the LDH activity when NADH and pyruvate concentrations were extremely low while NAD+ and lactate were abundant (Zhao et al.2013). Fig. 3A shows some of the electron recycling systems introduced for transformation of pyruvate to different organic compounds. Cloning and expression of Bacillus sphaericus alanine dehydrogenase gene under the control of nisin A promoter resulted in rerouting of 30%–40% from lactate to L-alanine, an amino acid used as a food sweetener. Introducing the gene in LDH-deficient mutant resulted in complete conversion of glucose to L-alanine (Hols et al.1999) (Fig. 3A). Figure 3. View largeDownload slide Native and engineered metabolic pathways in lactic acid bacteria to form various products. (A) Pathways diverging from the pyruvate node of central carbon metabolism. Enzyme abbreviations (Blue: native; purple: bold; underlined: heterologous): THL: thiolase, ADH: alcohol dehydrogenase, ALDH: aldehyde dehydrogenase, BCD/ETFAB: butyryl-CoA dehydrogenase complex, CRT: crotonase, HBD: 3-hydroxybutyryl-CoA dehydrogenase, LDH: lactate dehydrogenase, MGS: methylglyoxal synthase, LDH: lactate dehydrogenase, LADH: lactaldehyde dehydrogenase, MeGR: methylglyoxal reductase, AKR: Aldo-keto reductase, LDR: lactaldehyde reductase, PDC: pyruvate decarboxylase complex, ALS: acetolactate synthase, ALDC: acetolactate decarboxylase, ButA: acetoin reductase, ButB: 2,3-butanediol dehydrogenase, AlaD: alanine dehydrogenase. Abbreviations of intermediates: FBP: fructose-1,6-biphosphate, GAP: glyceraldehyde-3-phosphate, DHAP: dihydroxyacetone phosphate, 1,2-PDO: 1,2-propanediol, and 2,3-BDO: 2,3-butanediol. (B) Pathways for transformation of sugars to low calorie polyols. Abbreviations of intermediates: G-1-P: glucose-1-phosphate, G-6-P: glucose-6-phosphate, F-6-P: fructose-6-phosphate, FBP: fructose-1,6-biphosphate, GAP: glyceraldehyde-3-phosphate, DHAP: dihydroxyacetone phosphate, 6-PG: 6-phophogluconate, R-5-P: ribulose-5-phosphate, and X-5-P: xylulose-5-phophate. Enzyme abbreviations (Blue: native, Purple, bold, underlined: heterologous): MDH: mannitol dehydrogenase, MPDH: mannitol-1-phosphate dehydrogenase, M1P: mannitol-1-phosphatase, SPDH: sorbitol-6-phosphate dehydrogenase, S6P: sorbitol-6-phosphatase, and XR: xylose reductase. Figure 3. View largeDownload slide Native and engineered metabolic pathways in lactic acid bacteria to form various products. (A) Pathways diverging from the pyruvate node of central carbon metabolism. Enzyme abbreviations (Blue: native; purple: bold; underlined: heterologous): THL: thiolase, ADH: alcohol dehydrogenase, ALDH: aldehyde dehydrogenase, BCD/ETFAB: butyryl-CoA dehydrogenase complex, CRT: crotonase, HBD: 3-hydroxybutyryl-CoA dehydrogenase, LDH: lactate dehydrogenase, MGS: methylglyoxal synthase, LDH: lactate dehydrogenase, LADH: lactaldehyde dehydrogenase, MeGR: methylglyoxal reductase, AKR: Aldo-keto reductase, LDR: lactaldehyde reductase, PDC: pyruvate decarboxylase complex, ALS: acetolactate synthase, ALDC: acetolactate decarboxylase, ButA: acetoin reductase, ButB: 2,3-butanediol dehydrogenase, AlaD: alanine dehydrogenase. Abbreviations of intermediates: FBP: fructose-1,6-biphosphate, GAP: glyceraldehyde-3-phosphate, DHAP: dihydroxyacetone phosphate, 1,2-PDO: 1,2-propanediol, and 2,3-BDO: 2,3-butanediol. (B) Pathways for transformation of sugars to low calorie polyols. Abbreviations of intermediates: G-1-P: glucose-1-phosphate, G-6-P: glucose-6-phosphate, F-6-P: fructose-6-phosphate, FBP: fructose-1,6-biphosphate, GAP: glyceraldehyde-3-phosphate, DHAP: dihydroxyacetone phosphate, 6-PG: 6-phophogluconate, R-5-P: ribulose-5-phosphate, and X-5-P: xylulose-5-phophate. Enzyme abbreviations (Blue: native, Purple, bold, underlined: heterologous): MDH: mannitol dehydrogenase, MPDH: mannitol-1-phosphate dehydrogenase, M1P: mannitol-1-phosphatase, SPDH: sorbitol-6-phosphate dehydrogenase, S6P: sorbitol-6-phosphatase, and XR: xylose reductase. Diacetyl is a strong buttery flavor formed in many dairy products like fresh cheeses, butter-milk, etc. by specific strains of L. lactis and Leuconostoc species through spontaneous oxidative decrboxylation of α-acetolactate (AL) produced from citric acid, a minor component of milk (Hugenholtz 1993). AL is also formed by condensation of two molecules of pyruvate by the enzyme AL synthase (Swindell et al.1996), and is subsequently converted by α-AL decarboxylase (ALDB) to acetoin (Fig. 3A). Based on the earlier findings on overproduction of Streptococcus mutans water-forming Nox in L. lactis that resulted in decreased NADH/NAD+ ratio under aerobic conditions and conversion of pyruvate mostly to acetoin or diacetyl instead of lactate despite the retained LDH activity (Platteeuw et al.1995; de Felipe et al.1998), Hugenholtz et al. (2000) designed an efficient scheme for converting glucose to diacetyl by combining Nox overproduction and aldb gene inactivation (to prevent formation of acetoin), and using oxygen instead of pyruvate as the electron acceptor. The AL-acetoin pathway was also engineered using two different cofactor regeneration approaches for converting acetoin into 2,3-butanediol (2,3-BDO), an important chemical for the production of plastics, solvents, pharmaceuticals and cosmetics (Celinska and Grajek 2009). Overexpression of the native AL synthase and acetoin reductase (ButA) enzymes in L. lactis devoid of the ldh genes, enhanced pyruvate utilization to form 2,3-BDO at a maximum theoretical yield (67%) from glucose under anaerobic conditions (Gaspar et al.2011) (Fig. 3A). Furthermore, the engineered strain exhibited higher growth rate and biomass yield. In a more recent study, L. lactis was transformed into a respiration dependent strain producing large amounts of AL (83.5 mM), which could be converted to diacetyl by metal catalyzed oxidation. The diacetyl was further converted into 2,3-BDO (yield of 0.82 mol/mol glucose) by introducing two additional enzyme activities, diacetyl reductase from Klebsiella pneumonia and butanediol dehydrogenase from Enterobacter cloacae (Liu et al.2016) (Fig. 3A). The cofactor engineering strategy was also successfully employed for the production of acetaldehyde, yet another important aroma compound present in dairy products. It is also a commodity chemical used for the industrial production of a range of chemicals for paints, plasticizers, cosmetics and pharmaceuticals. Limited amounts of acetaldehyde are currently produced from bioethanol (http://www.agrobiobase.com/en/database/bioproducts/consumers-goods/acetaldehyde). Acetaldehyde is normally formed in heterofermentative bacteria as an intermediate during ethanol production from acetyl-CoA by the two-domain aldehyde/alcohol dehydrogenase (ADHE) (Bosma, Forster and Nielsen 2017), but not as an end-product of fermentation. Re-routing of pyruvate towards acetaldehyde in L. lactis was achieved by nisin-controlled overexpression of Zymomonas mobilis pyruvate decarboxylase (pdc) gene with that of endogeneous nox gene (Bongers, Hoefnagel and Kleerebezem 2005). Nox overproduction helped to maintain a very low ratio of NADH/NAD+ even in the absence of molecular oxygen, hence favoring the utilization of pyruvate by NADH independent PDC for acetaldehyde production rather than for lactate or ethanol production. Attempts have been made to develop LAB strains for production of biofuel candidates, ethanol, propanol and butanol production. Some LAB exhibit relatively high tolerance to the solvents, and are hence considered as promising hosts for their production. Although some increases in ethanol levels have been reported by expression of a heterologous PDC in LDH-negative Lb. plantarum, lactate has often turned out to be the main product due to activation of alternative LDHs (Liu et al.2006). The potential of developing LAB for production of ethanol as the main product was demonstrated by engineering of a L. lactis strain involving codon-optimized expression of Z. mobilis PDC and ADHE genes using synthetic promoters and knockout of genes encoding three LDHs, phosphotransacetylase and native ADHE (Solem, Dehli and Jensen 2013). For production of n-propanol by LAB, glucose was initially converted into 1,2-propanediol (1,2-PDO) (see next section), which is further converted into the desired alcohol by the action of enzymes encoded by genes of the propanediol-utilization (Pdu) operon (Christensen et al.2014). The feasibility of butanol production has been investigated by reconstructing butanol synthesis pathway of Clostridium acetobutylicum in Lb. brevis that possesses the genes and several enzyme activities crucial for the formation of butanol (Berezina et al.2010). A recombinant Lb. brevis strain carrying the clostridial bcs operon (comprising genes encoding 3-hydroxybutyryl-CoA dehydrogenase, crotonase, butyryl-CoA dehydrogenase and two subunits of the electron transfer flavoproteins) was able to produce 300 mg/L butanol from glucose, with participation of its own thiolase, aldehyde dehydrogenase and ADHE. Engineering enhanced polyol production Low-calorie polyols including mannitol and sorbitol are among the high value products produced naturally by several heterofermentative LAB (Fig. 3B), with applications in food and pharmaceutical industries, as well as in medicine. D-Sorbitol is also regarded as one of the top 10 renewable value added chemicals from biomass (Bozell and Petersen 2010); being an important precursor for L-ascorbic acid and long chain polyols. Furthermore, isosorbide, a dehydration product of sorbitol, is currently of great interest as a building block of polycarbonates, polyesters, polyurethanes and epoxides (Dussenne et al.2017). In some LAB, fructose is used as a carbon source for growth as well as an electron acceptor, getting converted into mannitol using mannitol dehydrogenase (Fig. 3B) (Saha and Racine 2011). Although produced in significant quantities, mannitol is normally formed along with lactic acid, acetic acid and other metabolites. Inactivation of ldhD and ldhL genes in Lb. fermentum led to the production of mannitol together with pyruvate, and under anaerobic conditions 2,3-BDO was formed as a co-product (Aarnikunnas et al.2003). In Leuconostoc pseudomesenteroides, the mannitol yield from fructose was increased from 74% to 85% by random mutagenesis that led to inactivation of fructokinase activity to about 10% of that in the parent strain, with a resultant reduced leakage of fructose into the PK pathway (Helando et al.2005). On the other hand, in Lb. reuteri, a truncated version of fructokinase from Aspergillus niger along with its activator was expressed, resulting in a strain with enhanced flux through EM pathway and improved ability to handle elevated glucose concentrations, resulting in improved biomass yield and NADH availability for the fructose to be more efficiently transformed to mannitol (Papagianni and Legisa 2014). Attempts have also been made to enhance mannitol production by homofermentative LAB. Highest mannitol yield of 50% from glucose was reported in ldh deficient L. lactis overexpressing mannitol-1-phosphate dehydrogenase and mannitol-1-phosphatase genes from Lb. plantarum and protozoan parasite Eimeria tenella, respectively (Fig. 3B) (Wisselink et al.2005). For sorbitol production, mannitol phosphate dehydrogenase and LDH enzymes were inactivated, and a sorbitol dehydrogenase gene was overexpressed in Lb. plantarum, allowing more fructose 6-phosphate to be reduced to sorbitol 6-phosphate (Fig. 3B) (Ladero et al.2007). This resulted in near theoretical yield of 0.65 mol sorbitol/mol glucose by resting cells. In Lb. casei, sorbitol 6-phosphate dehydrogenase gene was integrated into the lac operon; while the resting cells pre-grown on lactose produced sorbitol at low concentration from glucose, the production was increased by inactivation of ldhL gene (Nissen, Perez-Martinez and Yebra 2005). There has also been interest in the production of xylitol, a five-carbon low calorie sugar alcohol, which is not naturally produced by LAB. Xylitol production was demonstrated in a L. lactis strain engineered with xylose reductase from Pichia stiptis and a xylose transporter from Lb. brevis, and grown in a glucose-limited fed-batch cultivation mode with high xylose concentration (Nyyssola et al.2005) (Fig. 3B). Lb. buchneri, Lb. brevis and Lb. fermentum are able to convert lactate into 1,2-propanediol (1,2-PDO), a polyol widely used in anti-freeze fluids, polyester resins, non-ionic detergents, coolants and additive in cosmetics, nutritional products, pharmaceuticals and dyes (Bennett and San 2001). The two-step conversion occurs at low pH and involves hydrogenation to lactaldehyde catalyzed by lactaldehyde dehydrogenase followed by reduction to 1,2-PDO by 1,2-propanediol dehydrogenase, (Elferink et al.2001; Nishino et al.2003; Bosma, Forster and Nielsen 2017). Two moles of lactate are required for this reaction, one being converted to 1 mol 1,2-PDO and the second to 1 mol acetic acid maintaining cofactor balance (Elferink et al.2001). The titers are however too low. Introduction of methylglyoxal synthase gene in Lb. reuteri converted it into 1,2-PDO producer from glucose mediated through dihydroxyacetone phosphate, methylglyoxal and lactaldehyde, respectively, as intermediates. Simultaneously, n-propanol was obtained from 1,2-PDO by the action of the Pdu pathway (Christensen et al.2014). ENGINEERING ENHANCED PRODUCTION OF SPECIALTY PRODUCTS Production of EPS of different compositions by wide range of LAB is well known; they alter the rheological properties of the matrix in which they are dispersed and also provide prebiotic effect, stress tolerance and facililtate biofilm formation (Caggianiello, Kleerebezem and Spano 2016; Zeidan et al.2017). Increasing demand for EPS with interesting properties has led to efforts in improving the level of production by natural or metabolic engineering approaches. The latter has focused on synthesis of precursors such as UDP-glucose, UDP-galactose, etc. Overexpression of the enzymes involved in the synthesis of intermediates such as phosphoglucomutase and UDP-glucose phosphorylase has led to varying levels of improvements in different LAB (Boels et al.2001; Levander, Svensson and Rådström 2002). The biosynthetic routes of EPS have also been used for heterologous expression of polysaccharides in LAB for which the GRAS status is an advantage over other microbial hosts. An interesting example is that of hyaluronic acid (HA), a copolymer of UDP-glucuronic acid and UDP-acetylglucosamine, with applications in medicine, cosmetics and specialty foods. Combining the expression of a streptococcal HA synthase with UDP-glucose dehydrogenase and pyrophosphorylase in L. lactis resulted in good production of HA (Prasad, Ramachandran and Jayaraman 2012), and even the size of the polymer could be controlled by regulating the ratio of HA synthase and the dehydrogenase enzyme (Sheng et al.2009). Even though several LAB lack the ability to synthesize many vitamins, certain strains have been shown to produce or possess genes related to the biosynthesis of water soluble vitamins belonging to the B groups such as riboflavin, folate and Vitamin B12 that are essential cofactors for important metabolic activities. (Capozzi et al.2012). Riboflavin production has been noted in strains of Lb. fermentum, L. plantarum, L. lactis and L. acidophilus (Thakur, Tomar and De 2016), while folate is produced by L. lactis and other LAB. Pseudovitamin B12, a corrinoid like molecule, was isolated (Santos et al.2007), and also a complete biosynthetic gene cluster of Vitamin B12 was identified in Lb. reuteri CRL 1098 (Taranto et al.2003; Santos et al.2008a). Subsequently, production of active Vitamin B12 was demonstrated in two L. reuteri strains by supplementing the culture medium with 5,6-diemthylbenzimidazole and δ-aminolevulinic acid (ALA) and optimizing the fermentation conditions (Mohammed et al.2014). More recently, isolation of two Lb. plantarum with high extracellular yields of Vitamin B12 has been reported (Li et al.2017). LAB producing vitamins have primarily attracted interest due to the possibility of in situ fortification of the foods and for production of novel vitamin-enriched foods (Hugenholtz 2008; LeBlanc et al.2011; Li et al.2017). Overexpression of the genes involved in folate biosynthesis and of p-aminobenzoic acid has led to increased folate production in L. lactis (Wegkamp et al.2007). Different strategies have been applied to transform riboflavin consuming strains to producer strains by genetic engineering (by overexpression of riboflavin biosynthetic genes) (Burgess et al.2004; Sybesma et al.2004) or by exposure to purine analogues and/or toxic riboflavin analogues roseoflavin (Burgess et al.2004, 2006). LAB producing more than one vitamin were constructed by overexpressing folate biosynthesis genes in L. lactis overproducing riboflavin (Sybesma et al.2004), and in Lb. reuteri producing Vitamin B12, respectively (Santos et al.2008b). Several LAB isolated from traditional fermented foods including Lb. paracasei, Lb. buchneri and Lb. brevis have been found to produce gamma-aminobutyric acid (GABA), a non-protein amino acid (Li and Cao 2010; Lim et al.2017), which acts as an inhibitory neurotransmitter, and is used in functional foods and pharmaceuticals (Dhakal, Bajpai and Baek 2012). It is produced by decarboxylation of L-glutamate in a reaction catalyzed by glutamate decarboxylase, and high yields have been reported by fed-batch cultivation of Lb. brevis (Li et al.2010). Production of the biodegradable polymer, poly-β-hydro-xybutyrate (PHB) has been observed in several LAB although in low concentrations (6%–35.8% of cell dry weight), the Lactobacillus species, in general, accumulating more PHB than the others (Aslim et al.1998). Although expression of plant genes in LAB is still relatively scarce as compared to that in E. coli and yeasts, some examples of attempts on expression of biosynthetic pathways of plant metabolites including isoprenoids and phenylpropanoids in L. lactis are known (Hernández et al.2007; Song et al.2012). THE PROPANEDIOL-UTILIZATION PATHWAY AND PLATFORM CHEMICALS Glycerol acts as an electron acceptor (and not as a carbon source) in several heterofermentative LAB like Lb. reuteri, Lb. brevis, Lb. buchneri and Lb. diolivorans when present together with glucose and enables regeneration of NAD(P)H, being itself converted to 1,3-propanediol as the end product and also resulting in increased growth rate and biomass yield (Pflügl et al.2012). Industrial production of biobased 1,3-PDO is currently done from glucose using engineered E. coli (Sauer, Marx and Mattanovich 2008) and the product is used to produce a polyester fiber, polytrimethylene terephthalate under the tradename Sorona® (sorono.com/our-story/). 1,3-PDO is also used in solvents, adhesives, resins, detergents and cosmetics (Zeng and Sabra 2011). Production of 1,3-PDO from glycerol by wild type microbes including the Lactobacillus species and Klebsiella pneumonia led to an increasing interest in understanding the metabolic pathway involved. It is now known that several LAB metabolize glycerol as well as 1,2-PDO through Pdu pathway (Fig. 4), which has been known for decades in Salmonella enterica but has remained elusive in LAB despite its presence in some Lactobacillus spp., Streptococcus sanguinis, Enterococcus malodoratus and Listeria monocytogenes (Chen and Hatti-Kaul 2017). Figure 4. View largeDownload slide Overview of the propanediol-utilization pathway. (Left) Dehydration of glycerol to 3-hydroxypropionaldehyde (3-HPA) followed by reduction to 1,3-propanediol (1,3-PDO) and oxidation to 3-hydroxypropionic acid (3-HP) via 3-hydroxypropionyl-CoA (3-HP-CoA) and 3-hydroxypropionyl phosphate (3-HP-P) as intermediates. (Right) Dehydration of 1,2-propanediol (1,2-PDO) to propionaldehyde followed by reduction to n-propanol (n-POH) and oxidation to propionic acid via propionyl-CoA and propionyl phosphate as intermediates. Enzyme abbreviations (Blue capitals): PduCDE: glycerol dehydratase, PduQ: 1,3-propanediol oxidoreductase, PduP: propionaldehyde dehydrogenase, PduL: Phosphotransacylase, and PduW: propionate kinase. Figure 4. View largeDownload slide Overview of the propanediol-utilization pathway. (Left) Dehydration of glycerol to 3-hydroxypropionaldehyde (3-HPA) followed by reduction to 1,3-propanediol (1,3-PDO) and oxidation to 3-hydroxypropionic acid (3-HP) via 3-hydroxypropionyl-CoA (3-HP-CoA) and 3-hydroxypropionyl phosphate (3-HP-P) as intermediates. (Right) Dehydration of 1,2-propanediol (1,2-PDO) to propionaldehyde followed by reduction to n-propanol (n-POH) and oxidation to propionic acid via propionyl-CoA and propionyl phosphate as intermediates. Enzyme abbreviations (Blue capitals): PduCDE: glycerol dehydratase, PduQ: 1,3-propanediol oxidoreductase, PduP: propionaldehyde dehydrogenase, PduL: Phosphotransacylase, and PduW: propionate kinase. Studies with Lb. reuteri have shown that the the first step of glycerol metabolism is dehydration catalysed by a Vitamin B12 dependent diol dehydratase (PduCDE) to form 3-hydroxypropionaldehyde (3-HPA), which is reduced to 1,3-PDO by 1,3-PDO oxidoreductase (PduQ) (Sriramulu et al.2008) (Fig. 4). By producing Lb. reuteri mutants with individual deletions of PduQ and other cytosolic ADHEs, it was demonstrated by Chen et al. (2016) that PduQ is more active in generating NAD+ during glycerol metabolism by the resting cells, while ADH7 is responsible for maintaining the cofactor balance by converting 3-HPA to 1,3-PDO outside the microcompartment (MCP) in the growing cells. The genome sequence of Lb. reuteri revealed a parallel oxidative branch of the Pdu pathway for 3-HPA metabolism involving 3 consecutive reactions catalysed by coenzyme-A acylating propionaldehyde dehydrogenase (PduP), phosphotransacylase (PduL) and propionate kinase (PduW), respectively, to give 3-hydroxypropionic acid (3-HP), which has also been confirmed experimentally (Sabet-Azad et al.2013; Dishisha et al.2014, 2015). 3-HPA and 3-HP are not commercially available and are ranked as important platform chemicals for the biobased industry (Bozell and Petersen 2010). 3-HPA, also known as reuterin, can also be used as an antimicrobial agent in food and health products (Vollenweider and Lacroix 2004). The enzymes of the Pdu pathway are encoded by a pdu operon that also codes for the structural proteins making up the MCP housing the pathway (Sriramulu et al.2008). The MCP protects the cells from the toxic effects of the aldehyde intermediate. The diol dehydratase (PduCDE) is optimally active with glycerol and 1,2-PDO (C3 polyols) as substrates, and displays some activity with ethane-1,2-diol (C2) and 1,2-butanediol (C4) to produce corresponding aldehydes. Construction of a double mutant of the pduC gene (PduC-Ser302Ala/Gln337Ala) of the dehydratase extended the substrate range up to C6-diols. On the other hand, the enzymes PduQ and PduP of the reductive and oxidative branches of the Pdu pathway were optimally active with 3-HPA but also showed activity with C3-C10 aliphatic aldehydes, suggesting a broad substrate scope (Chen and Hatti-Kaul 2017). Efficient production of 1,3-PDO by co-feeding glucose or lignocellulose hydrolysate to Lb. diolivorans has been reported; optimized feeding resulting in yields up to 95% based on glycerol and titers of 92 g/l (Pflügl et al.2012, 2014; Lindibauer, Marx and Sauer 2017). A limitation with such a process however is the complex, expensive downstream processing for recovery of the pure polyol from the product mixture containing also organic acids and ethanol. This problem can be overcome by the use of resting cells, which requires first the production of cell biomass in which the Pdu pathway is induced by including low concentration of glycerol in the medium and then using the harvested cells to transform glycerol in aqueous solution. Such a two-step process has been used for production of the important metabolites, 3-HPA, 3-HP and 1,3-PDO, of the Pdu pathway by Lb. reuteri. Production of 3-HPA was achieved by trapping the aldehyde in situ to resins functionalised with bisulfite and semicarbazide, respectively, which helped to minimise the product inhibition and also its further conversion to 1,3-PDO/3-HP (Sardari et al.2013, 2014). Dishisha et al. (2014) demonstrated clean and efficient transformation of glycerol using the resting cells to an equimolar mixture of 3-HP and 1,3-PDO through the two branches of the Pdu pathway, confirming that the two routes maintained the cofactor balance and the process would continue as long as the Pdu enzymes were active. As 3-HP and 1,3-PDO can be easily separated from each other, their co-production is interesting even from an industrial perspective. Production of 1,3-PDO and 3-HP by Lb. reuteri was further integrated with another biotransformation step using Gluconobacter oxydans for selective oxidation of 1,3-PDO in the mixture to 3-HP, which, in turn, was catalytically dehydrated using TiO2 to acrylic acid (99 mol% from glycerol), thus providing a biobased route to a number of highly important industrial chemicals (Dishisha, Pyo and Hatti-Kaul 2015). LAB AND THEIR ENZYMES AS BIOCATALYSTS FOR SELECTIVE TRANSFORMATIONS The biocatalytic potential of LAB has been demonstrated in several reports, wherein the activity of an enzyme in the cells is utilized for transformations of natural/synthetic substrates to high value products such as pharmaceutical intermediates, nutraceuticals, specialty chemicals, etc. The whole cells are used as the biocatalyst, which lowers the cost of enzyme isolation and purification, and moreover the enzymes are more stable in the intracellular milieu. The GRAS and non-GMO status of these bacteria is of course an added advantage because of higher consumer acceptance even if they are not included in the final product. In some cases, however, the enzymes are isolated for use or expressed in a heterologous host for biotransformations. One of the most successful examples of biocatalysis is the use of ADH activity of different lactobacilli for catalyzing bioreduction of prochiral ketones to optically active alcohols (Scheme 1), which are important pharmaceutical intermediates. Scheme 1. View largeDownload slide Enantioselective reduction of a ketone to a corresponding R-alcohol by Lactobacillus sp. alcohol dehydrogenase. Regeneration of the cofactor NADPH is achieved by oxidation of a co-substrate using a suitable enzyme (Leuchs and Greiner 2011). Scheme 1. View largeDownload slide Enantioselective reduction of a ketone to a corresponding R-alcohol by Lactobacillus sp. alcohol dehydrogenase. Regeneration of the cofactor NADPH is achieved by oxidation of a co-substrate using a suitable enzyme (Leuchs and Greiner 2011). Recombinant ADHs from Lb. kefir and Lb. brevis have been extensively characterized (Weckbecker and Hummel 2006; Leuchs and Greiner 2011). The enzymes have broad substrate scope with high regio- and enantioselectivity, and are NADPH dependent, which needs to be regenerated for lowering the cofactor costs. Lb. brevis ADH has been used both in an isolated form or as a recombinant whole cell biocatalyst (E. coli), free or immobilized, with different means of cofactor regeneration, in homogeneous or biphasic reaction media (Leuchs and Greiner 2011). The enzyme remains active in the presence of organic solvents, supercritical fluids or gaseous reactants. Lb. kefir whole cells have been used for reduction of a prochiral ketoester to the corresponding (S)- alcohol in high optical purity and the intracellular cofactor (NADP) regeneration was achieved using 2-propanol as the co-substrate, which was oxidized to acetone (Amidjojo and Weuster-Botz 2005). Use of the recombinant Lb. kefir ADH has also been reported for stereoselective reduction of several aliphatic and aromatic ketones as well as β-ketoesters to their corresponding alcohols with >99% enantiomeric excess (Weckbecker and Hummel 2006). Here, the NADPH was regenerated using a coupled glucose dehydrogenase catalyzed oxidation of the co-substrate glucose. The bioreduction process was improved to give high selectivity and space-time yield by using immobilized Lb. kefir cells in a plug flow reactor (Tan et al.2006). More recently, resting cells of Lb. reuteri have been used for asymmetric reduction of prochiral aryl ketones into chiral and optically active alcohols with (R)-enantiopreference, which are important building blocks for the synthesis of drugs for neurological and neurodegenerative disorders (Perna et al.2016). The reactions were performed in aqueous medium with high yields, using pure glucose, lactose, spruce lignocellulose hydrolysate and cheese whey, respectively, as the sources of reducing equivalents. This work has been further extended to develop a chemo-enzymatic process for the synthesis of (S)-Rivastigmine (Scheme 2), an important drug used for the treatment of mild to moderate dementia of the Alzheimer´s type, in which R-regioselective reduction of an aromatic ketone by whole cells of L. reuteri DSM 20 016 constituted an important intermediate step (Vitale et al.2018). Scheme 2. View largeDownload slide Chemo-enzymatic synthesis of (S)-Rivastigmine (Vitale et al.2018). The key step is the bioreduction of 3-acetylphenyl-N-ethyl-N-methylcarbamate, obtained from commercially available 3-hydroxyacetophenone, using whole cells of Lb. reuteri to the corresponding secondary (R)-alcohol, which is finally converted into the final product, S-Rivastigmine by a two-step reaction. Scheme 2. View largeDownload slide Chemo-enzymatic synthesis of (S)-Rivastigmine (Vitale et al.2018). The key step is the bioreduction of 3-acetylphenyl-N-ethyl-N-methylcarbamate, obtained from commercially available 3-hydroxyacetophenone, using whole cells of Lb. reuteri to the corresponding secondary (R)-alcohol, which is finally converted into the final product, S-Rivastigmine by a two-step reaction. Another interesting example of a dehydrogenase enzyme is that of D-LDH from Leuconostoc mesenteroides, which was used as a biocatalyst in an efficient continuous process using a membrane reactor for synthesis of (R)-3-(4-fluorophenyl)-2-hydroxypropionic acid (Scheme 3), a building block for a rhinovirus protease inhibitor, with excellent enantiomeric excess (ee > 99.9%) at multikilogram scale (Tao and McGee 2002). Cofactor regeneration was achieved by coupling the reaction with Candida boidinii formate dehydrogenase catalyzed oxidation of ammonium formate to CO2 and NH3. Scheme 3. View largeDownload slide Synthesis of (R)-3-fluorophenyl-2-hydroxy propionic acid using D-LDH as catalyst in aqueous medium. Cofactor regeneration was achieved by oxidation of ammonium formate used as the co-substrate catalyzed by formate dehydrogenase (Tao and McGee 2002). Scheme 3. View largeDownload slide Synthesis of (R)-3-fluorophenyl-2-hydroxy propionic acid using D-LDH as catalyst in aqueous medium. Cofactor regeneration was achieved by oxidation of ammonium formate used as the co-substrate catalyzed by formate dehydrogenase (Tao and McGee 2002). As already described above, the H2O producing NADH oxidase present in lactobacilli provides a clean system for efficient recycling of NAD+ and is applicable for several oxidation systems. Lb. brevis NADH oxidase was used for NAD+ regeneration in a reaction catalyzed by Bacillus subtilis acetoin reductase/2,3-butanediol dehydrogenase expressed in E. coli cells for stereospecific oxidation of (2R,3R)-2,3-butanediol and meso-2,3-butanediol to chiral acetoins (3R and 3S), widely used to synthesize novel optically active α-hydroxyketone derivatives and liquid crystal composites (Xiao et al.2010). 2´-N-deoxyribosyltransferase (NDT) is an enzyme that catalyzes the cleavage of N-glycosidic bond of 2´-deoxyr-ibonucleoside and transfers the glycosyl moiety to a purine or a pyrimidine base, a reaction that can be used for the synthesis of nucleoside analogs (NAs) (Scheme 4), which constitute an important class of antiviral and antitumor drugs. NDTs from Lb. reuteri and Lb. animalis have been used for the synthesis of natural and non-natural NAs including new arabinonucleosides and halogenated pyrimidine and purine 2΄-deoxyribonucleosides (Fernández-Lucas et al.2010; Britos et al.2016). Covalent immobilization of the Lb. animalis NDT to a solid support resulted in enhanced process and storage stability of the enzyme (Méndez et al.2018). Scheme 4. View largeDownload slide General scheme of nucleoside 2΄-DT catalyzed exchange between the purine or pyrimidine base (Fernández-Lucase et al.2010). Scheme 4. View largeDownload slide General scheme of nucleoside 2΄-DT catalyzed exchange between the purine or pyrimidine base (Fernández-Lucase et al.2010). Deglycosylation of ginsenosides (ginseng saponins), the primary active components of ginseng roots used as a traditional herb in South East Asia, increases their biological potency. The whole cells of Lb. rhamnosus cells, induced for β-glucosidase production using cellobiose, were used for deglycosylation of ginsenoside Rb1 to form ginsenoside Rd, reported to have several beneficial effects for humans including anti-obesity, wound-healing and immunosuppression (Ku et al.2016). Microbial hydration of unsaturated fatty acids (oleic, linoleic and linolenic acid) present in vegetable oils provides a facile route for the production of hydroxyl fatty acids with a variety of applications as starting materials for industrial- and fine chemicals, cosmetics and pharmaceuticals. Biotransformation using an anaerobic culture of Lb. rhamnosus resulted in the formation of only 10-hydroxy derivatives of the fatty acids with very high enantiomeric purity (ee >99%) (Serra and De Simeis 2018) (Scheme 5). Scheme 5. View largeDownload slide Regio- and stereoselective hydration of oleic, linoleic and linolenic acid using whole cells of Lb. rhamnosus (Serra and De Simeis 2018). Scheme 5. View largeDownload slide Regio- and stereoselective hydration of oleic, linoleic and linolenic acid using whole cells of Lb. rhamnosus (Serra and De Simeis 2018). Furthermore, several LAB, particularly the Lactobacillus species, catalyze the transformation of linoleic acid through linoleate isomerase activity to conjugated linoleic acid isomers, which are reported to possess beneficial effects on humans including anti-inflammatory, anti-obesity, anti-carcinogenic activities (Kishino et al.2002; Kuhl and Lindner 2016; Yang et al.2017). CONCLUDING REMARKS LAB have a unique position in being important tools for combining the production of food and non-food products in the biobased economy. This review has provided a glimpse of the increasing trend in utilizing LAB and their enzymes for the production of platform-, specialty-, fine chemicals, nutraceuticals and pharmaceutical intermediates. LAB have a number of advantages as industrial microbes in terms of high growth rates, tolerance to various stress conditions, uncoupling of growth and production, ability to use diverse feedstocks as carbon sources, ease of scaling up based on their microaerophilic/anaerobic characteristics. With the availability of genome sequences and the high throughput omics tools, their use will benefit from the systems biology approach through combining mathematical modeling techniques with functional genomics data (Teusink and Smid 2006; de Vos 2011). In order to be competitive with other established organisms as industrial production hosts, LAB cells need to be designed for growth on simple media, direct hydrolysis of biomass polysaccharides (Okano et al.2010) and even combine the synthesis of two or more products in a cell biorefinery (Cheirsilp et al.2018). In some cases, two-steps process involving cell growth and product(s) formation, respectively, with dynamic metabolic control can provide an optimal strategy for better control and improved volumetric rates, titers and yields (Dishisha, Pyo and Hatti-Kaul 2015; Burg et al.2016). Acknowledgements RHK and LC are supported by Mistra (The Swedish Foundation for Strategic Environmental Research) through grant number 2016/1489. HEE would like to acknowledge the support of MOHE and UTM-RMC through HICOE grant no. R.J130000.7846.4J262. Conflict of interest. None declared. REFERENCES Aarnikunnas J , von Weymarn N , Rönnholm K et al. Metabolic engineering of Lactobacillus fermentum for production of mannitol and pure L-lactic acid or pyruvate . Biotechnol Bioeng 2003 ; 82 : 653 – 63 . Google Scholar Crossref Search ADS PubMed Abdel-Rahman MA , Tashiro Y , Sonomoto K . Lactic acid production from lignocellulose-derived sugars using lactic acid bacteria: Overview and limits . J Biotechnol 2011 ; 156 : 286 – 301 . Google Scholar Crossref Search ADS PubMed Abdel-Rahman MA , Tashiro Y , Zendo T et al. Efficient homofermentative l-(+)-lactic acid production from xylose by a novel lactic acid bacterium, Enterococcus mundtii QU 25 . Appl Environ Microbiol 2011 ; 77 : 1892 – 5 . Google Scholar Crossref Search ADS PubMed Abdel-Rahman MA , Sonomoto K . Opportunities to overcome the current limitations and challenges for efficient microbial production of optically pure lactic acid . J Biotechnol 2016 ; 236 : 176 – 92 . Google Scholar Crossref Search ADS PubMed Amidjojo M , Weuster-Botz D . Asymmetric synthesis of the chiral synthon ethyl (S)-4-chloro-3-hydroxybutanoate using Lactobacillus kefir . Tetrahedron: Asymmetry 2005 ; 16 : 899 – 901 . Google Scholar Crossref Search ADS Andersen HW , Solem C , Hammer K et al. Twofold reduction of phosphofructokinase activity in Lactococcus lactis results in strong decreases in growth rate and in glycolytic flux . J Bacteriol 2001 ; 183 : 3458 – 67 . Google Scholar Crossref Search ADS PubMed Aslim B , Caliskan F , Beyatli Y et al. Poly-β-hydroxybutyrate production by lactic acid bacteria . FEMS Microbiol Lett 1998 ; 159 : 293 – 7 . Google Scholar Crossref Search ADS PubMed Axelsson L , Lindstad G , Naterstad K . Development of an inducible gene expression system for Lactobacillus sakei . Lett Appl Microbiol 2003 ; 37 : 115 – 20 . Google Scholar Crossref Search ADS PubMed Bai DM , Zhao XM , Li XG et al. Strain improvement and metabolic flux analysis in the wild-type and a mutant Lactobacillus lactis strain for L(+)-lactic acid production . Biotechnol Bioeng 2004 ; 88 : 681 – 9 . Google Scholar Crossref Search ADS PubMed Bai DM , Yan ZH , Wei Q et al. Ammonium lactate production by Lactobacillus lactis BME5-18M in pH-controlled fed-batch fermentations . Biochem Eng J 2004 ; 19 : 47 – 51 . Google Scholar Crossref Search ADS Bai Z , Gao Z , Sun J et al. D-Lactic acid production by Sporolactobacillus inulinus YBS1-5 with simultaneous utilization of cottonseed meal and corncob residue . Bioresour Technol 2016 ; 207 : 346 – 52 . Google Scholar Crossref Search ADS PubMed Barre P . Identification of thermobacteria and homofermentative, thermophilic, pentose-utilizing Lactobacilli from high temperature fermenting grape musts . J Appl Bacteriol 1978 ; 44 : 125 – 9 . Google Scholar Crossref Search ADS Barrangou R , van Pijkeren JP . Exploiting CRISPR-Cas immune systems for genome editing in bacteria . Curr Opin Biotechnol 2016 ; 37 : 61 – 68 . Google Scholar Crossref Search ADS PubMed Barrangou R , Fremaux C , Deveau H et al. CRISPR provides acquired resistance against viruses in prokaryotes . Science 2007 ; 315 : 1709 – 12 . Google Scholar Crossref Search ADS PubMed Bennett GN , San KY . Microbial formation, biotechnological production and applications of 1,2-propanediol . Appl Microbiol Biotechnol 2001 ; 55 : 1 – 9 . Google Scholar Crossref Search ADS PubMed Berezina OV , Zakharova NV , Brandt A et al. Reconstructing the clostridial n-butanol metabolic pathway in Lactobacillus brevis . Appl Microbiol Biotechnol 2010 ; 87 : 635 – 46 . Google Scholar Crossref Search ADS PubMed Bhowmik T , Steele JI . Cloning, characterization and insertional inactivation of the Lactobacillus helveticus D(-) lactate dehydrogenase gene . Appl Microbiol Biotechnol 1994 ; 41 : 432 – 9 . Google Scholar PubMed Boels JC , Ramos A , Kreerebezem M et al. Functional analysis of the Lactococcus lactis galU and galE genes and their impact on sugar nucleotide and exopolysaccharide biosynthesis . Appl Environ Microbiol 2001 ; 67 : 3033 – 40 . Google Scholar Crossref Search ADS PubMed Bongers RS , Hoefnagel MH , Kleerebezem M . High-level acetaldehyde production in Lactococcus lactis by metabolic engineering . Appl Environ Microbiol 2005 ; 71 : 1109 – 13 . Google Scholar Crossref Search ADS PubMed Bosma EF , Forster J , Nielsen AT . Lactobacilli and pediococci as versatile cell factories – Evaluation of strain properties and genetic tools . Biotechnol Adv 2017 ; 35 : 419 – 42 . Google Scholar Crossref Search ADS PubMed Bozell JJ , Petersen GR . Technology development for the production of biobased products from biorefinery carbohydrates - the US Department of Energy's ‘Top 10’ revisited . Green Chem 2010 ; 12 : 539 – 54 . Google Scholar Crossref Search ADS Britos CN , Lapponi MJ , Cappa VA et al. Biotransformation of halogenated nucleosides by immobilized Lactobacillus animalis 2΄-N -deoxyribosyltransferase J Fluorine Chem 2016 ; 186 : 91 – 96 . Google Scholar Crossref Search ADS Burg JM , Cooper CB , Ye Z et al. Large-scale bioprocess competitiveness: The potential of dynamic metabolic control in two-stage fermentations . Curr Opin Chem Eng 2016 ; 14 : 121 – 36 . Google Scholar Crossref Search ADS Burgess CM , O´Connell-Motherway M , Sybesma W et al. Riboflavin production in Lactococcus lactis: potential for in situ production of vitamin-enriched foods . Appl Environ Microbiol 2004 ; 70 : 5769 – 77 . Google Scholar Crossref Search ADS PubMed Burgess CM , Smid EJ , Rutten G et al. A general method for selection of riboflavin-overproducing food grade micro-organisms . Microb Cell Fact 2006 ; 5 : 24 Google Scholar Crossref Search ADS PubMed Caggianiello G , Kleerebezem M , Spano G . Exopolysaccharides produced by lactic acid bacteria: from health-promoting benefits to stress tolerance mechanisms . Appl Microbiol Biotechnol 2016 ; 100 : 3877 – 86 . Google Scholar Crossref Search ADS PubMed Campo N , Daveran-Mingot ML , Leenhouts K , Ritzenthaler P , Le Bourgeois P . Cre-loxP recombination system for large genome rearrangements in Lactococcus lactis . Appl Environ Microbiol 2002 ; 68 : 2359 – 67 . Google Scholar Crossref Search ADS PubMed Capozzi V , Russo P , Duenas MT et al. Lactic acid bacteria producing B-group vitamins: a great potential for functional cereals products . Appl Microbiol Biotechnol 2012 ; 96 : 1383 – 94 . Google Scholar Crossref Search ADS PubMed Carvalho AL , Cardoso FS , Bohn A et al. Engineering trehalose synthesis in Lactococcus lactis for improved stress tolerance . Appl Environ Microbiol 2011 ; 77 : 4189 – 99 . Google Scholar Crossref Search ADS PubMed Castro-Aguirre E , Iniguez-Franco F , Samsudin H et al. Poly(lactic acid) – Mass production, processing, industrial applications, and end of life . Adv Drug Deliv Rev 2016 ; 107 : 333 – 66 . Google Scholar Crossref Search ADS PubMed Celinska E , Grajek W . Biotechnological production of 2,3-butanediol – Current state and prospects . Biotechnol Adv 2009 ; 27 : 715 – 25 . Google Scholar Crossref Search ADS PubMed Cheirsilp B , Suksawang S , Yeesang J et al. Co-production of functional exopolysaccharides and lactic acid by Lactobacillus kefiranofaciens originated from fermented milk, kefir . J Food Sci Technol 2018 ; 55 : 331 – 40 . Google Scholar Crossref Search ADS PubMed Chen L , Bromberger PD , Niuwenhuiys G et al. Redox balance in Lactobacillus reuteri DSM20016: Roles of iron-dependent alcohol dehydrogenases in glucose/ glycerol metabolism . PLoS One . 2016 ; 11 : e0168107 . Google Scholar Crossref Search ADS PubMed Chen L , Hatti-Kaul R . Exploring Lactobacillus reuteri DSM20016 as a biocatalyst for transformation of longer chain 1,2-diols: Limits with microcompartment . PLoS One 2017 ; 12 : e0185734 . Google Scholar Crossref Search ADS PubMed Christensen B , Olsen PB , Regueira TB et al. Propanol production by lactobacillus bacterial hosts . 2014 ; WO2014/102180A1 . Corona-Hernandez RI , Alvarez-Parrilla E , Lizardi-Mendoza J et al. Structural stability and viability of microencapsulated probiotic bacteria: A review . Compr Rev Food Sci Food Saf 2013 ; 12 : 614 – 28 . Google Scholar Crossref Search ADS Cui F , Li Y , Wan C . Lactic acid production from corn stover using mixed cultures of Lactobacillus rhamnosus and Lactobacillus brevis . Bioresour Technol 2011 ; 102 : 1831 – 6 . Google Scholar Crossref Search ADS PubMed de Felipe FL , Starrenburg MJC , Hugenholtz J . The role of NADH-oxidation in acetoin and diacetyl production from glucose in Lactococcus lactis subsp. lactis MG1363 . FEMS Microbiol Lett 1997 ; 156 : 15 – 19 . Google Scholar Crossref Search ADS de Felipe FL , Kleerebezem M , de Vos WM et al. Cofactor engineering: A novel approach to metabolic engineering in Lactococcus lactis by controlled expression of NADH oxidase . J Bacteriol 1998 ; 180 : 3804 – 8 . Google Scholar PubMed de Vos WM . Systems solutions by lactic acid bacteria: From paradigms to practice . Microb Cell Fact 2011 ; 10 : S2 . Google Scholar Crossref Search ADS PubMed Derkx PMF , Janzen T , Sörensen KI et al. The art of strain improvement of industrial lactic acid bacteria without the use of recombinant DNA technology . Microb Cell Fact 2014 ; 13 : 55 . Google Scholar Crossref Search ADS PubMed Dhakal R , Bajpai VK , Baek K-H . Production of gaba (γ-aminobutyric acid) by microorganisms: A review . Braz J Microbiol 2012 ; 43 : 1230 – 41 . Google Scholar Crossref Search ADS PubMed Ding SF , Tan TW . L-Lactic acid production by Lactobacillus casei fermentation using different fed-batch feeding strategies . Process Biochem 2006 ; 41 : 1451 – 4 . Google Scholar Crossref Search ADS Dishisha T , Pereyra LP , Pyo SH et al. Flux analysis of the Lactobacillus reuteri propanediol-utilization pathway for production of 3-hydroxypropionaldehyde, 3-hydroxypropionic acid and 1,3-propanediol from glycerol . Microb Cell Fact 2014 ; 13 : 76 . Google Scholar Crossref Search ADS PubMed Dishisha T , Pyo S-H , Hatti-Kaul R . Bio-based 3-hydroxypropionic- and acrylic acid production from biodiesel glycerol via integrated microbial and chemical catalysis . Microb Cell Fact 2015 ; 14 : 200 . Google Scholar Crossref Search ADS PubMed Douillard FP , de Vos WM . Functional genomics of lactic acid bacteria: From food to health . Microb Cell Fact 2014 ; 13 : S8 . Google Scholar Crossref Search ADS PubMed Douillard FP , Mahony J , Campanacci V et al. Construction of two Lactococcus lactis expression vectors combining the Gateway and the NIsin Controlled Expression systems . Plasmid 2011 ; 66 : 129 – 35 . Google Scholar Crossref Search ADS PubMed Dussenne C , Delaunay T , Wiatz V et al. Synthesis of isosorbide: An overview of challenging reactions . Green Chem 2017 ; 19 : 5332 – 44 . Google Scholar Crossref Search ADS Elferink SJWHO , Krooneman J , Gottschal JC et al. Anaerobic conversion of lactic acid to acetic acid and 1,2-propanediol by Lactobacillus buchneri . Appl Environ Microbiol 2001 ; 67 : 125 – 32 . Google Scholar Crossref Search ADS PubMed Fernández-Lucas J , Acebal C , Sinisterra JV et al. Lactobacillus reuteri 2΄-deoxyribosyltransferase, a novel biocatalyst for tailoring of nucleosides . Appl Environ Microbiol 2010 ; 76 : 1462 – 70 . Google Scholar Crossref Search ADS PubMed Fiocco D , Capozzi V , Goffin P et al. Improved adaptation to heat, cold, and solvent tolerance in Lactobacillus plantarum . Appl Microbiol Biotechnol 2007 ; 77 : 909 – 15 . Google Scholar Crossref Search ADS PubMed Frese SA , Benson AK , Tannock GW et al. The evolution of host specialization in the vertebrate gut symbiont Lactobacillus reuteri . PLoS Genet 2011 ; 7 : e1001314 . Google Scholar Crossref Search ADS PubMed Fukui S , Oi A , Obayashi A et al. Studies on the pentose metabolism by microorganism. 1. A new type-lactic acid fermentation of pentose by lactic acid bacteria . J Gen Appl Microbiol 1957 ; 3 : 258 – 68 . Google Scholar Crossref Search ADS Gandini C , Tarraran L , Kalemasi D et al. Recombinant Lactococcus lactis for efficient conversion of cellodextrins into L-lactic acid . Biotechnol Bioeng 2017 ; 114 : 2807 – 17 . Google Scholar Crossref Search ADS PubMed Garrigues C , Johansen E , Pedersen MB et al. Getting high (OD) on heme . Nat Rev Microbiol 2006 ; 4 : 318- . Google Scholar Crossref Search ADS Gaspar P , Neves AR , Shearman CA et al. The lactate dehydrogenases encoded by the ldh and ldhB genes in Lactococcus lactis exhibit distinct regulation and catalytic properties — comparative modeling to probe the molecular basis . FEBS J 2007 ; 274 : 5924 – 36 . Google Scholar Crossref Search ADS PubMed Gaspar P , Neves AR , Gasson MJ et al. High yields of 2,3-butanediol and mannitol in Lactococcus lactis through engineering of NAD + cofactor recycling . Appl Environ Microbiol 2011 ; 77 : 6826 – 35 . Google Scholar Crossref Search ADS PubMed Gaspar P , Carvalho AL , Vinga S et al. From physiology to systems metabolic engineering for the production of biochemicals by lactic acid bacteria . Biotechnol Adv 2013 ; 31 : 764 – 88 . Google Scholar Crossref Search ADS PubMed Goh YJ , Azcárate-Peril MA , O’Flaherty S et al. Development and application of a upp-based counterselective gene replacement system for the study of the S-layer protein SlpX of Lactobacillus acidophilus NCFM . Appl Environ Microbiol 2009 ; 75 : 3093 – 105 . Google Scholar Crossref Search ADS PubMed Guandalini S . Probiotics for prevention and treatment of diarrhea . J Clin Gastroenterol 2011 ; 45 : S149 – 53 . Google Scholar Crossref Search ADS PubMed Guo W , Jia W , Li Y et al. Performances of Lactobacillus brevis for producing lactic acid from hydrolysate of lignocellulosics . Appl Biochem Biotechnol 2010 ; 161 : 124 – 36 . Google Scholar Crossref Search ADS PubMed Guo Y , Pan D , Li H et al. Antioxidant and immunomodulatory activity of selenium exopolysaccharide produced by Lactococcus lactis subsp. lactis . Food Chem 2013 ; 138 : 84 – 9 . Google Scholar Crossref Search ADS PubMed Halbmayr E , Mathiesen G , Nguyen TH et al. High-level expression of recombinant β-galactosidases in Lactobacillus plantarum and Lactobacillus sakei using a sakacin P-based expression system . J Agric Food Chem 2008 ; 56 : 4710 – 9 . Google Scholar Crossref Search ADS PubMed Heiss S , Hörmann A , Tauer C et al. Evaluation of novel inducible promoter/repressor systems for recombinant protein expression in Lactobacillus plantarum . Microb Cell Fact 2016 ; 15 : 50 . Google Scholar Crossref Search ADS PubMed Helando M , Aarnikunnas J , von Weymarn N et al. Improved mannitol production by a random mutant of Leuconostoc pseudomesenteroides . J Biotechnol 2005 ; 116 : 283 – 94 . Google Scholar Crossref Search ADS PubMed Hernández I , Molenaar D , Beekwilder J et al. Expression of plant flavor genes in Lactococcus lactis . Appl Environ Microbiol 2007 ; 73 : 1544 – 52 . Google Scholar Crossref Search ADS PubMed Hidalgo-Cantabrana C , O´Flaherty S , Barrangou R . CRISPR-based engineering of next-generation lactic acid bacteria . Curr Opin Microbiol 2017 ; 37 : 79 – 87 . Google Scholar Crossref Search ADS PubMed Hols P , Kleerebezem M , Schanck AN et al. Conversion of Lactococcus lactis from homolactic to homoalanine fermentation through metabolic engineering . Nat Biotechnol 1999 ; 17 : 588 – 92 . Google Scholar Crossref Search ADS PubMed Hugenholtz J . Citrate metabolism in lactic acid bacteria . FEMS Microbiol Rev 1993 ; 12 : 165 – 78 . Google Scholar Crossref Search ADS Hugenholtz J . The lactic acid bacterium as a cell factory for food ingredient production . Int Dairy J 2008 ; 18 : 466 – 75 . Google Scholar Crossref Search ADS Hugenholtz J , Kleerebezem M , Starrenburg M et al. Lactococcus lactis as a cell factory for high-level diacetyl production . Appl Environ Microbiol 2000 ; 66 : 4112 – 4 . Google Scholar Crossref Search ADS PubMed Jensen PR , Hammer K . The sequence of spacers between the consensus sequences modulates the strength of prokaryotic promoters . Appl Environ Microbiol 1998 ; 64 : 82 – 87 . Google Scholar PubMed Karimi S , Ahl D , Vågesjö E et al. In vivo and in vitro Detection of luminescent and fluorescent Lactobacillus reuteri and application of red fluorescent mCherry for assessing plasmid persistence . PLoS One 2016 ; 11 : e0151969 . Google Scholar Crossref Search ADS PubMed Kim J-H , Block DE , Mills DA . Simultaneous consumption of pentose and hexose sugars: An optimal microbial phenotype for efficient fermentation of lignocellulosic biomass . Appl Microbiol Biotechnol 2010 ; 88 : 1077 – 85 . Google Scholar Crossref Search ADS PubMed Kishino S , Ogawa J , Omura Y et al. Conjugated linoleic acid production from linoleic acid by lactic acid bacteria . J Amer Oil Chem Soc 2002 ; 79 : 159 – 63 . Google Scholar Crossref Search ADS Ku S , You HJ , Park MS et al. Whole-Cell biocatalysis for producing ginsenoside Rd from Rb1 using Lactobacillus rhamnosus GG . J Microbiol Biotechnol 2016 ; 26 : 1206 – 15 . Google Scholar Crossref Search ADS PubMed Kuhl GC , Lindner JDD . Biohydrogenation of linoleic acid by lactic acid bacteria for the production of functional cultured dairy products: A review . Foods 2016 ; 5 : 13 . Google Scholar Crossref Search ADS Kuipers OP , Beerthuyzen MM , de Ruyter PG et al. Autoregulation of nisin biosynthesis in Lactococcus lactis by signal transduction . J Biol Chem 1995 ; 270 : 27299 – 304 . Google Scholar Crossref Search ADS PubMed Kuo YC , Yuan SF , Wang CA et al. Production of optically pure l -lactic acid from lignocellulosic hydrolysate by using a newly isolated and d -lactate dehydrogenase gene-deficient Lactobacillus paracasei strain . Bioresour Technol 2015 ; 198 : 651 – 7 . Google Scholar Crossref Search ADS PubMed Kwon S , Lee PC , Lee EG et al. Production of lactic acid by Lactobacillus rhamnosus with vitamin-supplemented soybean hydrolysate . Enzyme Microb Technol 2000 ; 26 : 209 – 15 . Google Scholar Crossref Search ADS PubMed Kwon HO , Wee YJ , Kim JN et al. Production of lactic acid from cheese whey by batch and repeated batch cultures of Lactobacillus sp. RKY2 . Appl Biochem Biotechnol 2006 ; 131 : 694 – 704 . Google Scholar Crossref Search ADS PubMed Kylä-Nikkilä K , Hujanen M , Leisola M et al. Metabolic engineering of Lactobacillus helveticus CNRZ32 for production of pure L-(+)-lactic acid . Appl Environ Microbiol 2000 ; 66 : 3835 – 41 . Google Scholar Crossref Search ADS PubMed Ladero V , Ramos A , Wiersma A et al. High-level production of the low-calorie sugar sorbitol by Lactobacillus plantarum through metabolic engineering . Appl Environ Microbiol 2007 ; 73 : 1864 – 72 . Google Scholar Crossref Search ADS PubMed Lambert JM , Bongers RS , Kleerebezem M . Cre-lox-based system for multiple gene deletions and selectable-marker removal in Lactobacillus plantarum . Appl Environ Microbiol 2007 ; 73 : 1126 – 35 . Google Scholar Crossref Search ADS PubMed Law J , Buist G , Haandrikman A et al. A system to generate chromosomal mutations in Lactococcus lactis which allows fast analysis of targeted genes . J Bacteriol 1995 ; 177 : 7011 – 8 . Google Scholar Crossref Search ADS PubMed Lawley B , Sims IM , Tannock GW . Whole-transcriptome shotgun sequencing (RNA-seq) screen reveals upregulation of cellobiose and motility operons of Lactobacillus ruminis L5 during growth on tetrasaccharides derived from barley β-glucan . Appl Environ Microbiol 2013 ; 79 : 5661 – 9 . Google Scholar Crossref Search ADS PubMed Lazzi C , Meli F , Lambertini F et al. Growth promotion of Bifidobacterium and Lactobacillus species by proteinaceous hydrolysates derived from poultry processing leftovers . Int J Food Sci Technol 2013 ; 48 : 341 – 9 . Google Scholar Crossref Search ADS LeBlanc JG , Laino JE , Juarez del Valle M et al. B-Group vitamin production by lactic acid bacteria - current knowledge and potential applications . J Appl Microbiol 2011 ; 111 : 1297 – 309 . Google Scholar Crossref Search ADS PubMed Lechardeur D , Cesselin B , Fernandez A et al. Using heme as an energy boost for lactic acid bacteria . Curr Opin Biotechnol 2011 ; 22 : 143 – 9 . Google Scholar Crossref Search ADS PubMed Leroy F , De Vuyst L . Lactic acid bacteria as functional starter cultures for the food fermentation industry . Trends Food Sci Technol 2004 ; 15 : 67 – 78 . Google Scholar Crossref Search ADS Leauches S , Greiner L . Alcohol dehydrogenase from Lactobacillus brevis: a versatile robust catalyst for enantioselective transformations . Chem Biochem Eng Q 2011 ; 25 : 267 – 81 . Leenhouts K , Buist G , Bolhuis A et al. A general system for generating unlabelled gene replacements in bacterial chromosomes . Molecular and General Genetics MGG 1996 ; 253 : 217 – 24 . Google Scholar Crossref Search ADS PubMed Levander F , Svensson M , Rådström P . Enhanced exopolysaccharide production by metabolic engineering of Streptococcus thermophilus . Appl Environ Microbiol 2002 ; 68 : 784 – 90 . Google Scholar Crossref Search ADS PubMed Li HX , Cao YS . Lactic acid bacterial cell factories for gamma-aminobutyric acid . Amino Acids 2010 ; 39 : 1107 – 16 . Google Scholar Crossref Search ADS PubMed Li HX , Qiu T , Huang GD et al. Production of gamma-aminobutyric acid by Lactobacillus brevis NCL912 using fed-batch fermentation . Microb Cell Fact 2010 ; 9 : 85 . Google Scholar Crossref Search ADS PubMed Li P , Gu Q , Yang L et al. Characterization of extracellular vitamin B 12 producing Lactobacillus plantarum strains and assessment of the probiotic potentials . Food Chem 2017 ; 234 : 494 – 501 . Google Scholar Crossref Search ADS PubMed Lim HS , Cha IT , Roh SW et al. Enhanced production of gamma-aminobutyric acid by optimizing culture conditions of Lactobacillus brevis HYE1 Isolated from Kimchi, a Korean fermented Food . J Microbiol Biotechnol 2017 ; 27 : 450 – 9 . Google Scholar Crossref Search ADS PubMed Lindibauer KA , Marx H , Sauer M . Effect of carbon pulsing on the redox household of Lactobacillus diolivorans in order to enhance 1,3-propanediol oriduction . Nat Biotechnol 2017 ; 34 : 32 – 39 . Liu S , Nichols NN , Dien BS et al. Metabolic engineering of a Lactobacillus plantarum double ldh knockout strain for enhanced ethanol production . J Ind Microbiol Biotechnol 2006 ; 33 : 1 – 7 . Google Scholar Crossref Search ADS PubMed Liu J , Chan SHJ , Brock-Nannestad T et al. Combining metabolic engineering and biocompatible chemistry for high-yield production of homo-diacetyl and homo-(S,S)-2,3-butanediol . Metab Eng 2016 ; 36 : 57 – 67 . Google Scholar Crossref Search ADS PubMed Mack DR . D-(−)-Lactic acid-producing probiotics, d(−)-lactic acidosis and infants . Can J Gastroenterol 2004 ; 18 : 671 – 5 Google Scholar Crossref Search ADS PubMed Mathiesen G , Sveen A , Piard J-C et al. Heterologous protein secretion by Lactobacillus plantarum using homologous signal peptides . J Appl Microbiol 2008 ; 105 : 215 – 26 . Google Scholar Crossref Search ADS PubMed Mazzoli R , Bosco F , Mizrahi I et al. Towards lactic acid bacteria-based biorefineries . Biotechnol Adv 2014 ; 32 : 1216 – 36 . Google Scholar Crossref Search ADS PubMed Méndez MB , Rivero CW , López-Gallego F et al. Development of a high efficient biocatalyst by oriented covalent immobilization of a novel recombinant 2΄-N-deoxyribosyltransferase from Lactobacillus animalis . J Biotechnol 2018 ; 270 : 39 – 43 . Google Scholar Crossref Search ADS PubMed Michon C , Langella P , Eijsink VGH et al. Display of recombinant proteins at the surface of lactic acid bacteria: strategies and applications . Microb Cell Fact 2016 ; 15 : 70 . Google Scholar Crossref Search ADS PubMed Mierau I , Kleerebezem M . 10 years of the nisin-controlled gene expression system (NICE) in Lactococcus lactis . Appl Microbiol Biotechnol 2005 ; 68 : 705 – 17 . Google Scholar Crossref Search ADS PubMed Mohammed Y , Lee B , Kang Z et al. Capability of Lactobacillus reuteri to produce an active form of Vitamin B12 under optimized fermentation conditions . J Acad Indust Res 2014 ; 2 : 617 – 21 . Morais S , Shterzer N , Grinberg IR et al. Establishment of a simple Lactobacillus plantarum cell consortium for cellulase-xylanase synergistic interactions . Appl Environ Microbiol 2013 ; 79 : 5242 – 9 . Google Scholar Crossref Search ADS PubMed Morais S , Shterzer N , Lamed R et al. A combined cell-consortium approach for lignocellulose degradation by specialized Lactobacillus plantarum cells . Biotechnol Biofuels 2014 ; 7 : 112 . Google Scholar Crossref Search ADS PubMed Neves AR , Pool WA , Castro R et al. The alpha-phosphoglucomutase of Lactococcus lactis is unrelated to the alpha-D-phosphohexomutase superfamily and is encoded by the essential gene pgmH . J Biol Chem 2006 ; 281 : 36864 – 73 . Google Scholar Crossref Search ADS PubMed Nguyen TT , Nguyen HM , Geiger B et al. Heterologous expression of a recombinant lactobacillal β-galactosidase in Lactobacillus plantarum: effect of different parameters on the sakacin P-based expression system . Microb Cell Fact 2015 ; 14 : 30 . Google Scholar Crossref Search ADS PubMed Nguyen H-M , Mathiesen G , Stelzer EM et al. Display of a β-mannanase and a chitosanase on the cell surface of Lactobacillus plantarum towards the development of whole-cell biocatalysts . Microb Cell Fact 2016 ; 15 : 169 . Google Scholar Crossref Search ADS PubMed Nguyen T-T , Nguyen T-H , Maischberger T et al. Quantitative transcript analysis of the inducible expression system pSIP: comparison of the overexpression of Lactobacillus spp. β-galactosidases in Lactobacillus plantarum . Microb Cell Fact 2011 ; 10 : 46 . Google Scholar Crossref Search ADS PubMed Nishino N , Yoshida M , Shiota H et al. Accumulation of 1,2-propanediol and enhancement of aerobic stability in whole crop maize silage inoculated with Lactobacillus buchneri . J Appl Microbiol 2003 ; 94 : 800 – 7 . Google Scholar Crossref Search ADS PubMed Nissen I , Perez-Martinez G , Yebra MJ . Sorbitol synthesis by an engineered Lactobacillus casei strain expressing a sorbitol-6-phosphate dehydrogenase gene within the lactose operon . FEMS Microbiol Lett 2005 ; 249 : 177 – 83 . Google Scholar Crossref Search ADS PubMed Nyssola A , Pihlajaniemi A , Palva A et al. Production of xylitol from D-xylose by recombinant Lactococcus lactis . J Biotechnol 2005 ; 118 : 55 – 66 . Google Scholar Crossref Search ADS PubMed Oh JH , van Pijkeren JP . CRISPR-Cas9-assisted recombineering in Lactobacillus reuteri . Nucleic Acids Res 2014 ; 42 : e131- . Google Scholar Crossref Search ADS PubMed Ohara H , Owaki M , Sonomoto K . Xylooligosaccharide fermentation with Leuconostoc lactis . J Biosci Bioeng 2006 ; 101 : 415 – 20 . Google Scholar Crossref Search ADS PubMed Okano K , Yoshida S , Tanaka T et al. Homo-D-Lactic acid fermentation from arabinose by redirection of the phosphoketolase pathway to the pentose phosphate pathway in L-Lactate dehydrogenase gene-deficient Lactobacillus plantarum . Appl Environ Microbiol 2009a ; 75 : 5175 – 8 . Google Scholar Crossref Search ADS Okano K , Yoshida S , Yamada R et al. Improved production of homo-D-lactic acid via xylose fermentation by introduction of xylose assimilation genes and redirection of the phosphoketolase pathway to the pentose phosphate pathway in L-lactate dehydrogenase gene-deficient Lactobacillus plantarum . Appl Environ Microbiol 2009b ; 75 : 7858 – 61 . Google Scholar Crossref Search ADS Okano K , Zhang Q , Shinkawa S et al. Efficient production of optically pure D- lactic acid from raw corn starch by using a genetically modified L- lactate dehydrogenase gene-deficient and alpha-amylase-secreting Lactobacillus plantarum strain . Appl Environ Microbiol 2009c ; 75 : 462 – 7 . Google Scholar Crossref Search ADS Okano K , Zhang Q , Yoshida S et al. D-Lactic acid production from cellooligosaccharides and β-glucan using L-LDH gene deficient and endoglucanase-secreting Lactobacillus plantarum . Appl Microbiol Biotechnol 2010 ; 85 : 643 – 50 . Google Scholar Crossref Search ADS PubMed Okano K , Hama S , Kihara M et al. Production of optically pure D-lactic acid from brown rice using metabolically engineered Lactobacillus plantarum . Appl Microbiol Biotechnol 2017 ; 101 : 1869 – 75 . Google Scholar Crossref Search ADS PubMed Oshiro M , Shinto H , Tashiro Y et al. Kinetic modeling and sensitivity analysis of xylose metabolism in Lactococcus lactis IO-1 . J Biosci Bioeng 2009 ; 108 : 376 – 84 . Google Scholar Crossref Search ADS PubMed Overbeck T , Steele JL , Broadbent JR . Fermentation of de-oiled algal biomass by Lactobacillus casei for production of lactic acid . Bioprocess Biosyst Eng 2016 ; 39 : 1817 – 23 . Google Scholar Crossref Search ADS PubMed Papadimitriou K , Alegria A , Bron PA et al. Stress physiology of lactic acid bacteria . Microbiol Mol Biol Rev 2016 ; 80 : 837 – 90 . Google Scholar Crossref Search ADS PubMed Papagianni M , Legisa M . Increased mannitol production in Lactobacillus reuteri ATCC 55730 production strain with a modified 6-phosphofructo-1-kinase . J Biotechnol 2014 ; 181 : 20 – 26 . Google Scholar Crossref Search ADS PubMed Patnaik R , Louie S , Gavrilovic V et al. Genome shuffling of Lactobacillus for improved acid tolerance . Nat Biotechnol 2002 ; 20 : 707 – 12 . Google Scholar Crossref Search ADS PubMed Perna FM , Ricci MA , Scilimati A et al. Cheap and environmentally sustainable stereoselective arylketones reduction by Lactobacillus reuteri whole cells . J Mol Catal B: Enzym 2016 ; 124 : 29 – 37 . Google Scholar Crossref Search ADS Pflügl S , Marx H , Mattanovich D et al. 1,3-Propanediol production from glycerol with Lactobacillus diolivorans . Bioresour Technol 2012 ; 110 : 133 – 40 . Google Scholar Crossref Search ADS Pflügl S , Marx H , Mattanovich D et al. Heading for an economic industrial upgrading of crude glycerol from biodiesel production to 1,3-propanediol by Lactobacillus diolivorans . Bioresour Technol 2014 ; 152 : 499 – 504 Google Scholar Crossref Search ADS PubMed Platteeuw C , Hugenholtz J , Starrenburg M et al. Metabolic engineering of Lactococcus lactis: influence of the overproduction of α-acetolactate synthase in strains deficient in lactate dehydrogenase as a function of culture conditions . Appl Environ Microbiol 1995 ; 61 : 3967 – 71 . Google Scholar PubMed Prasad SB , Ramachandran KB , Jayaraman G . Transcription analysis of hyaluronan biosynthesis genes in Streptococcus zooepidemicus and metabolically engineered Lactococcus lactis . Appl Microbiol Biotechnol 2012 ; 94 : 1593 – 607 . Google Scholar Crossref Search ADS PubMed Rallu F , Gruss A , Ehrlich SD et al. Acid- and multistress-resistant mutants of Lactococcus lactis: identification of intracellular stress signals . Mol Microbiol 2000 ; 35 : 517 – 28 . Google Scholar Crossref Search ADS PubMed Reddy G , Altaf M , Naveena BJ et al. Amylolytic bacterial lactic acid fermentation – A review . Biotechnol Adv 2008 ; 26 : 22 – 34 . Google Scholar Crossref Search ADS PubMed Rud I , Jensen PR , Naterstad K et al. A synthetic promoter library for constitutive gene expression in Lactobacillus plantarum . Microbiology 2006 ; 152 : 1011 – 9 . Google Scholar Crossref Search ADS PubMed Russell WM , Klaenhammer TR . Efficient system for directed integration into the Lactobacillus acidophilus and Lactobacillus gasseri chromosomes via homologous recombination . Appl Environ Microbiol 2001 ; 67 : 4361 – 4 . Google Scholar Crossref Search ADS PubMed Sabet-Azad R , Linares-Pastén JA , Torkelson L et al. Coenzyme A-acylating propionaldehyde dehydrogenase (PduP) from Lactobacillus reuteri: Kinetic characterization and molecular modeling . Enzyme Microb Technol 2013 ; 53 : 235 – 42 . Google Scholar Crossref Search ADS PubMed Sabet-Azad R , Sardari RRR , Linares-Pastén JA et al. Production of 3-hydroxypropionic acid from 3-hydroxypropionaldehyde by recombinant Escherichia coli co-expressing Lactobacillus reuteri propanediol utilization enzymes . Bioresour Technol 2015 ; 180 : 214 – 21 . Google Scholar Crossref Search ADS PubMed Saha BC , Racine FM . Biotechnological production of mannitol and its applications . Appl Microbiol Biotechnol 2011 ; 89 : 879 – 91 . Google Scholar Crossref Search ADS PubMed Salgado JM , Rodríguez N , Cortés S et al. Development of cost-effective media to increase the economic potential for larger-scale bioproduction of natural food additives by Lactobacillus rhamnosus, Debaryomyces hansenii, and Aspergillus niger . J Agric Food Chem 2009 ; 57 : 10414 – 28 . Google Scholar Crossref Search ADS PubMed Santos F , Vera JL , Lamosa P et al. Pseudovitamin B12 is the corrinoid produced by Lactobacillus reuteri CRL1098 under anaerobic conditions . FEBS Lett 2007 ; 581 : 4865 – 70 . Google Scholar Crossref Search ADS PubMed Santos F , Vera JL , van der Heijden R et al. The complete coenzyme B12 biosynthesis gene cluster of Lactobacillus reuteri CRL1098 . Microbiology 2008 ; 154 : 81 – 93 Google Scholar Crossref Search ADS PubMed Santos F , Wegkamp A , de Vos WM et al. High-level folate production in fermented foods by the B12 producer Lactobacillus reuteri JCM1112 . Appl Environ Microbiol 2008 ; 74 : 3291 – 4 . Google Scholar Crossref Search ADS PubMed Sardari RRR , Dishisha T , Pyo S-H et al. Improved production of 3-hydroxypropionaldehyde by complex formation with bisulfite during biotransformation of glycerol . Biotechnol Bioeng 2013 ; 110 : 1243 – 8 . Google Scholar Crossref Search ADS PubMed Sardari RRR , Dishisha T , Pyo S-H et al. Semicarbazide-functionalized resin as a new scavenger for in situ recovery of 3-hydroxypropionaldehyde during biotransformation of glycerol by Lactobacillus reuteri . J Biotechnol 2014 ; 192 : 223 – 30 Google Scholar Crossref Search ADS PubMed Sauer M , Marx H , Mattanovich D . Microbial production of 1,3-propanediol . Recent Pat Biotechnol 2008 ; 2 : 191 – 7 . Google Scholar Crossref Search ADS PubMed Sauer M , Russmayer H , Grabherr R et al. The efficient clade: Lactic acid bacteria for industrial chemical production . Trends Biotechnol 2017 ; 35 : 756 – 69 . Google Scholar Crossref Search ADS PubMed Schroeter J , Klaenhammer T . Genomics of lactic acid bacteria . FEMS Microbiol Lett 2009 ; 292 : 1 – 6 . Google Scholar Crossref Search ADS PubMed Serra S , De Simeis D . Use of Lactobacillus rhamnosus (ATCC 53103) as whole-cell biocatalyst for the regio- and stereoselective hydration of oleic, linoleic, and linolenic acid . Catalysts 2018 ; 8 : 109 Google Scholar Crossref Search ADS Shahab RL , Luterbacher JS , Brethauer S et al. Consolidated bioprocessing of lignocellulosic biomass to lactic acid by a synthetic fungal-bacterial consortium . Biotechnol Bioeng 2018 ; 115 : 1207 – 15 . Google Scholar Crossref Search ADS PubMed Sheng JZ , Ling PX , Zhu XQ et al. Use of induction promoters to regulate hyaluronan synthase and UDP-glucose-6-dehydrogenase of Streptococcus zooepidemicus expression in Lactococcus lactis: A case study of the regulation mechanism of hyaluronic acid polymer . J Appl Microbiol 2009 ; 107 : 136 – 44 . Google Scholar Crossref Search ADS PubMed Solem C , Dehli T , Jensen PR . Rewiring Lactococcus lactis for ethanol production . Appl Environ Microbiol 2013 ; 79 : 2512 – 8 . Google Scholar Crossref Search ADS PubMed Solem C , Jensen PR . Modulation of gene expression made easy . Appl Environ Microbiol 2002 ; 68 : 2397 – 403 . Google Scholar Crossref Search ADS PubMed Song AA , Abdullah JO , Abdullah MP et al. Functional expression of an orchid fragrance gene in Lactococcus lactis . IJMS 2012 ; 13 : 1582 – 97 . Google Scholar Crossref Search ADS PubMed Song L , Cui H , Tang L et al. Construction of upp deletion mutant strains of Lactobacillus casei and Lactococcus lactis based on counterselective system using temperature-sensitive plasmid . J Microbiol Methods 2014 ; 102 : 37 – 44 . Google Scholar Crossref Search ADS PubMed Sriramulu DD , Liang M , Hernandez-Romero D et al. Lactobacillus reuteri DSM 20016 produces cobalamin-dependent diol dehydratase in metabolosomes and metabolizes 1,2-propanediol by disproportionation . J Bacteriol 2008 ; 190 : 4559 – 67 . Google Scholar Crossref Search ADS PubMed Stern J , Morais S , Ben-david Y et al. Assembly of synthetic functional cellulosomal structures onto the Lactobacillus plantarum cell surface – a potent member of the gut microbiome . Appl Environ Microbiol 2018 ; 84 : e00282 – 18 . Google Scholar Crossref Search ADS PubMed Sun Z , Harris HMB , McCann A et al. Expanding the biotechnology potential of lactobacilli through comparative genomics of 213 strains and associated genera . Nat Commun 2015 ; 6 : 8322 . Google Scholar Crossref Search ADS PubMed Swindell SM , Benson KH , Griffin HG et al. Genetic manipulation of the pathway for diacetyl metabolism in Lactococcus lactis . Appl Environ Microbiol 1996 ; 62 : 2641 – 3 . Google Scholar PubMed Sybesma W , Burgess C , Starrenburg M et al. Multivitamin production in Lactococcus lactis using metabolic engineering . Metab Eng 2004 ; 6 : 109 – 15 . Google Scholar Crossref Search ADS PubMed Sørvig E , Grönqvist S , Naterstad K et al. Construction of vectors for inducible gene expression in Lactobacillus sakei and L. plantarum . FEMS Microbiol Lett 2003 ; 229 : 119 – 26 . Google Scholar Crossref Search ADS PubMed Tan AWI , Fischbach M , Huebner H et al. Synthesis of enantiopure (5R)-hydroxyhexane-2-one with immobilised whole cells of Lactobacillus kefiri . Appl Microbiol Biotechnol 2006 ; 71 : 289 – 93 . Google Scholar Crossref Search ADS PubMed Tanaka K , Komiyama A , Sonomoto K et al. Two different pathways for D -xylose metabolism and the effect of xylose concentration on the yield coefficient of L -lactate in mixed-acid fermentation by the lactic acid bacterium Lactococcus lactis IO-1 . Appl Microbiol Biotechnol 2002 ; 60 : 160 – 7 . Google Scholar Crossref Search ADS PubMed Taniguchi M , Tokunaga T , Horiuchi K et al. Production of L-lactic acid from a mixture of xylose and glucose by co-cultivation of lactic acid bacteria . Appl Microbiol Biotechnol 2004 ; 66 : 160 – 5 . Google Scholar Crossref Search ADS PubMed Tao J , McGee K . Development of a continuous enzymatic process for the preparation of ( R )-3-(4-fluorophenyl)-2-hydroxy propionic Acid . Org Process Res Dev 2002 ; 6 : 520 – 4 . Google Scholar Crossref Search ADS Taranto MP , Vera JL , Hugenholtz J et al. Lactobacillus reuteri CRL1098 produces cobalamin . J Bacteriol 2003 ; 185 : 5643 – 7 . Google Scholar Crossref Search ADS PubMed Tarraran L , Mazzoli R . Alternative strategies for lignocellulose fermentation through lactic acid bacteria: The state of the art and perspectives . FEMS Microbiol Lett 2018 ; 365 , DOI: 10.1093/femsle/fny126. . Teusink B , Smid EJ . Modelling strategies for the industrial exploitation of lactic acid bacteria . Nature Rev 2006 ; 4 : 46 – 56 . Teusink B , Wiersma A , Jacobs L et al. Understanding the adaptive growth strategy of Lactobacillus plantarum by in silico optimisation . PLoS Comput Biol 2009 ; 5 : e1000410 . Google Scholar Crossref Search ADS PubMed Thakur K , Tomar SK , De S . Lactic acid bacteria as a cell factory for riboflavin production . Microb Biotechnol 2016 ; 9 : 441 – 51 . Google Scholar Crossref Search ADS PubMed Trip H , Mulder NL , Lolkema JS . Improved acid stress survival of Lactococcus lactis expressing the histidine decarboxylation pathway of Streptococcus thermophilus CHCC1524 . J Biol Chem 2012 ; 287 : 11195 – 204 . Google Scholar Crossref Search ADS PubMed van Pijkeren JP , Britton RA . High efficiency recombineering in lactic acid bacteria . Nucleic Acids Res 2012 ; 40 : e76- . Google Scholar Crossref Search ADS PubMed van Pijkeren JP , Britton RA . Precision genome engineering in lactic acid bacteria . Microb Cell Fact 2014 ; 13 : S10 . Google Scholar Crossref Search ADS PubMed Vázquez JA , Murado MA . Mathematical tools for objective comparison of microbial cultures . Biochem Eng J 2008 ; 39 : 276 – 87 . Google Scholar Crossref Search ADS Viana R , Yebra MJ , Galán JL et al. Pleiotropic effects of lactate dehydrogenase inactivation in Lactobacillus casei . Res Microbiol 2005 ; 156 : 641 – 9 . Google Scholar Crossref Search ADS PubMed Vitale P , Perna FM , Agrimi G et al. Whole-cell biocatalyst for chemoenzymatic total synthesis of Rivastigmine . Catalysts 2018 ; 8 : 55 . Google Scholar Crossref Search ADS Vollenweider S , Lacroix C . 3-Hydroxypropionaldehyde: Applications and perspectives of biotechnological production . Appl Microbiol Biotechnol 2004 ; 64 : 16 – 27 . Google Scholar Crossref Search ADS PubMed Wang X , Wang G , Yu X et al. Pretreatment of corn stover by solid acid for d-lactic acid fermentation . Bioresour Technol 2017 ; 239 : 490 – 5 . Google Scholar Crossref Search ADS PubMed Weckbecker A , Hummel W . Cloning, expression, and characterization of an (R)-specific alcohol dehydrogenase from Lactobacillus kefir . Biocatal Biotransform 2006 ; 24 : 380 – 9 . Google Scholar Crossref Search ADS Wegkamp A , van Oorschot W , de Vos WM et al. Characterization of the role of para-aminobenzoic acid biosynthesis in folate production by Lactococcus lactis . Appl Environ Microbiol 2007 ; 73 : 2673 – 81 . Google Scholar Crossref Search ADS PubMed Wisselink H , Moers AP , Mars AE et al. Overproduction of heterologous mannitol 1-phosphatase: A key factor for engineering mannitol production by Lactococcus lactis . Appl Environ Microbiol 2005 ; 71 : 1507 – 14 . Google Scholar Crossref Search ADS PubMed Wolken WAM , Lucas PM , Lonvaud-Funel A et al. The mechanism of the tyrosine transporter TyrP supports a proton motive tyrosine decarboxylation pathway in Lactobacillus brevis . J Bacteriol 2006 ; 188 : 2198 – 206 . Google Scholar Crossref Search ADS PubMed Wu C , Zhang J , Wang M et al. Lactobacillus casei combats acid stress by maintaining cell membrane functionality . J Ind Microbiol Biotechnol 2012 ; 39 : 1031 – 9 . Google Scholar Crossref Search ADS PubMed Wu C , Huang J , Zhou R . Progress in engineering acid stress resistance of lactic acid bacteria . Appl Microbiol Biotechnol 2014 ; 98 : 1055 – 63 . Google Scholar Crossref Search ADS PubMed Wu C , Huang J , Zhou R . Genomics of lactic acid bacteria: Current status and potential applications . Crit Rev Microbiol 2017 ; 43 : 393 – 404 . Google Scholar Crossref Search ADS PubMed Xiao Z , Lv C , Gao C et al. A novel whole-cell biocatalyst with NAD+ regeneration for production of chiral chemicals . PLoS One 2010 ; 5 : e8860 . Google Scholar Crossref Search ADS PubMed Yang B , Gao H , Stanton C et al. Bacterial conjugated linoleic acid production and their applications . Prog Lipid Res 2017 ; 68 : 26 – 36 . Google Scholar Crossref Search ADS PubMed Ye L , Zhao H , Wu JC . Improved acid tolerance of Lactobacillus pentosus by error-prone whole genome amplification . Bioresour Technol 2013 ; 135 : 459 – 63 . Google Scholar Crossref Search ADS PubMed Yi X , Zhang P , Sun J et al. Engineering wild-type robust pediococcus acidilactici strain for high titer l- and d-lactic acid production from corn stover feedstock . J Biotechnol 2016 ; 217 : 112 – 21 . Google Scholar Crossref Search ADS PubMed Yoshida S , Okano K , Tanaka T et al. Homo-D-lactic acid production from mixed sugars using xylose-assimilating operon-integrated Lactobacillus plantarum . Appl Microbiol Biotechnol 2011 ; 92 : 67 – 76 . Google Scholar Crossref Search ADS PubMed Yu L , Pei X , Lei T et al. Genome shuffling enhanced L-lactic acid production by improving glucose tolerance of Lactobacillus rhamnosus . J Biotechnol 2008 ; 134 : 154 – 9 . Google Scholar Crossref Search ADS PubMed Zeidan AA , Poulsen VK , Janzen T et al. Polysaccharide production by lactic acid bacteria: From genes to industrial applications . FEMS Microbiol Rev 2017 ; 41 : S168 – 200 . Google Scholar Crossref Search ADS PubMed Zeng AP , Sabra W . Microbial production of diols as platform chemicals: recent progresses . Curr Opin Biotechnol 2011 ; 22 : 749 – 57 . Google Scholar Crossref Search ADS PubMed Zhang H , Cai Y . Lactic acid bacteria . Fundamentals and Practice . Dordrecht : Springer ; 2014 . Zhang J , Fu R-Y , Hugenholtz J et al. Glutathione protects Lactococcus lactis against acid stress . Appl Environ Microbiol 2007 ; 73 : 5268 – 75 . Google Scholar Crossref Search ADS PubMed Zhang J , Wu C , Du G et al. Enhanced acid tolerance in Lactobacillus casei by adaptive evolution and compared stress response during acid stress . Biotechnol Bioproc Eng 2012 ; 17 : 283 – 9 . Google Scholar Crossref Search ADS Zhao R , Zheng S , Duan C et al. NAD-dependent lactate dehydrogenase catalyses the first step in respiratory utilization of lactate by Lactococcus lactis . FEBS Openbio 2013 ; 3 : 379 – 86 . Google Scholar Crossref Search ADS PubMed © FEMS 2018. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - Lactic acid bacteria: from starter cultures to producers of chemicals
JF - FEMS Microbiology Letters
DO - 10.1093/femsle/fny213
DA - 2018-10-01
UR - https://www.deepdyve.com/lp/oxford-university-press/lactic-acid-bacteria-from-starter-cultures-to-producers-of-chemicals-afy0S0P6fb
VL - 365
IS - 20
DP - DeepDyve
ER -