Spaceflight and ground-based microgravity analog experiments have suggested that microgravity can affect microbial growth and metabolism. Although the effects of microgravity and its analogs on microorganisms have been studied for more than 50 years, plausible conflicting and diverse results have frequently been reported in different experiments, especially regarding microbial growth and secondary metabolism. Until now, only the responses of a few typical microbes to microgravity have been investigated; systematic studies of the genetic and phenotypic responses of these microorganisms to microgravity in space are still insufficient due to technological and logistical hurdles. The use of different test strains and secondary metabolites in these studies appears to have caused diverse and conflicting results. Moreover, subtle changes in the extracellular microenvironments around microbial cells play a key role in the diverse responses of microbial growth and secondary metabolisms. Therefore, “indirect” effects represent a reasonable pathway to explain the occurrence of these phenomena in microorganisms. This review summarizes current knowledge on the changes in microbial growth and secondary metabolism in response to spaceflight and its analogs and discusses the diverse and conflicting results. In addition, recommendations are given for future studies on the effects of microgravity in space on microbial growth and secondary metabolism. Keywords: Microbial growth, Secondary metabolism, Spaceflight, Microgravity, Simulated microgravity, Microgravity analogs Background [21–23], and microbial mutations and relation to adapta- Microbes are highly evolved  and can survive in tion to LSMMG . Considerable effort has been fo- many extreme environments [2, 3], including outer cused on cell growth and secondary metabolism. space [4, 5]. However, the different mechanisms by The significance of exploring the effects of space which they respond and adapt to these environments microgravity on microbial growth and metabolism in- (especially to microgravity in space) remain unclear. Re- cludes two important implications. First, the growth of cently, spaceflight and ground simulated microgravity microorganisms (especially pathogenic microbes) in a (SMG) or low-shear modeled microgravity (LSMMG) ex- space capsule could be a threat to astronaut health and periments have demonstrated that microgravity can affect be detrimental to their immune systems [10, 11, 25, 26]. cellular processes and functions in microorganisms, such Second, microorganisms can produce many special sec- as cell growth [6–9], gene expression [10–12], cell morph- ondary metabolites that could be utilized as medicine ology and development [13, 14], virulence and resistance for both humans and animals [5, 23, 27] as well as some [15–18], biofilm formation [19, 20], secondary metabolism toxic secondary metabolites that may threaten the health of astronauts . Investigations into whether the pro- duction of secondary metabolites by these microorganisms * Correspondence: email@example.com; firstname.lastname@example.org is altered in the space environment are worthwhile. Bing Huang and Dian-Geng Li contributed equally to this work. Although studies on the responses of microbes to State Key Laboratory of Microbial Resources, Institute of Microbiology, microgravity date back to the 1960s, many basic ques- Chinese Academy of Sciences, Beijing 100101, China Nanlou Respiratory Diseases Department, Chinese PLA General Hospital/ tions concerning the effects of microgravity on microbial Chinese PLA Postgraduate Medical School, Beijing 100853, China © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Huang et al. Military Medical Research (2018) 5:18 Page 2 of 14 behavior are far from being fully resolved [29, 30]. More- level (i.e., the value of equivalent accelerated speed) over, our systematic and in-depth understanding of the needs to be specified in this context. Generally, the − 3 genetic and phenotypic responses of a variety of micro- “microgravity” level ranges from approximately 10 to − 6 organisms to microgravity environments in space is in- 10 g and is dependent on the location within the sufficient due to technological and logistical hurdles. To spacecraft and the frequency of vibrations [33, 34]. date, only a few typical microbes, including streptomy- Therefore, the term “microgravity” has been suggested cetes, have been investigated in terms of their responses to be exclusively restricted to experiments performed in to microgravity and its analogs . Interestingly, plaus- an environment such as drop towers, parabolic flights, ible but conflicting results for cellular growth rates were sounding rockets, recoverable satellites, spaceships, and reported in different spaceflight and clinorotation exper- the space station (spacelab). Furthermore, the usage of iments . In addition to the microbial growth rate, the term “microgravity” should be independent of the secondary metabolism was also found to be similarly sen- interfering factors of the actual acceleration of the sitive to microgravity and simulated microgravity [22, 31]. spacecraft in space and cosmic radiation, while the term Furthermore, the results of these studies have been mixed, “spaceflight” should contain “microgravity” and the other without conclusive assertions and suggestions for future inherent factors in space (i.e., cosmic radiation). antibiotic production in space environments [22, 32]. Generally, each type of spaceflight opportunity has its Thus, nothing conclusive or concrete is known about the own time range of duration and corresponding “micro- effects of microgravity or simulated microgravity on mi- gravity” level based on the various spaceflight technolo- crobial growth and secondary metabolism; thus, this area gies  (Table 1). To date, many studies associated of research remains open to further exploration. with the responses of terrestrial life have been conducted In this review, we compare the technological methods in space microgravity conditions by recoverable satellites, of microgravity experiments used for spaceflight and space shuttles and on the space station (spaceship) . ground-based simulated microgravity. We also analyzed The effect of an organism in response to the microgravity the similarities and differences in their effects on micro- of a space experiment in these studies is frequently de- bial growth and secondary metabolism as well as the scribed as the “spaceflight effect” due to considerations of causes of the inconsistent results. Based on the analysis the interference of cosmic radiation, spacecraft vibrations of previous studies, it is clear that the experiments per- and hypervelocity; the effects of microgravity and space- formed under spaceflight and SMG conditions differed flight are different. Earlier studies often lacked on board in some procedures, including in the use of different controls during spaceflight due to restrictions in the use strains, growth media, and types of ground-based facilities of centrifuges and sample fixation in orbit. Recently, these (GBFs), which may lead to conflicting results. We also drawbacks have been gradually overcome by using an propose that subtle differences in the microenvironment incubator-centrifuge in orbit that could simulate 1 g Earth could play a key role in the diverse responses observed for gravity and thus separate other space environmental fac- microbial growth and secondary metabolism. Finally, we tors during spaceflight. Furthermore, real-time sample fix- provide recommendations for future studies on the effects ation in orbit could avoid the interference of spacecraft of microgravity in a space environment on microbial landing [36, 37]. growth and secondary metabolism. Although some of these studies were conducted in a space environment by means of the spacecraft and space Space microgravity and its analogs on the ground station, microgravity experiments in space are costly and A large proportion of the experiments were performed performed infrequently due to technological and logis- under simulated microgravity conditions using ground- tical hurdles. Hence, several GBFs with different physical based microgravity simulators due to the scarcity and cost- concepts have been constructed to simulate microgravity liness of spaceflight opportunities. However, it should be on the ground [38, 39] (Table 2). The term “simulated noted that the real microgravity in space is not equivalent to microgravity analogs using ground-based simulators. Table 1 Several flight opportunities and their characteristics Therefore, questions remain concerning the similarities Flight opportunities Time of duration Gravity level (g) and relationships between real space microgravity and − 5 − 2 Drop tower 2–9s 10 -10 simulated microgravity by ground-based simulators. − 3 − 2 Parabolic flight 15–30 s 10 -10 The use of the term “microgravity” in most studies re- − 4 − 3 Sounding rockets 6–15 min 10 -10 fers to the conditions of “weightlessness” or “zero-g” that − 5 − 3 Recoverable satellites/space 1–2 mon 10 -10 only exist in a space environment. In fact, microgravity shuttle is labeled “μg”’, referring to the fact that the gravita- −6 − 5 Space station (spacelab) Several years or 10 -10 tional forces are not entirely equal to zero but are just permanent very small and that its corresponding “microgravity” Huang et al. Military Medical Research (2018) 5:18 Page 3 of 14 Table 2 Several ground-based facilities (GBFs) and their characteristics Ground-based facilities (GBFs) Simulative effect Suitable organism 2-D clinostats Simulated microgravity effect Plant tissue Random positioning machines (RPMs) Simulated microgravity effect Plant tissue (3-D clinostats) High-aspect rotating vessels (HARVs) Low-shear modeled microgravity Human cells, animal cells, (LSSMG) microorganisms Rotating-wall vessel (RWV) Rotating-wall bioreactor (RWB) Rotary cell culture system (RCCS) Diamagnetic levitation apparatus Simulated microgravity Protozoan, plants, mammals, microorganisms microgravity” emerged from microgravity analogs using Random positioning machines (RPMs) GBFs. It has been suggested that the term should be used A clinostat with two axes is called a three-dimensional (3-D) only for experiments performed in GBFs in which the dir- clinostat. The use of 3-D clinostats was hypothesized to be ection of the gravity vector has undergone a continuous capable of increasing the quality of simulations, especially and constant change, with the gravity level averaged to for larger organisms. Often, 3-D clinostats rotate bio- near zero along with rotation and time . Indeed, real logical samples along two independent axes to change microgravity cannot be achieved with a ground-based their orientations at constant speeds and directions rela- simulator because the magnitude of the Earth’sgravity tive to the gravity vector, thereby eliminating the effect of vector cannot be changed, although its effect can be chan- gravity. Specifically, 3-D clinostats that rotate with ran- ged. In this respect, the microgravity analog of “simulated dom changing speed and direction relative to the gravity microgravity” created by GBFs has been regarded as vector are called “RPMs” [40, 47]. There is increasing evi- “functionally near weightlessness” and was not equal dence that the use of the RPMs can generate effects simi- to the “μg” in space or “weightlessness”.Inother lar and comparable to the effects of real microgravity in words, the effects generated by GBFs may be similar space when they rotate fast enough that the organism can- to those of microgravity in space, and preliminary ex- not perceive and experience the gravity vector (i.e., the periments before launch should first be performed changes in direction are faster than the organism’smini- using GBFs. Here, several GBFs that are frequently mum response time (MRT) to the gravity vector). Rela- used are described. tively responsive living organisms, such as plants and other higher organisms, have been observed to be more suitable and ideal candidates for investigations involving Clinostats RPMs . In its early stages, the clinostat was a very well- established paradigm to simulate microgravity on the Rotating wall vessels (RWVs) ground . In principle, a clinostat is a device that en- The RWVs were initially developed by the NASA ables the rotation of the samples with one or two axes Johnson Space Center (Houston, TX, USA) for cell cul- and prevents the organism from perceiving the gravity tures. RWVs can generate LSMMG and have been used vector by continuously breaking the direction of the as an optimized suspension culture technology [49, 50]. gravity vector. The number of rotational axes, the speed, Usually, RWVs consist of a hollow disk or cylinder that and the direction of rotation were designed according to is entirely filled with a liquid medium with almost no different organisms and experimental requirements for bubbles; this disk or cylinder rotates perpendicularly to the practical application of clinostats [41–43]. Two- the direction of the gravity vector with one rotational dimensional (2-D) clinostats rotate perpendicularly to the axis. The cells are maintained in suspension under spe- direction of the gravity vector, with one rotation axis cial culture conditions when the RWVs run in solid- representing a classical and well-established model to body rotation, establishing a continuous low-shear, low- simulate microgravity; these clinostats are widely applied turbulence environment for cell growth that is similar to to study the effects of microgravity on biological samples the space microgravity environment. RWV analogues, today. Moreover, several studies have shown that the re- such as rotating wall bioreactors (RWBs), rotating cell sults obtained from various model systems using 2-D culture systems (RCCSs) and high-aspect rotating vessels clinorotation were similar to those found under real (HARVs), were designed according to similar physical microgravity conditions [44–46]. principles using different configurations and have also Huang et al. Military Medical Research (2018) 5:18 Page 4 of 14 been frequently applied to study the effects of micro- similar to those found in real microgravity environments gravity on biological samples . . These results suggest that GBFs are feasible and cheap tools that can be used on the ground and that Diamagnetic levitation could play an important role in microgravity experi- Diamagnetic levitation is an emerging technology that ments. Moreover, the effects of “simulated microgravity” uses a strong, spatially varying magnetic field produced on the organism could be used as preliminary and by a Bitter solenoid or a superconducting solenoid mag- screening experiments of the microgravity effects during net to simulate an altered gravity environment and gen- spaceflight, and the “simulated microgravity” experi- erate aspects of weightlessness similar to the conditions ments performed using GBFs could also be used as the observed in space . Interestingly, the diamagnetic ground controls of the spaceflight or microgravity exper- force opposes the force of gravity on a levitating object, iments in space. similar to the manner in which the centrifugal force bal- ances the gravitational force on an orbiting spacecraft. Microbial growth responses to spaceflight and The purpose of diamagnetic levitation is to lessen the in- simulated microgravity ternal stresses induced by the force of gravity to as close An increasing number of studies have investigated the to zero as possible to simulate a near weightless environ- growth responses of bacteria, fungi and archaea to ment. Normally, the diamagnetic repulsion of the object microgravity in space and to microgravity analogs on the or living organism exactly balances its gravity through- ground. In these studies, the bacteria mainly included out the body. This was demonstrated to make investigat- Escherichia coli [17, 24, 56, 59–71], Bacillus subtilis ing the effects of weightlessness on small organisms [7, 59, 67, 68], Salmonella typhimurium [18, 72], feasible without going into space. In previous studies, Pseudomonas aeruginosa [8, 73, 74], Staphylococcus diamagnetic levitation was used to successfully levitate [28, 56], Streptococcus , Streptomyces [76, 77], etc. protozoans , plants , animals [53, 54], and micro- [78–80]; the fungi mainly included Saccharomyces cer- organisms [55, 56]. Thus, the use of diamagnetic evisiae [13, 55, 81]and Candida albicans ; and the levitation as a ground-based tool has been frequently ap- archaea included Haloarchaea . An increasing plied to investigate the effects of microgravity on living amount of evidence has suggested that microorganisms organisms, including cell growth and gene expression are just as ubiquitous in space habitats as they are on , secondary metabolism , and plasmid transfer Earth [83–93]. However, inconsistent results concerning . Although some advantages have been demonstrated cellular growth rates have frequently been reported in for diamagnetic levitation, it must be noted that the these studies. From a comprehensive analysis of these strong magnetic field itself may influence the organism results in their contexts, we found that microbial growth and that the effects of simulated weightlessness may re- responses to microgravity and its analogs were dependent ceive some interference from the strong magnetic field on two dominating aspects. First, the selection of strains . Accordingly, the effects of interference should be for these studies varied based on different experimental evaluated prior to the simulated microgravity experi- objectives, with the most inherent characteristic of these ment. Moreover, distinguishing between the effects of strains embodied in the property of cell motility. Second, magnetically simulated weightlessness and any other ef- the experimental conditions varied between studies, in- fects of the strong magnetic field by performing careful cluding in the microgravity conditions, culture methods control experiments is important. (suspension and agar cultures), and the medium nutrient Although the physical principles among these GBFs concentrations (high and low nutrient concentrations) are not exactly the same, they can achieve a similar func- (Additional file 1: Table S1). tion of simulated microgravity on the ground. As dis- cussed above, it should be noted that the physical The growth effects are associated with the microbial principle of microgravity simulators on the ground is species and strains used different from that of the real microgravity experienced As described previously, many investigations reported in space, although the effects of microgravity on the or- seemingly diverse results and effects on microbes ex- ganism can be simulated by microgravity analogs on the posed to microgravity and simulated microgravity . ground to a certain extent. Indeed, reports have indi- In these investigations, the enumeration of the final cell cated that the effects of “simulated microgravity” using population as an indicator of the growth rate was re- GBFs on the organism are not all the same as the effects ported more frequently than any other measured vari- induced by microgravity in space . However, an in- able for comparisons of the differences in the spaceflight creasing number of studies have shown that the results cultures and controls. Generally, most studies found from various model systems using 2-D clinostats, RPMs, that spaceflight increased the microbial growth rate RWV or RWB analogs, and diamagnetic levitation were under microgravity and simulated microgravity Huang et al. Military Medical Research (2018) 5:18 Page 5 of 14 conditions [7, 24, 59, 60]. However, several microbes Ralstonia pickettii (which lacked flagella) were found in a exposed to simulated microgravity were reported to high nutrient broth under simulated microgravity condi- grow more slowly compared to the controls [76, 77], tions compared to normal gravity conditions. Benoit et al. while other studies reported that no significant differ-  reviewed the experimental results from previous in- ences were found in the growth rates of microbes sub- vestigations and suggested a strong correlation between jected to spaceflight cultures and those of the ground the growth effects of bacteria grown in suspension cul- control group [67, 68]. The discrepant effects were tures exposed to spaceflight or microgravity analogs on largely dependent on the microbes and strains used in the ground and cell motility. This review indicated that these studies. For example, E. coli ZK650 cultures were spaceflight and microgravity analogs increased microbial found to have a higher dry cell weight under LSSMG in growth of non-motile bacteria in suspension cultures. In the HARVs , while no differences were found in the light of these findings, recent studies regarding microbial dry cell weight of Bacillus brevis Nagano cultures in the growth under microgravity conditions and its analogs HARV . In addition, the streptomycetes (Streptomy- gradually took cell motility into consideration. However, ces clavuligerus ATCC 27064 and Streptomyces hygro- an exception was recently reported, which examined the scopicus ATCC 29253) were both found to have lower growth of Pseudomonas aeruginosa PA14 and its Δmo- dry cell weights in their cultures in the HARVs and tABCD mutant, which is deficient in swimming motility RWBs, respectively [76, 77]. . As described in this study, the final cell densities ob- Interestingly, differences in growth effects under micro- served with the motility mutant were consistent with gravity and its analog conditions have been reported for those observed with the wild type during spaceflight. different strains of E. coli [56, 59, 62–71]. Surprisingly, dif- ferent growth responses were demonstrated in some The growth effects are associated with the culture methods studies for the same strain of E. coli (Escherichia coli In most studies, microbial growth experiments under ATCC 4157) [7, 59, 67, 68]. The type strain of E. coli has microgravity or its analog conditions were performed been the frequent focus of investigations that showed that using suspension cultures, while only a few studies were the microbial growth rate was increased under micro- performed on solid or semi-solid media. Of the studies gravity or its analog conditions. A series of experiments described above, microbial growth in suspension cul- was performed using suspension cultures of E. coli tures was frequently shown to exhibit an increased final aboard several US Space Shuttle missions. The results cell density under microgravity or its analog conditions, of these studies showed not only that the final cell while no distinct differences in the final cell popula- population density of E. coli was approximately doubled tions were found for growth on solid or semi-solid in the spaceflight cultures but also that the lag phase was media [7, 59, 67, 68]. Previous spaceflight experiments shortened and that the duration of exponential growth demonstrated that suspension cultures of E. coli and B. was extended [7, 59]. Other experiments performed under subtilis exhibited increased cell growth in the space- simulated microgravity conditions on the ground using flight environment [7, 59, 69]. However, other studies RWVs, RWBs, HARVs, RCCSs and diamagnetic levitation showed that E. coli and B. subtilis grown on solid agar indicated that the growth rate of E. coli was similarly in- during Space Shuttle Mission STS-63 did not experi- creased [56, 62, 65, 66, 71, 78]. However, the E. coli growth ence an increased final cell mass, but that changes in rate was also found to be unchanged under microgravity other growth characteristics might have occurred when or simulated microgravity conditions similar to those de- the bacteria were grown under various gravitational scribed in these studies [17, 63, 66, 71, 78]. In fact, a sub- conditions [67, 68]. In suspension cultures, cells are sequent set of studies using B. subtilis found similar likely to experience microgravity both simulated and results in the exposure of cultures to microgravity during real, because they are on a “free fall” inside the suspen- spaceflight [7, 59, 67, 68]. sion and subject to a gravity vector. However, in agar (solid) substratum, the cells are already “attached” to a The growth effects are associated with the strain’s surface, and the gravity vector effect cannot occur. This inherent properties (cell motility) finding also indicated that fluid dynamics and extracel- The study of Baker et al.  provided substantial evi- lular transport phenomena but not cellular dynamics dence that microbial growth under simulated microgravity were the most likely causes of the previously reported conditions using RCCSs apparatus varied with the cellular increases in bacterial growth under microgravity condi- motility of the strains used. In their study, simulated tions. Surprisingly, Van Mulders et al. reportedthat microgravity did not affect the motile strain Sphingobac- the model eukaryotic organism Saccharomyces cerevisiae terium thalpophilium (which had flagella) regardless of S1278b (laboratory strain) exhibited reduced invasive the method of enumeration and the medium used, while growth on semi-solid agar medium under microgravity significantly higher numbers of the non-motile strain Huang et al. Military Medical Research (2018) 5:18 Page 6 of 14 conditions, while no differences in invasive growth were two microorganisms grew faster and yielded more bio- observed for the CMBSESA1 industrial strain. mass in liquid suspension cultures under microgravity Our lab investigated the effects of spaceflight and SMG conditions during spaceflight [7, 69] but exhibited no (Fig. 1) on the growth of Streptomyces coelicolor A3(2), visible differences in growth rates in agar or semi-solid which were incubated in “SIMBOX” (Science in Micro- cultures [67, 68]. The growth rate of the strains was gravity Box, Astrium, Germany) during the Shenzhou-8 speculated to be related to fluid dynamics and the dis- − 3 − 4 space mission (μG=10 –10 g). SIMBOX (Fig. 2)is tribution of the liquid medium rather than to cellular an advanced space incubator with 42 separate EUEs effects induced by the microgravity environment. Des- (Experiment Unique Equipments, Astrium, Germany) pite the fact that it is difficult to interpret the puzzling for experimental containers and a 1 g centrifuge to results described in these studies, the culture condi- simulate gravity in space [36, 37]. Our results showed tions of microorganisms in microgravity experiments that the growth rate of strains cultured on yeast-starch should be emphasized due to their importance for the agar medium (JCM42) was not influenced by either effects of microgravity. SMG or spaceflight; however, the cell biomass grown in liquid cultures was increased under SMG or spaceflight The growth effects are associated with the nutrition conditions (Fig. 3). The increased final cell biomass concentrations and oxygen availability of the liquid media under both SMG and spaceflight conditions was found to Interestingly, recent studies have demonstrated that the be due to the fluidic turbulence of the liquid medium, effects of microgravity or its analogs on microbial which did not fill the whole culture chamber. These re- growth differed when the microbes were cultured in li- sults are similar to those obtained from previous studies quid media with high or low nutrient levels. Baker et al. on E. coli and B. subtilis, which also showed that the latter  found that there were few differences in the cell Fig. 1 The clinostats used in the SMG experiments and their corresponding principle models. a The 2-D clinostats with a horizontal axis used to generate SMG condition on ground. b The 2-D clinostats with a vertical axis used to generate 1G control condition on ground. c The schematic diagram of mechanical principle of the different clinostats. SMG: Axis of rotation is perpendicular to the direction of the gravity vector, which is used to simulate microgravity effects on ground; NG: The samples fixed in the metal support are rotating, used as the dynamic control of SMG; 1G: The samples fixed in the metal support are static and are used as the static control of SMG Huang et al. Military Medical Research (2018) 5:18 Page 7 of 14 Fig. 2 Shenzhou-8 spaceflight experiments. Microbial samples cultured in solid (a) and liquid (b) media were loaded into Experiment Unique Equipments (EUEs) before launching. c The inside of SIMBOX. Red circles indicate the two EUEs in a static slot (microgravity position, μG) and a centrifuge slot (simulated 1G position, S-1G), respectively. d SIMBOX as an advanced space incubator numbers and that the cell size was not affected by the study revealed that the growth responses to modeled modeled reduced gravity when E. coli was grown in min- reduced gravity varied with nutrient conditions; the au- imal medium. However, the total cell numbers were thors also speculated that larger surface-to-volume ratios higher and cells were smaller when grown under re- may have helped to compensate for the zone of nutrient duced gravity in higher nutrient conditions compared to depletion around the cells under modeled reduced those of the normal gravity controls. In summary, this gravity. Fig. 3 Colony and cultural features of S. coelicolor after the 16.5-day spaceflight experiment . a Colony features after the 16.5-day spaceflight experiment. “SCOA3(2)” framed in red indicates the pure S. coelicolor A3(2) culture, “SCOA3(2)*” framed in blue indicates the S. coelicolor A3(2) co-cultured with the indicator strain B. subtilis, and “SCOM145*” framed in yellow indicates S. coelicolor M145 co-cultured with B. subtilis. b Cultural features of S. coelicolor A3(2) grown in a JCM42 liquid medium after the 16.5-day spaceflight experiment. “SCOA3(2)” framed in red indicates the pure S. coelicolor A3(2) culture, and “SCOA3(2)*” framed in blue indicates S. coelicolor A3(2) co-cultured with B. subtilis Huang et al. Military Medical Research (2018) 5:18 Page 8 of 14 In the study of Kim et al. , wild-type Pseudomonas could change their growing status; thus, the effect in- aeruginosa PA14 grown during spaceflight showed an in- duced by the elevated CO levels is likely to dominate all creased final cell density compared to the normal gravity responses. Therefore, other factors, such as cosmic radi- controls when low concentrations of phosphate in the ation and slightly elevated CO , may also lead to con- medium were combined with decreased oxygen avail- flicting results in microbial growth. ability; in contrast, no difference in the final cell density Taken together, the different effects observed using the was observed between spaceflight and normal gravity same species in these studies may be due to differences when the availability of either phosphate or oxygen was in the species’ inherent attributes and the experimental increased. These results indicated that differences in conditions. The strain’s inherent attributes were mainly the final bacterial cell density observed between space- specified according to cell motility [31, 78]. Additionally, flight and normal gravity cases were due to an interplay the extrinsic experimental conditions were speculated to between microgravity conditions and the availability of play a role in microbial responses. In other words, it is substrates essential for growth. Similarly, a study by highly likely that the cellular surroundings affect their Dijkstra et al.  showed that diamagnetic levitation growth responses to microgravity and its analogs on the increased the growth rate and reduced the sedimenta- ground [59, 67, 68]. It was hypothesized that indirect tion rate in liquid cultures of both Escherichia coli K12 physical effects, such as changes in fluid dynamics and MG1655 and Staphylococcus epidermidis NCTC11047. extracellular transport, rather than a direct microgravity Further experiments revealed that the increased growth effect were the most likely cause of the differences ob- rate was due to enhanced oxygen availability as a result served in bacterial growth during spaceflight. Thus, the of convective stirring in the liquid induced by the extracellular surroundings represented by the microenvi- magnetic field. ronments encountered by the organisms varied under different experimental conditions in previous studies The growth effects are associated with the rotation speed (i.e., the culture mode, the medium nutrient concen- of the simulator tration, and the rotation speed). The majority of the Recent studies demonstrated that the effects of micro- proposed mechanisms focused on physical factors, gravity or its analogs on microbial growth were different such as decreased mass diffusion or shear levels or the when the microbes were cultured at higher speeds and development of microenvironments (i.e., changes in lower speeds in the RCCS. A study by Baker et al.  the distribution of nutrients and by-products due to a showed that significant differences in the final cell popu- lack of cell sedimentation) that directly affected the lations between modeled reduced gravity in the RCCSs organisms and therefore could alter cell growth. and normal gravity controls were observed only at Although conflicting results of the growth effects fol- higher speeds (30–50 r/min) when E. coli ATCC 26 was lowing the exposure of microbes to spaceflight or its an- subjected to different rotation speeds during clinorota- alogs on the ground were found for the different strains tion. However, the differences were found to be non- used in these studies, the microenvironments around significant in the final cell populations grown at lower the cells should be considered in the interpretation of speeds (less than 20 r/min). the results and phenomena. In our opinion, external Factors other than microgravity may influence micro- physical factors would play a more dominant role in the bial growth in space. Cosmic radiation and slightly ele- effects of microbial growth under spaceflight or its ana- vated CO levels are both other factors influenced by log conditions. spaceflight, and the effect of spaceflight is an integration of multiple results in response to space-based factors. Effect of spaceflight and simulated microgravity Generally, it is difficult to distinguish the factor from on microbial secondary metabolism which the effect comes. The biological effects of cosmic Generally, the biosynthesis and yield of microbial sec- radiation may be induced through direct energy absorp- ondary metabolites are sensitive to extracellular environ- tion. For example, DNA is damaged after exposure to mental signals and stresses, including nutrients, heat, ionizing radiation, leading to an increase in mutation. osmotic stress and shear stress [96–98]. Therefore, it However, these effects are also induced indirectly via in- would be interesting to look into the yield of secondary teractions of those molecules with radiation-induced metabolites produced in the cultures under microgravity radicals, also leading to an increase in cell damage. and its analog conditions . Similar to the effects of the During spaceflight, microgravity could interfere with the conditions on the microbial growth rate, diverse results operation of the cellular repair processes of DNA when involved with the yields of secondary metabolites were damaged by cosmic radiation, bringing an increase in frequently reported [22, 32], which are summarized in the radiation response . Some microbial cells that are Table 3. A review of these responses is both significant susceptible to slightly elevated CO levels in spacecraft and timely for scientists considering the application of 2 Huang et al. Military Medical Research (2018) 5:18 Page 9 of 14 Table 3 Responses of the yields of secondary metabolites to simulated microgravity and spaceflight Strain Antibiotics Response Equipment Streptomyces clavuligerus ATCC β-lactam antibiotic cephalosporin Inhibited High-aspect rotating vessels (HARV) 27064  Streptomyces hygroscopicus ATCC Rapamycin Inhibited Rotating-wall bioreactor (RWB) 29253  Streptomyces ansochromogenus Nikkomycin X, Z Increased Unmanned satellite 7100  Streptomyces avermitilis PE1  Avermectin Increased Diamagnetic levitation Streptomyces plicatus WC56452  Actinomycin D Increased US Space Shuttle mission STS-80 Streptomyces plicatus WC56452  Actinomycin D Increased and then decreased International space station (ISS) Escherichia coli ZK650  Microcin B17 Inhibited High-aspect rotating vessels (HARV) Bacillus brevis Nagano  Gramicidin S Unaffected High-aspect rotating vessels (HARV) Humicola fuscoatra WC5157  Monorden Increased US Space Shuttle mission STS-77 Microcystis aeruginosa PCC7806  Microcystin Increased Rotary cell culture system (RCCS) Cupriavidus metallidurans LMG Poly-β-hydroxybutyrate (PHB) Increased and then decreased Rotating wall vessel (RWV) 1195  Streptomyces coelicolor A3(2)  Undecylprodigiosin (RED) Increased slightly 2D-clinostat (SM-1) Streptomyces coelicolor A3(2)  Actinorhodin (ACT) Inhibited 2D-clinostat (SM-1), Shenzhou-8 Space mission simulated microgravity on the ground to explore the ef- environment simulated by diamagnetic levitation and fects of the space microgravity environment on secondary under a strong magnetic field. However, this study indi- metabolites, many of which are used in human and veter- cated that the magnetic field was a more dominant fac- inary medicine (i.e., antibiotics, anti-tumor agents and tor in influencing changes in secondary metabolite immunosuppressants) . production than altered gravity. In the studies of Fang et al. [76, 77], the production of The yields of secondary metabolites are in/de-creased or the β-lactam antibiotic cephalosporin and the polyketide uninfluenced in response to spaceflight and simulated macrolide rapamycin by S. clavuligerus ATCC 27064 and microgravity S. hygroscopicus ATCC 29253, respectively, were shown In the study of Lam et al. , the specific productivity to be inhibited by LSMMG. Further analysis showed that (pg/CFU) of actinomycin D produced by Streptomyces growth under LSMMG conditions favored the extracel- plicatus WC56452 was increased during spaceflight lular production of rapamycin. Moreover, the site of aboard the US Space Shuttle mission STS-80. In another rapamycin accumulation was modified to a moderate ex- study , Lam reported that the production of mono- tent towards an extracellular location, while the total rden by Humicola fuscoatra WC5157 grown on two yields of rapamycin were decreased. Fang et al.  also types of agar media (T8 and PG) was also increased demonstrated that the production of the peptide anti- during spaceflight aboard the US Space Shuttle Mission biotic microcin B17 by E. coli ZK650 was inhibited by STS-77. In a study by Luo et al. , the productivity LSMMG in HARVs. The site of microcin accumulation of nikkomycins by Streptomyces ansochromogenus in- was found to be markedly different depending on whether creased by 13–18% during 15 days of spaceflight aboard E. coli was grown in shaking flasks or RWBs. The accu- a satellite, and the proportion of nikkomycin X and Z mulation of microcin was intracellular when the bacteria increased correspondingly. A study by Xiao et al.  were grown in flasks, whereas in HARVs, the majority demonstrated that the production of the toxin micro- of the microcin was found in the extracellular medium. cystin (MC) by the cyanobacteria Microcystis aerugi- It should be noted that the shift in the localization of nosa PCC7806 was enhanced by simulated microgravity microcin from intracellular to extracellular was prob- (SMG), which acted as a novel environmental signal, ably due to the much lower degree of shear stress in while its growth was inhibited. Moreover, enhanced the bioreactors because the addition of a single glass MC production was reported to be associated with pig- bead to the RWB medium created enough shear to ment and nitrogen metabolism. Liu et al. reported change the site of microcin accumulation from the that the production of the important anthelmintic agent medium to the cells . avermectin produced by Streptomyces avermitilis in a In another study by Fang et al. , gramicidin S (GS) solid culture was increased in an altered gravity production by B. brevis Nagano in NASA HARVs was Huang et al. Military Medical Research (2018) 5:18 Page 10 of 14 found to be unaffected. Interestingly, this finding indi- secondary metabolism to microgravity are likely to be cates that LSMMG does not have a universally negative due to the indirect physical effects of microgravity, such effect on secondary metabolism and suggests that mi- as changes in fluid dynamics and the extracellular trans- crobes respond to LSMMG in specific ways. port of metabolites. For suspension cultures, fluid dy- namics may be more responsible for the effects of The yields of secondary metabolites fluctuate over time simulated microgravity on microbial cells, while for solid in response to spaceflight and simulated microgravity cultures, the gas dynamics in the cultivating vessel may In the study of De Gelder et al. , the yield of poly- influence the effects of simulated microgravity on micro- β-hydroxybutyrate (PHB) produced by Cupriavidus bial cells. Hence, based on the abovementioned studies metallidurans LMG 1195 under LSSMG conditions in and theory, it is reasonable to envision that the re- the RWVs was reported to be increased after 24 h of sponses of microbial secondary metabolism to micro- culture, while after 48 h of culture, the PHB concentra- gravity could be interpreted and elucidated in this tions were reduced in SMG compared to in the control. way. Various extracellular stress signals have been In the study of Benoit et al. , the production of acti- found to induce or promote secondary metabolisms in nomycin D by Streptomyces plicatus WC56452 in space a variety of microbes, and the extracellular stresses are was reported to be increased by 15.6 and 28.5% on days transferred to the downstream responsive genes by a 8 and 12, respectively, but decreased in all subsequent cascade of complex signal transduction steps [96–98]. matched sample points (16–72 d at intervals of 4 d) Thus, despite the fact that the extracellular stress sig- compared to the ground controls. Finally, the maximum nals induced by microgravity were the same, the pro- production levels were found at day 24 in the ground con- cesses and steps experienced were different for trol and at day 12 in the spaceflight samples, respectively. different secondary metabolites when the extracellular Collectively, these studies suggest that microgravity or stress signals were transmitted to the pathway-specific its analogs on the ground could alter secondary metabo- regulatory gene of the specific secondary metabolite. lisms in microorganisms. However, corresponding mo- Meanwhile, the extracellular microenvironment affects lecular evidence for the metabolic phenotypes has not the secretion and transport of secondary metabolites. been reported. Our latest study revealed the effect of Furthermore, the cellular surroundings (i.e., micro- simulated microgravity and spaceflight on the secondary environment) in different experiments may have con- metabolism of S. coelicolor A3(2) at both the phenotypic tributed to these differences. For the experiments and whole transcriptome levels . For S. coelicolor under ground-based microgravity analog conditions, A3(2), the secondary metabolite undecylprodigiosin (RED) the use of various types of GBFs with different phys- was produced earlier and accumulated to a slightly higher ical concepts contributed to the observed differences concentration under SMG conditions in agar culture in effects, most likely as a result of the different extra- compared to the ground control, while the production cellular microenvironments around the microbial of actinorhodin (ACT) was delayed and markedly de- cells. In summary, the differences in the effects on mi- creased. The gray spore pigment TW95a (whiE) accu- crobial secondary metabolism were largely due to both mulated faster and higher under SMG conditions the different secondary metabolites assayed in the test than under 1 g. During spaceflight, the production of strains and the subtle differences in the microenviron- RED and ACT both decreased, while TW95a accumu- ments around the microbial cells; these factors collectively lated to a concentration higher than the control. The influenced the final results. phenotypic responses of secondary metabolites were Although it is true that these studies have provided further supported by the whole transcriptome and substantial evidence that microorganisms alter their qRT-PCR analysis . secondary metabolic properties under spaceflight and Based on both previous studies and our studies and ground-based analog conditions, the specific cause- considering the complex regulatory network involved in and-effect mechanisms of the microbial secondary me- antibiotic production, it is highly likely that the alter- tabolism responses to microgravity remain unclear. To ation of the secondary metabolism of streptomycetes by identify specific cause-and-effect pathways involved in microgravity is strain-, medium-, pathway- and/or case- the effects of microgravity on microbial secondary me- specific and therefore lacks directed and consistent tabolism, the analysis of the corresponding gene ex- behavior. As discussed above, the extracellular micro- pression and regulation will be indispensable, and the environment around microbial cells is involved in the microenvironments around the microbial cells should processes of their growth responses to microgravity and be emphasized and characterized in future studies. This its analogs on the ground and is likely also involved in approach will help to develop an in-depth understand- their secondary metabolism responses. Similarly, it ing of the life processes of microorganisms under space should be noted that the responses of microbial microgravity as well as earth gravity. Huang et al. Military Medical Research (2018) 5:18 Page 11 of 14 Conclusions effect of microgravity and its analogs on microbial With the human exploration of space accelerating, the growth and secondary metabolism should be intensively growth and metabolic responses of microorganisms to studied, and the specific cause-and-effect mechanisms of the extreme environment of space are becoming the microbial responses to microgravity should be disclosed at subjects of increasing concern. The progress of space- the molecular level. For preciseness, future studies should flight and ground-based simulated microgravity technol- take two important aspects into consideration: the strains ogy has promoted our understanding of the effects of used and their cell motility properties as well as the micro- microgravity in space on microbes. It should be noted environment around the microbial cells within the given that there are several cases in which the effects of simu- experimental conditions, such as the culture methods lated microgravity using GBFs are not the same as the (suspension culture or agar culture), medium nutrient effects of exposure to microgravity during spaceflights. concentrations (high or low nutrient concentration), and Additionally, there may be major influencing factors rotation speeds (fast rotating or slow rotating). other than microgravity that can affect the growth and metabolisms of microorganisms during spaceflight, such Additional file as cosmic radiation and the vibrations generated by the Additional file 1: The different responses of microbial growth to rocket and acceleration during the launch and landing of simulated microgravity and spaceflight. (DOC 92 kb) the spacecraft . Meanwhile, cosmic radiation is an important interfering factor that results in a difference Abbreviations in the effects of spaceflight and SMG using GBFs be- 1G: Ground gravity; 2-D: Two-dimensional; 3-D: Three-dimensional; cause there is often no cosmic radiation condition in μG: Microgravity; AC: Agar culture or semi-solid media; ACT: Actinorhodin; CFU: Colony-forming units; DL: Diamagnetic levitation; EUEs: Experiment the latter case. Unique Equipments; GBFs: Ground-based facilities; GS: Gramicidin S; Currently, it is widely believed that the responses of HARVs: High-aspect rotating vessels; LSMMG: Low-shear modeled microgravity; cells to gravity are attained by three possible pathways. MC: Microcystin; MRT: Minimum response time; NG: Normal gravity; PHB: Poly- β-hydroxybutyrate; RCCSs: Rotating cell culture systems; The first is based on the actions of a special molecules RED: Undecylprodigiosin; RPMs: Random positioning machines; RWBs: Rotating or organelles that function as a gravireceptor (i.e., the wall bioreactors; RWVs: Rotating wall vessels; SC: Suspension culture; “direct” effect). The second is based on the adaptive re- SF: Spaceflight; SIMBOX: Science in microgravity box; SM: Simulated microgravity; SMG: Simulated microgravity sponse of the cells to the changing microenvironment (i.e., the “indirect” effect), including extracellular nutri- Acknowledgements ent distribution and transport of metabolites by fluid We are grateful to the German Aerospace Center’s (DLR) Space Administration, EADS (Astrium), the China Manned Space Agency (CMSA) and the General dynamics. The third is based on the actions of the inte- Establishment of Space Science and Application, Chinese Academy of Sciences grated effects of the first two pathways–the “bifurcation (GESSA, CAS) for their technical and logistical support of the SIMBOX-Shenzhou- theory” (symmetry breaking) [102, 103]. It should be 8 space mission. noted that this theory was largely based on studies of Funding higher organisms (i.e., plants and animals). Microor- This work was supported by the China Manned Space Engineering Program ganisms are low and simple lifeforms that do not pos- (CMSE, 921–2), National Program on Key Basic Research Project (973 Program, No.2014CB744400) and the General Financial Grant from the China Postdoctoral sess a gravireceptor, unlike higher plants and animals Science Foundation (No.2016 M602971). [104, 105]; however, they could respond to changes of gravity. Thus, the microorganisms may respond to Availability of data and materials All data generated or analyzed during this study are included in this microgravity or reduced gravity via the changing micro- published article. environment in the medium. An increasing number of studies have shown that spaceflight and simulated Authors’ contributions BH and DGL reviewed the articles and drafted the manuscript. YH and CTL microgravity experiments induced alterations in the conceptualized the review, participated in its design and reviewed the growth rates and the productions of secondary metabo- manuscript. All authors read and approved the final manuscript. lites as well as global alterations in gene expression, Ethics approval and consent to participate protein regulation, and transport of metabolites. The Not applicable. complicated alterations were correlated with microbial adaptation to microgravity conditions; in addition to Competing interests the specific cell motility and secondary metabolites and The authors declare that they have no competing interests. pathways, changes in the extracellular microenviron- Received: 23 November 2017 Accepted: 26 April 2018 ment around the microbial cells induced different responses. References To predict the growth behavior and response of patho- 1. Kussell E. Evolution in microbes. Annu Rev Biophys. 2013;42:493–514. genic bacteria and the production of microbial drugs 2. Bronikowski AM, Bennett AF, Lenski RE. Evolutionary adaptation to such as antibiotics on long-term missions in space, the temperature. VIII. Effects of temperature on growth rate in natural isolates of Huang et al. Military Medical Research (2018) 5:18 Page 12 of 14 Escherichia coli and Salmonella enterica from different thermal environments. 26. 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Military Medical Research – Springer Journals
Published: May 14, 2018
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