349
A selection of interesting papers that were published in
the two months before this issue in major journals most
likely to report significant results in microbiology.
• of special interest
•• of outstanding interest
Current Opinion in Microbiology 2001, 4:349–360
Selected by Ariel Blocker
Sir W Dunn School of Pathology, Oxford, UK
e-mail: ariel.blocker@pathology.oxford.ac.uk
•• A link between virulence and ecological abundance in
natural populations of Staphylococcus aureus. Day NPJ,
Moore CE, Enright MC, Berendt AR, Maynard Smith J,
Murphy MF, Peacock SJ, Spratt BG, Feil EJ: Mol Science
2001, 292:114-116.
Significance: For decades, we have believed and taught
(although the relevant literature was mostly silent on the sub-
ject!) that virulence in bacterial pathogens arises by ‘mistake’.
That is, that free-living bacteria have vast environmental reser-
voirs and only ‘by accident’ become virulent: upon acquiring or
expressing one or a few new virulence genes (for example,
genes encoding toxins or antibiotic resistance) when their envi-
ronment changes or growth resources become limiting. In this
landmark study of the ‘bacterial population genetics’ of a very
threatening human pathogen, S. aureus, the authors have finally
tested this dogma by replacing disease carrying isolates such as
methicillin-resistant S. aureus (MRSA) and gentamycin-resistant
S. aureus (GISA) in the crucial context of their epidemiologically
relevant carriage population. Their results suggest that hypervir-
ulent clones are naturally abundant in the bacterial carriage
population and that there is a genetic correlation between fit-
ness in the environment and virulence in man, i.e. a similar set of
genetic traits may be responsible for both. This implies that
S. aureus is not solely an ‘opportunistic’ pathogen. This change
in perspective should rapidly have repercussions on disease
prevention and treatment in hospitals.
Findings: The authors simply chose appropriate sampling meth-
ods and environments, generating isolates from patients with
serious community-acquired disease, from nostrils of healthy indi-
viduals living in the same community and from cases of
hospital-acquired disease in hospitals serving the same commu-
nity. By using conventional genotyping techniques combinatorially
for multilocus sequence typing (MLST at seven unlinked house-
keeping loci) and computerised versions of basic methods of
population genetics analysis, they have identified a number of fre-
quently carried genotypes that were disproportionately common in
disease-causing strains, even taking into account their relative
abundance among carriage isolates. They defined these genotype
sets as ‘clonal complexes’ derived in the recent past as from a
common ancestor, the ‘clonal ancestor’, shared the most individual
genotypes with each of the other clones by single locus variant
(SLV) analysis. Moreover, the most virulent clones were often the
clonal ancestors themselves which then diversified mostly by
recombination (rather than point mutation which appears to be
used mainly upon acute environmental stress) and rapidly seemed
to lose their environmental fitness and their ability to cause inva-
sive disease. An association between recombination and loss of
virulence is evident, as SLVs recovered from asymtomatic carriage
were more likely to have arisen by recombination than those recov-
ered from invasive disease. Why would this be and what is the
cause of the presumably temporarily increased recombination
rates? Interestingly, loss of virulence was much more closely asso-
ciated with recombination at arcC, tpi and pta loci than with the
four other loci, indicating that there may be virulence factors or
pathogeneticity islands closely linked to these loci (note: the full
S. aureus genome is available!). Furthermore, the authors
obtained evidence that isolates corresponding to virulent geno-
types and those belonging to ancestral clones are more
successful colonizers than other genotypes, as they are more likely
to be recovered from both, rather than a single nostril of carrier
individuals. This leads them to speculate that greater propensity
for aggressively successful colonization and greater virulence
(such as entrance into the bloodstream via tissue damage) are
pleitropic effects of the same genetic change(s). There is evidence
for such a link already between the presence of fimbraie in E. coli
and urinary tract infection, suggesting that the authors’ findings
may be generalisable to other ‘clonal’ disease-causing bacteria.
•• Invasion and persistent intracellular colonization of ery-
throcytes: a unique parasitic strategy of the emerging
pathogen Bartonella. Schülein R, Seubert A, Gille C, Lanz C,
Hansmann Y, Piémont Y, Dehio C: J Exp Med 2001,
193:1077-1086.
Significance: Highly elegant study of the intraerythrocytic life
style of the Bartonella spp. in a newly developed rat infection
model (R Heller et al., Int J Syst Bacteriol 1998, 48:1333-1339).
B. quintana, transmitted by body lice, is the bacteriological
agent of ‘trench’ or ‘five-day’ fever, famous during World War I
and now plaguing the poor and homeless in the Western hemi-
sphere. B. bacilliformis causes Carrion’s fever in South America,
a disease with up to 80% mortality in its acute phase. The very
detailed information gained from this work will permit the fitting
of genetic mutation data (to come soon, hopefully, thanks to the
molecular biology tools recently developed for Bartonella spp.,
Microbiology
Paper alert
Contents (chosen by)
349 Host–microbe interactions: bacteria
(Blocker and
Peppler)
351 Cell regulation
(McFall-Ngai)
352 Ecology and industrial microbiology
(Normand)
353 Microbial techniques
(Brisse)
354 Host–microbe interactions: fungi
(Spitzer)
354 Host–microbe interactions: viruses
(Rosenthal)
355 Host–microbe interactions: parasites
(Sibley)
356 Genomics
(Labedan and Forterre)
357 Antimicrobials
(Brown)
358 Growth and development
(Gold et al. and Feucht)
Host–microbe interactions: bacteria