Nutrient conditions are primary drivers of bacterial capsule maintenance in Klebsiella

The fitness cost associated with the production of bacterial capsules is
considered to be offset by the protection provided by these extracellular
structures against biotic aggressions or abiotic stress. Yet, it is
unknown if the capsule contributes to fitness in the absence of these.
Here, we explored conditions favouring the maintenance of the capsule
inKlebsiella pneumoniae, where the capsule is known to be a major
virulence factor. Using short term experimental evolution on
different Klebsiellastrains, we showed that small environmental
variations have a strong impact on the maintenance of the capsule. Capsule
inactivation is frequent in nutrient-rich, but scarce in nutrient-poor
media. Competitions between wild type and capsule mutants in nine
different strains confirmed that the capsule is costly in nutrient-rich
media. Surprisingly, they also showed that the presence of a capsule
provides a clear fitness advantage in nutrient-poor conditions by
increasing both growth rates and population yields. The comparative
analyses of the wild-type and capsule mutants reveal complex interactions
between the environment, genetic background and serotype even in relation
to traits known to be relevant during pathogenesis. In conclusion, our
data suggests there are novel roles for bacterial capsules yet to be
discovered and further supports the notion that the capsule role in
virulence may be a by-product of its contribution to bacterial adaptation
outside the host.