Abstract
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. However, it is unknown if the capsule contributes to fitness in the absence of these. Here, we explored conditions favouring the maintenance of the capsule in Klebsiella pneumoniae, where the capsule is known to be a major virulence factor. Using short-term experimental evolution on different Klebsiella strains, 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, these results 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 suggest there are novel roles for bacterial capsules yet to be discovered and further supports the notion that the capsule's role in virulence may be a by-product of its contribution to bacterial adaptation outside the host.
Highlights
Most environments in which bacteria thrive are complex and can be temporally or spatially heterogeneous, both in their abiotic or biotic compositions
We generated a panel of nine capsule mutants to address the precise fitness effects and functions of the capsule in a range of conditions. The integration of these results revealed that the Kpn SC capsule can be selected in the absence of biotic stresses, suggesting that its contribution to bacterial adaptation to non-biotic stress and nutrient conditions outside the host may be an important driver of its evolution and maintenance
The capsule is an extracellular structure commonly associated with the ability to resist biotic or abiotic aggressions, like the immune system, phages or desiccation [9,25,26]
Summary
Most environments in which bacteria thrive are complex and can be temporally or spatially heterogeneous, both in their abiotic ( pH, nutrients, chemicals...) or biotic compositions (niche invasions, extinction events). The costs and benefits of the capsule when bacteria are not under biotic stress has not been thoroughly addressed It is currently unknown whether all capsules (or all serotypes) provide the same advantages and whether their function is evolutionarily conserved. We generated a panel of nine capsule mutants to address the precise fitness effects and functions of the capsule in a range of conditions The integration of these results revealed that the Kpn SC capsule can be selected in the absence of biotic stresses, suggesting that its contribution to bacterial adaptation to non-biotic stress and nutrient conditions outside the host may be an important driver of its evolution and maintenance
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