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Selection for the bacterial capsule in the absence of biotic and abiotic aggressions depends on growth conditions
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Extracellular capsules protect the cell against both abiotic and biotic stresses such as bacteriophages and the host immune system. Yet, it is unclear if capsules contribute to fitness in the absence of external aggressions, in spite of the cost of production. Here, we enquire if there are conditions favouring the presence of the capsule in Klebsiella , where it is a major virulence factor. We shortly experimentally evolved 19 different strains, and show that small changes in growth media have a strong impact on the maintenance of the capsule. Competitions with capsule mutants in nine different strains showed that capsules provide ∼20% of fitness advantage in nutrient-poor conditions, due to faster growth rates and higher yields. In contrast, the capsule was readily lost in nutrient-rich media. The growth environment, as well as the capsule serotype, strongly influenced the role of the capsule in major virulence traits like hypermucoviscosity and biofilm formation. Our data shows that the capsule is selected for in situations lacking abiotic, but also biotic stresses and further supports that the capsule role in virulence may be a by-product of adaptation outside the host, hinting that it may have important roles in bacterial physiology yet to be discovered.
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