Latent evolution of biofilm formation depends on life-history and genetic background

Abstract

Adaptation to one environment can often generate phenotypic and genotypic changes which impact the future ability of an organism to thrive in other environmental conditions. In the context of host-microbe interactions, biofilm formation can increase survival rates in vivo upon exposure to stresses, like the hosts immune system or antibiotic therapy. However, how the generic process of adaptation impacts the ability to form biofilm and how it may change through time has seldomly been studied. To do so, we used a previous evolution experiment with three strains of the Klebsiella pneumoniae species complex, in which we did not specifically select for biofilm formation. We observed that changes in the ability to form biofilm happened very fast at first and afterwards reverted to ancestral levels in many populations. Biofilm changes were associated to phenotypic changes in population yield and surface polysaccharide production. Genotypically, mutational targets in the tip adhesin of type III fimbriae (mrkD) or the fim switch of type I fimbriae were driven by nutrient availability during evolution, and their impact on biofilm formation was dependent on capsule production. Analyses of natural isolates revealed similar mutations in mrkD, suggesting that they also play an important role in adaptation outside the laboratory. Our work reveals that the latent evolution of biofilm formation, and its evolutionary dynamics, depend on nutrient availability, the genetic background and other intertwined phenotypic and genotypic changes. Ultimately, it suggests that small differences in the environment can alter an organisms fate in more complex niches like the host.