DSSS - The importance of being sweet: how the extracellular capsule impacts horizontal gene transfer

Surface structures are the first cellular components to interact with the environment and thus play a major impact in bacterial fitness, adaptation and in bacteria-host interactions. One such structure, the bacterial capsule, encoded in more than half of the bacterial species, including many facultative pathogens, including all ESKAPE pathogens. Capsules are best known for their role in clinical settings, and are considered a major virulence factor. However, capsules also play an important role outside a host because they protect the cells from physical and chemical stresses.

In our laboratory, we use Klebsiella pneumoniae as a model species. It is a gut commensal but also a ubiquitous bacterium. We combine molecular genetics, experimental evolution and comparative genomics to understand how the capsule impacts K. pneumoniae physiology and evolution across different time scales and at different levels of biological organization from the single-cell to communities.

Specifically, I will show that the capsule type qualitatively influences bacteriophage-driven genetic exchanges, whereas the amount of capsule quantitatively alters routes of exchange via plasmids. I will show that the least capsule-sensitive conjugation machineries are the most frequent in the species’ plasmids, and are the only ones associated with both antibiotic resistance and virulence factors, driving the convergence between virulence and antibiotics resistance in the population. Finally, I will speculate how an epigenetic switch responsible for generating phenotypic heterogeneity in capsule production could be at the origin of lineage diversification in K. pneumoniae. Overall, our results help explain the distribution of antibiotic-resistance plasmids and could explain the transition from commensalism to pathogenesis in K. pneumoniae.