Capsular Polysaccharides in Bacteroides thetaiotaomicron and Their Role in Mediating Interactions with Host Immunity and Bacteriophage

Abstract

Members of the Bacteroides and Parabacteroides genera, a prevalent portion of the gut microbiota, typically encode multiple loci for the synthesis of capsular polysaccharides (CPS). Strains may possess more than a dozen of these loci in a single genome, and locus expression is tightly regulated and may be highly dynamic. While some individual CPS have been characterized and confer advantageous properties on the bacterium (e.g. host immunomodulation), the functions of the majority of CPS are unknown. The model gut commensal Bacteroides thetaiotaomicron (B. theta) encodes for 8 distinct CPS synthesis loci, and these are coordinately regulated with other cellular functions such as carbohydrate utilization. I created a novel set of isogenic B. theta strains that each express one of the bacterium’s eight CPS. These strains enable the identification of advantages conferred by individual CPS, as well as the role that encoding and expressing multiple CPS may play in optimal survival within the host organism.

This collection of single CPS-expressing strains was pooled and inoculated into germ-free mice, in which we could alter host genetic and dietary parameters. We found that specific CPS provided an advantage over others when in direct competition, and that adjusting levels of host immune response (especially adaptive immunity and IgA levels) altered the stringency of intra-strain competition. The CPS5-expressing strain outcompeted all other single CPS-expressing strains in the presence of adaptive immunity and even successfully competed with the wild-type strain until the community was perturbed with antibiotics, which selected for the wild-type strain.

Additionally, I tested the ability of different CPS-expressing strains to be infected by bacteriophages. I determined that a large collection of bacteriophages isolated on this panel of B. theta strains are only able to infect a subset of the bacterial strains, directly linking CPS to bacteriophage resistance. Wild-type B. theta alters its CPS locus expression when exposed to bacteriophage and is able to quickly recover from phage infection, indicating that another purpose of encoding for the synthesis of multiple CPS is to render subpopulations of the bacterium phage resistant. My work describes roles for specific CPS, identifies advantages of switching between multiple CPS types, and provides a unique resource to further study CPS produced by human-associated commensal bacteria.