Cullender, Tyler2013-09-052018-05-272013-05-26bibid: 8267658https://hdl.handle.net/1813/34086The mammalian gut contains an incredibly diverse and abundant bacterial community that makes essential contributions to host metabolism and immune system priming. The close proximity of the intestinal epithelium with the gut microbiota poses serious challenges to the host, who in turn has developed a complex set of physical and immunological mechanisms to effectively manage this host-microbe relationship. A defect in any one of these mechanisms can result in opportunistic invasion of host tissues by otherwise commensal bacteria and this breakdown in homeostasis may contribute to numerous metabolic and gastrointestinal pathologies. Here, I investigate the role of two such mechanisms in maintaining homeostasis: Toll-like receptor 5 (TLR5), the innate immune receptor for bacterial flagellin, as well as IgA, which has previously been indicated to be aberrant in abundance and antigen specificity in the absence of TLR5. Using metatranscriptomics and biochemical assays, I found that TLR5-/- mice harbor a bacterial community that exhibits increased flagellation. I also found that TLR5 is required to synthesize the amount of anti-flagellin IgA observed in healthy WT mice, and flagellin load appears to share an inverse relationship with levels of anti-flagellin IgA. I performed 16S rRNA analysis of the gut microbiota in mice with or without the capacity for producing IgA to better understand how IgA influences the stability and resilience of the bacterial community. Indeed, the lack of IgA resulted in reduced temporal stability and reduced community evenness, as well as increased susceptibility to antibiotic-induced perturbation. To specifically investigate the influence of TLR5 on IgA coating of the gut microbiota, I used fluorescence activated cell sorting (FACS) combined with 16S rRNA analysis and found that the bacterial community of TLR5-/- mice displays an aberrant profile of IgA coating, including reduced coating of Proteobacteria and increased coating of Firmicutes. With a combination of immunohistochemistry, fluorescent in situ hybridization, and fluorescent microscopy of intestinal tissues, I observed that TLR5-/- mice have a defective mucosal barrier that results in bacterial invasion of villi in the small intestine and penetration deep into the colonic mucus layer. Together, these results highlight the critical role of TLR5 in maintaining gut homeostasis through directing the synthesis of anti-flagellin IgA and thus reducing levels of flagellation and motility in the bacterial community.en-USTLR5IgAflagellindissertation or thesis