The role and regulation of CsrA in Vibrio cholerae pathogenesis

Vibrio cholerae is a natural inhabitant of the aquatic environment; however, if ingested, it can be a deadly human pathogen. A V. cholerae infection requires a rapid change in gene expression in response to host-specific environmental cues. This response enables the bacterium to overcome microbial deterrents found throughout the gastrointestinal tract while promoting colonization within the small intestine. This study shows that the post-transcriptional global regulator, CsrA, has an instrumental role in this process. In this study, I show that in response to amino acid supplementation, the levels of the CsrA-antagonistic Csr small RNAs (sRNAs) decrease. This decrease in the Csr sRNAs likely shifts the equilibrium from more sequestered CsrA to more available CsrA, enabling CsrA to regulate more direct RNA targets. This shift is reflected in the CsrA-dependent increase in ToxR protein levels in the presence, but not absence, of nutrient supplementation. This change in CsrA availability in response to nutrient supplementation is one example of V. cholerae altering gene expression in response to environmental cues. Additionally, I demonstrate that CsrA autoregulates its availability by controlling the expression of the Csr sRNAs through their primary regulator, VarA. Because the activity of CsrA must be tightly regulated, this intrinsic regulatory feedback loop prevents major fluctuations in CsrA availability, which would be deleterious to the cell. I also show through transcriptomic approaches that CsrA-mediated regulation controls the gene expression of nearly 25% of the V. cholerae transcriptome. From this analysis, I found that CsrA represses the expression of genes required for survival in the aquatic environment, including biofilm production, while simultaneously activating expression of genes required for host colonization, such as motility and virulence factors. Approximately 70% of all flagellar assembly genes were downregulated in the csrA mutant, and the csrA mutant had impaired motility compared to the wild-type, suggesting that CsrA positively regulates motility. Additionally, CsrA directly activates the protein production of the quorum sensing master regulator, AphA, which is required for toxin production. Taken together, these data indicate that CsrA regulation controls multiple pathways that are important both outside and inside the host