Butyrate Sensing by Campylobacter jejuni Impacts Bacterial-Host Interactions
The intestinal microbial ecosystem aids the host in digestion and nutrition by breaking down goods and providing beneficial vitamins and metabolites, including short-chain fatty acids (SCFAs) and lactate. Due to the abundance and intestinal distribution of these metabolites, bacterial pathogens can use them as biogeographical cues to discriminate among different regions of the host intestines. Indeed, Campylobacter jejuni, a commensal bacterium of the lower intestinal tract of avian species and a leading cause of bacterial diarrheal disease in humans, recognizes intestinal niches that support growth by sensing molecular cues produced by the microbiota. How C. jejuni senses and responds to microbiota-generated SCFAs and organic acids is not understood. Herein, I identified and characterized the C. jejuni BumSR two-component signal transduction system (TCS) that specifically directs a response to butyrate. Deletion of either C. jejuni bumS or bumR abolishes butyrate-modulated transcriptional changes in gene expression. Analysis of ΔbumS and ΔbumR mutants in a chick model of commensalism indicated that bumR is important for early colonization. This contrasts with a human volunteer infection study that demonstrated bumR is essential for infection of humans. Mutational analyses of genes within the BumSR regulon in the natural avian host revealed additional colonization factors including peb3, a putative glycoprotein adhesin/substrate binding protein, and Cjj0580, a putative d- and tri-carboxylate transporter. Through multiple biochemical assays, I discovered that BumS lacks kinase activity in vitro but possesses specific phosphatase activity towards BumR. These activities are not directly influenced by butyrate, suggesting that other metabolites, perhaps resulting from butyrate catabolism, are the direct cues sensed by BumS to modulate butyrate-dependent responses. By site-directed mutagenesis, I identified residues in the conserved H box that are required for BumS phosphatase activity. Consistent with previous work, phospho-BumR exhibits enhanced binding of target promoters in electrophoretic mobility shift assays, indicating that phosphorylated BumR likely has higher affinity to bind DNA at target promoters in vivo to either enhance or repress gene expression. Overall, this highlights BumSR as a non-canonical and first-identified TCS that directs a response to butyrate to modulate colonization gene expression through a phosphatase-dependent mechanism.