Signals and Sensory Mechanisms that Impact Campylobacter jejuni-Host Interactions

Campylobacter jejuni is a leading cause of bacterial diarrheal disease worldwide and a frequent commensal organism of the intestinal tract of poultry and other agriculturally-important animals. Upon infection of the avian host, C. jejuni likely responds to external stimuli present within the intestinal tract to establish commensalism. The sensing mechanisms and subsequent physiological responses by C. jejuni can be crucial for initial growth and colonization and long-term persistence within the infected host. However, how many of the signals and sensing mechanisms affecting C. jejuni biology are not fully understood. In this work, I explored signal transduction mechanisms and possible in vivo signals that may influence the colonization capacity of C. jejuni. One method C. jejuni employs to monitor environmental stimuli are two-component regulatory systems (TCSs). I analyzed the potential of C. jejuni Cjj81176_1484 (Cjj1484) and Cjj81176_1483 (Cjj1483) to encode a cognate TCS that influences expression of genes possibly important for C. jejuni growth and colonization. Through transcriptome analysis, I discovered that the regulons of the Cjj1484 histidine kinase and the Cjj1483 response regulator contain many common genes, which suggests these proteins likely form a cognate TCS. I found that this TCS generally functions to repress expression of specific proteins with roles in metabolism, iron/heme acquisition, and respiration. Furthermore, the TCS repressed expression of Cjj81176_0438 and Cjj81176_0439, which had previously been found to encode a gluconate dehydrogenase complex required for commensal colonization of the chick intestinal tract. However, the TCS and other specific genes whose expression is repressed by the TCS were not required for colonization of chicks. I observed that the Cjj1483 response regulator binds target promoters both in unphosphorylated and phosphorylated forms and influences expression of some specific genes independently of the Cjj1484 histidine kinase. I propose that this TCS may sense signals found in the host intestinal tract, wherein repression of genes may be relieved. In addition to characterizing the Cjj1484/Cjj1483 TCS, I explored the role of metabolites that are commonly found in the intestines -- organic acids and short chain fatty acids (SCFAs) -- in C. jejuni commensal colonization. C. jejuni has both acetate and lactate utilization pathways, as well as for acetate production. I observed that acetogenesis mutants incapable of producing acetate were deficient for colonization of the avian intestinal tract early during infection, but not at later points during infection. Furthermore, I found that an acetogenesis mutant was impaired during growth in a defined media containing solely amino acids and organic acids as carbon sources. Transcriptome analysis of the acetogenesis mutant identified the SCFA-induced regulon which contains metabolically important genes, many of which have been implicated in C. jejuni colonization and virulence. In addition, I found that peb1C, which was downregulated in the acetogenesis mutant, was important for colonization of the chick ceca. I further confirmed in vitro that physiological concentrations of the SCFAs acetate and butyrate activated expression of the SCFA-induced regulon whereas the organic acid lactate repressed these genes. I found that in vivo expression of the SCFA-induced regulon was highest in regions of the intestinal tract where SCFAs are present in the greatest concentration. Furthermore, butyrate counteracted the inhibitory effects of lactate when the two compounds were combined in culture in vitro. I propose that C. jejuni senses the concentration of SCFAs and organic acids to discriminate between different regions of the intestinal tract and to coordinate expression of colonization genes in the preferred niche for colonization. In effect, SCFA sensing and signaling allows C. jejuni to home to appropriate sites of the host for colonization and long-term persistence.