Indole is an extracellular cue that regulates V.cholerae virulence

Vibrio cholerae is an aquatic organism and facultative human pathogen that causes the acute diarrheal disease cholera. The two important virulence determinants that are required for disease are the toxin co-regulated pilus (TCP) and cholera toxin (CT), which are required for intestinal colonization and the production of the secretory diarrhea that is the hallmark of cholera, respectively. The expression of the genes that encode for CT and TCP production is governed by a hierarchal regulatory system called the ToxR regulon, which activates virulence genes in response to unknown in vivo stimuli. Indole is signaling molecule produced by a variety of bacteria and regulates diverse physiological processes including antibiotic resistance, biofilm formation, and virulence. In this study, the role of indole in V. cholerae virulence was examined. We report that indole inversely regulated virulence gene expression and biofilm formation. Indole repressed virulence factor production by activating the expression of leuO, a ToxR-regulated global regulatory protein. Upregulation of leuO repressed the expression of the toxT virulence activator, which resulted in attenuated CT and TCP production. In addition to inhibiting virulence genes, it was determined using RNA sequencing that indole affected the expression of many other genes, including those involved in biofilm formation and metabolism. Our collective results have uncovered a role for indole as an extracellular cue that inversely modulates the expression of the ToxR virulence regulon and biofilm formation in V. cholerae.