Indole Sensing Regulator (IsrR) Promotes Virulence Gene Expression in Enteric Pathogens.

Enteric pathogens such as enterohemorrhagic E. coli (EHEC) and its surrogate murine model Citrobacter rodentium sense indole levels within the gut to navigate its biogeography and modulate virulence gene expression. Indole is a microbiota-derived signal that is more abundant in the intestinal lumen, with its concentration decreasing at the epithelial lining where it is absorbed. E. coli, but not C. rodentium, produces endogenous indole because it harbors the tnaA gene. Microbiota-derived exogenous indole is sensed by the CpxAR two-component system, where CpxA is a membrane-bound histidine-sensor-kinase (HK) and CpxR is a response regulator (RR). Indole inhibits CpxAR function leading to decreased expression of the locus of enterocyte effacement (LEE) pathogenicity island, which is essential for these pathogens to form lesions on enterocytes. In our transcriptome studies comparing wild-type (WT) EHEC and Δ tnaA ± indole, one of the most upregulated genes by indole is ygeV , which is a predicted orphan RR. Because of the role YgeV plays in the indole signaling cascade, we renamed this gene indole sensing regulator ( isrR ). In the absence of endogenous indole, IsrR activates LEE gene expression. IsrR only responds to endogenous indole, with exogenous indole still blocking virulence gene expression independently from IsrR. Notably, a C. rodentium isrR mutant is attenuated for murine infection, depicting delayed death, lower intestinal colonization, and LEE gene expression. IsrR aids in discriminating between microbiota-derived (exogenous) and endogenous self-produced indole in fine-tuning virulence gene expression by enteric pathogens in the intestine. IMPORTANCE Enteric pathogens sense the complex intestinal chemistry to find a suitable colonization niche. The microbiota plays an important part in shaping this chemistry. Here we show that the abundant microbiota-derived exogenous signal indole impacts host-pathogen interactions by allowing enteric pathogens to discriminate between the luminal environment, where expression of virulence genes is an unnecessary energy burden, from the epithelial surface, where this gene expression is needed for host colonization. We describe a new signaling node through the regulator IsrR that allows for this shift. These findings establish a mechanism through which pathogens discriminate from self- and microbiota-derived signaling to establish infection.