Mechanism of Salmonella-dependent disruption of propionate-mediated colonization resistance

PROJECT SUMMARY Infection with non-typhoidal Salmonella is a significant cause of diarrheal disease worldwide, causing approximately 150 million illnesses and 60,000 deaths each year. In the gastrointestinal tract, S. Tm encounters the resident commensal bacteria (gut microbiota). The gut microbiota protects the host against invading pathogens (colonization resistance) and limits pathogen expansion. Propionate, a short-chain fatty acid produced by members of the gut microbiota, is predicted to mediate colonization resistance against S. Tm by down-regulating invasion and inhibiting growth. As a successful pathogen, S. Tm may possess mechanisms to mitigate the toxic effects of propionate. Discovery of the prpBCDE operon, which enables S. Tm to metabolize propionate into pyruvate, provided initial insight into the ability of S. Tm to overcome propionate inhibition. However, it remains unknown under what conditions S. Tm uses the prpBCDE operon to eliminate intracellular propionate. During infection, the host's inflammatory response provides electron acceptors that allow S. Tm to metabolize diverse nutrients into carbon sources to support pathogen growth. My data suggests that propionate serves as a carbon source for S. Tm, specifically during anaerobic respiration when inflammation-derived electron acceptors are present. In preliminary experiments, I determined that inflammation-derived electron acceptors also alter propionate-dependent changes in expression of S. Tm invasion machinery. Thus, inflammation-derived electron acceptors may provide S. Tm with the opportunity to eliminate the inhibitory effects of propionate and fuel growth during infection. Indeed, my pilot studies demonstrate that propionate metabolism benefits S. Tm in vivo as a wildtype strain of S. Tm had a growth advantage over a prpC mutant strain in mouse models of S. Tm gastroenteritis. Therefore, the central hypothesis of this proposal is that S. Tm metabolizes propionate in the inflamed gut to regulate its invasion and support its luminal growth, ultimately allowing this pathogen to overcome propionate-dependent colonization resistance. To test this hypothesis, I will use an innovative combination of commensal members of the gut microbiota and S. Tm mutant strains along with germ- free and conventional mouse models to explore how S. Tm contends with propionate during infection. Experiments proposed in Aim 1 will determine if propionate serves as a carbon source to fuel S. Tm growth in vitro and in vivo. In Aim 2, I will identify if gut inflammation and anaerobic respiration are required for propionate metabolism to be beneficial to S. Tm during infection. Aim 3 will investigate if propionate metabolism not only fuels growth by providing a carbon source during respiration but by signaling to S. Tm to decrease invasion. If successful, this research will challenge the dogma that short-chain fatty acids inhibit Salmonella growth in the gut. This project will describe how S. Tm mitigates the detrimental effects of propionate by metabolizing this metabolite into a usable carbon source to fuel growth. Expected findings will provide a deeper understanding of a novel mechanism used by this bacterial pathogen to evade the intestinal microbiota and establish infection.