Mechanisms of Salmonella-mediated disruption of colonization resistance in the inflamed gut

PROJECT SUMMARY Infection with non-typhoidal Salmonella is 1 of 4 most prevalent global causes of diarrheal disease. In the United States, Salmonella enterica serovar Typhimurium (S. Tm) infection results in 1.35 million illnesses annually. To infect the gastrointestinal tract, S. Tm contends with the resident commensal bacteria (gut microbiota). The gut microbiota benefits the host by limiting enteric pathogen expansion (colonization resistance), partially via the production of inhibitory metabolites such as short-chain fatty acids (SCFA) (e.g., propionate) and nutrient sequestration (e.g., amino acids). Thus, successful bacterial pathogens must possess mechanisms to survive in the competitive ecosystem of the gut. S. Tm uses a Type III secretion system (T3SS- I) to invade intestinal epithelial cells (EICs) and induce intestinal inflammation. As a result, S. Tm disrupts the host-microbiota ecosystem and overcomes microbiota-mediated colonization resistance by using inflammation- derived electron acceptors such as fumarate and nitrate for anaerobic respiration. However, the mechanisms that drive Salmonella-induced disruption of the microbial ecosystem in the gut and how this disruption affects host physiology and promotes pathogen expansion remain largely unknown. In this application, we will elucidate the mechanisms by which S. Tm-induced intestinal inflammation enables the pathogen to (i) overpower SCFA- mediated colonization resistance and (ii) gain access to microbiota-derived aspartate for anaerobic fumarate respiration. Our robust preliminary data obtained from in vitro studies and murine models demonstrate that the pathogen may use propionate metabolism to fine-tune virulence through modulation of T3SS-I expression. Our studies further reveal that S. Tm-induced inflammation causes an increase in Bacteroides-derived aspartate in the intestinal lumen and that aspartate conversion into fumarate fuels S. Tm fumarate respiration in vitro and in vivo. Our preliminary data support our central hypothesis that pathogen-induced intestinal inflammation allows S. Tm to overcome mechanisms of colonization resistance established by the microbiota by (i) downregulating invasion of EICs via catabolism of Bacteroides-derived propionate and (ii) promoting the release of aspartate by commensal Bacteroides, which S. Tm uses to outcompete commensal Enterobacteriaceae. To test this hypothesis, we will define the impact of propionate catabolism on S. Tm pathogenesis in the inflamed gut (Aim 1). Aim 2 will identify the mechanism by which intestinal inflammation promotes increased aspartate availability in the inflamed gut. In Aim 3, we will determine how aspartate enables S. Tm to overcome colonization resistance by Enterobacteriaceae, a bacterium taxon that plays a critical role in protecting the host against S. Tm infection. If successful, this research will establish critical conceptual advances in understanding how enteric pathogens exploit the gut microbiota for expansion during gastroenteritis. Expected findings will provide a deeper understanding of a novel mechanism used by this bacterial pathogen to evade the intestinal microbiota and establish infection.