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.

Grant Number: 5R01AI168302-04
NIH Institute/Center: NIH
Principal Investigator: Mariana Byndloss