FIGURE 4.

Salmonella outcompetes resident microbiota in the gut. Host and resident microbiota protects against Salmonella colonization: indole secreted by commensals promotes intestinal health by decreasing secretion of proinflammatory cytokines and by decreasing Salmonella virulence gene expression and certain bacteria (B. thermophilum and Bacteroidetes) are associated with increased resistance to Salmonella. Salmonella uses microbiota-produced hydrogen for initial establishment in the intestine. Salmonella induces secretion of ROS and antimicrobial peptides by neutrophils and epithelial cells. While Salmonella is resistant to ROS, they are harmful to commensal microbes. In the inflamed intestine, microbiota-produced hydrogen sulfate is converted by the host to tetrathionate, which is used by Salmonella as an electron acceptor for anaerobic respiration and allows Salmonella to utilize ethanolamine, produced by the host as carbon source. Additionally, Salmonella – produced bacteriocin, colicin, inhibits resident Escherichia coli in the intestine. Thus Salmonella-induced inflammation leads to dysbiosis and allows Salmonella to propagate in the gut. Some resident bacterial species contribute to Salmonella infection. For example, A. muciphila inhibits mucin production and aids Salmonella in decreasing goblet cell population. Additionally, fucose, which is cleaved from mucin by B. thetaiotaomicron is utilized by Salmonella at the initial colonization stage.