Investigating Indole-Mediated Modulation of Salmonella Virulence and Chemotaxis

The microbial community present in the gastrointestinal tract is an important component of the host defense against pathogen infections. Prior work from our lab demonstrated that indole, a microbial metabolite of tryptophan, reduces enterohemorrhagic Escherichia coli O157:H7 attachment to intestinal epithelial cells and biofilm formation, suggesting that indole may be an effector/attenuator of colonization for a number of enteric pathogens. Here, we show that indole attenuates Salmonella Typhimurium (Salmonella) virulence and invasion as well as increases resistance of host cells to Salmonella invasion. Indole-exposed Salmonella colonized mice less effectively compared to solvent-treated controls, as evident by competitive index values less than 1 in multiple organs. Indole-exposed Salmonella demonstrated 160-fold less invasion of HeLa epithelial cells and 2-fold less invasion of J774A.1 macrophages, compared to solvent-treated controls. However, indole did not affect Salmonella intracellular survival in J774A.1 macrophages, suggesting that indole primarily affects Salmonella invasion. The decrease in invasion was corroborated by a decrease in expression of multiple Salmonella Pathogenicity Island-1 (SPI-1) genes. Indole also reduced Salmonella motility and acts as a chemo-repellent through the Tsr chemoreceptor. We also identified that the effect of indole on Salmonella virulence was mediated by both PhoPQ-dependent and independent mechanisms. Further investigation of PhoPQ-dependent mechanism using Autodock Vina, Molecular Dynamic simulations and in vitro mutagenesis experiments revealed that indole does not bind to the periplasmic domain of PhoQ. Computational analysis predicted indole-binding to the cytoplasmic catalytic domain. Indole also synergistically enhanced the inhibitory effect of a short chain fatty acid cocktail on SPI-1 gene expression. Lastly, indole-treated HeLa cells were 70% more resistant to Salmonella invasion suggesting that indole also increases resistance of epithelial cells to colonization. Our results demonstrate that indole is an important microbiota metabolite that has direct anti-infective effects on Salmonella and host cells, revealing novel mechanisms of pathogen colonization resistance