LuxS/AI-2 signaling in Salmonella Typhimurium regulates phoP/phoQ to protect against acidic pH for enhanced survival in macrophages

The intestinal milieu is largely characterized by the complex array of chemical compounds produced through the metabolic activity of resident microbiota. Enteric pathogens like Salmonella, which have evolved refined mechanisms to persist within this environment, utilize these microbial metabolites and self-produce quorum molecules as molecular cues to identify ecological niches and modulate their survival and virulence strategies. Salmonella quorum sensing involves producing and detecting Autoinducer-2 (AI-2) signaling molecules. Our research reveals that Salmonella Typhimurium enhances AI-2 biosynthesis and transport under acidic conditions, aiding environmental adaptation and facilitating pathogenesis in macrophages. AI-2 signaling regulates the pH-sensing two-component system genes, phoP/phoQ, ensuring cytosolic pH homeostasis, survival, and acid tolerance. It also involves regulating the lysine/cadaverine-mediated acid tolerance response and maintaining bacterial cytosolic pH. Furthermore, we show that the repressor LsrR protein, apart from the lsr promoter, binds to only one strand of the phoP promoter via its Y25 and R43 amino acids to negatively modulate phoP expression. Additionally this signaling ameliorates the intracellular survival by modulating Salmonella Pathogenicity Island-2 (SPI-2) regulators (ssrA/ssrB) and SPI-2 effector expression via PhoP. Mouse models demonstrate that AI-2 signaling is essential for colonizing the primary and secondary infection sites, showcasing its critical role in pathogenesis and low pH survival mechanisms in Salmonella.