LuxS/AI-2 regulates phoP/phoQ by a non-canonical mechanism to enhance acid stress survival in Salmonella Typhimurium.

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 universal 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 investigated the mechanism of AI-2-mediated gene regulation and demonstrated that, in addition to the lsr promoter, the repressor LsrR binds the phoP promoter via its Y25 and R43 residues, thereby negatively regulating 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. Therefore, quorum sensing facilitates survival in hostile host environments by modulating multiple genetic targets through the AI-2/LsrR-mediated feedback loop in pathogenic bacteria.