Salmonella endorses a dormant state within human epithelial cells for persistent infection

Abstract

Salmonella Typhimurium (S. Typhimurium) is an enteric bacterium capable of invading a wide range of hosts, including rodents and humans. It targets different host cell types showing different intracellular lifestyles. Within the infected cells S. Typhimurium colonizes multiple intracellular niches, and it is able to either actively divide at various rates, or remain dormant to persist. A comprehensive tool to monitor these distinct S. Typhimurium lifestyles has not been available so far. Here we developed a novel fluorescent reporter, Salmonella Intracellular Analyzer (SINA), compatible for fluorescence microscopy and flow cytometry for quantification at the single-bacterium level. Using SINA, we identified a S. Typhimurium subpopulation in infected epithelial cells that exhibits a unique phenotype in comparison to the previously documented vacuolar or cytosolic S. Typhimurium. This newly identified subpopulation remained dormant within a vesicular compartment distinct from either conventional Salmonella-containing vacuoles (SCV) or the previously reported niche of dormant S. Typhimurium inside macrophages. The dormant S. Typhimurium inside enterocytes were viable and expressed Salmonella Pathogenicity Island 2 (SPI-2) virulence factors at later infection time points. We found that the formation of these dormant S. Typhimurium is not triggered by the loss of SPI-2 expression but it is regulated by (p)ppGpp-mediated stringent response through RelA and SpoT. We predict that intraepithelial dormant S. Typhimurium represents an important pathogen niche as it provides an alternative strategy for S. Typhimurium pathogenicity and persistence. Author SummarySalmonella Typhimurium is a clinically relevant bacterial pathogen that causes Salmonellosis. It can actively or passively invade various host cell types and reside in a Salmonella-containing vacuole (SCV) within host cells. The SCV can be remodeled into a replicative niche with the aid of Salmonella Type III Secretion System 2 (T3SS2) effectors or else, the SCV is ruptured for the access of the nutrient-rich host cytosol. Depending on the infected host cell type, S. Typhimurium undertake different lifestyles that are distinct by their subcellular localization, replication rate and metabolic rate. We present here a novel fluorescent reporter system that rapidly detects S. Typhimurium lifestyles using fluorescence microscopy and flow cytometry. We identified a dormant S. Typhimurium population within enterocytes that displays capacities in host cell persistence, dormancy exit and antibiotic tolerance. We found that the molecular pathway suppressing S. Typhimurium dormancy in enterocytes is the one that has been shown to promote dormancy in macrophages. This suggests a divergent physiological consequence regulated by the same set of S. Typhimurium molecular mediators depending on the challenged host cell type. Altogether, our work demonstrates the potential of fluorescence reporters in facile bacterial characterization, and revealed a dormant S. Typhimurium population in human enterocytes that is distinct from those observed in macrophages and fibroblasts.