The resilience of Salmonella to bile stress is impaired due to the reduced efflux pump activity mediated by the antioxidant enzyme YqhD.

Bile salts play a critical role in modulating the host gut. They possess antimicrobial properties wherein they disrupt the bacterial membrane and produce reactive oxygen species (ROS), causing DNA damage. Pathogens like Salmonella regulate their metabolic activity to counteract the effects of bile. This study investigates the role of YqhD, an aldehyde reductase, in Salmonella's bile salt susceptibility. Our findings reveal increased survival of the yqhD mutant in the in-vitro studies in LB media with bile, liver cell line HepG2 and C57BL/6 mice on treatment with 8% sodium cholate in the cecum. Bile salts, physiologically produced for the digestion of fat, enhanced the organ burden of the yqhD mutant in C57BL/6 mice on replacing the chow diet with a high-fat diet (HFD). The yqhD mutation, on bile salt exposure, also leads to increased ROS levels and modulation of antioxidant genes in the bacteria. The addition of the antioxidant glutathione during bile stress enhances the survival of STM WT and makes it similar to the survival of STM ΔyqhD. Similarly, in the gp91^-/-phox mice, the organ burden and pathology of the liver and spleen were increased on STM WT infection, while it remained similar for the yqhD mutant on exposure to HFD compared to that of chow-fed mice. Furthermore, the yqhD mutant exhibited increased AcrAB efflux pump activity, regulated by RamA/R regulon.

Importance: Foodborne pathogen Salmonella can tolerate high concentrations of bile and even survive the harsh environment of the gall bladder. This study is significant as it explores the role of a novel antioxidant gene yqhD in bile salt susceptibility of Salmonella Typhimurium and Typhi. It highlights how the presence of gene yqhD, though advantageous in macrophages, reduces the Salmonella survival on bile salt exposure in vitro and in liver cell line HepG2. Deletion of yqhD increased the survival on bile stress exposure, which was attributed to its ability to induce the AcrAB efflux pump of Salmonella. A deeper understanding of how Salmonella modulates gene expression in response to bile stress could provide valuable insights into addressing the chronic carriage of Salmonella.