Deletion of both anaerobic regulator genes fnr and narL compromises the colonization of Salmonella Typhimurium in mice model.

Salmonella Typhimurium (STM), a zoonotic pathogen, can adjust its metabolic pathway according to the variations in the partial pressure of atmospheric oxygen and nitrate via fumarate nitrate reductase regulator (Fnr) and NarL, the response regulator for nitrate reductase. Both Fnr and NarL have been individually reported to be the contributors of virulent phenotypes of STM. Hypoxia along with nitrate-rich environment are prevalent in macrophages and the Salmonella-induced inflammatory lumen of the host's large intestine activates both fnr and narL genes. In this study, the double (fnr and narL) knockout STM showed a synergistic reduction in the swimming (62%), swarming (84%) and biofilm density (86%) phenotypes anaerobically in association with its significant aerobic attenuation. The intracellular replication of the double mutant was reduced by 2.3 logs in chicken monocyte-derived macrophages. Furthermore, the competitive index of the double mutant in liver and spleen was found to be 0.3 and 0.44 respectively at 120 h post-infection (PI) in mice. Surprisingly, no double mutant could be recovered from the infected mouse liver 3 days PI. Histopathological findings showed moderate infiltration of mononuclear cells in the large intestine of mice infected with double mutant, but severe infiltration was seen with the wild-type strain.