Effects of ompR Deletion on Stress Tolerance and Virulence in Salmonella Typhimurium Monophasic Variant

Salmonella Typhimurium monophasic variant (STm) has emerged as a novel predominant serovar in global foodborne Salmonella outbreaks. In our previous study, the two-component regulator OmpR was significantly upregulated in STm under disinfectant stress. However, effects of ompR deletion on stress tolerance and pathogenicity of this pathogen remained unclear. Therefore, this study aimed to systematically investigate the function of ompR in the adaptation of STm to food/food processing environments (e.g., disinfectants, thermal treatment, and high-salt preservation) and virulence. An isogenic ΔompR mutant was constructed using homologous recombination, and its phenotypic changes relative to the wild-type strain were evaluated, including bacterial growth, motility, stress resistance (thermal, osmotic, acid, and oxidative stress), and in vivo virulence in ICR mice. The results revealed that the deletion of ompR significantly decreased bacterial tolerance to multiple tested stresses, including thermal (50 °C, 55 °C, 60 °C), osmotic (10%, 20%, 30% NaCl), acid (pH = 3, 4, 5), and oxidative (1 mM, 2 mM, 5 mM H2O2) stresses. Additionally, the ΔompR mutant exhibited markedly reduced swimming and swarming motility compared to the wild-type strain. In the ICR mice infection model, the ΔompR mutant showed significantly attenuated virulence (100% survival rate within 7 days post-infection), with significantly lower bacterial loads in the liver, spleen and intestine. Transcriptome analysis showed that compared with the wild-type strain, the ΔompR mutant strain exhibited significant upregulation of fimbriae-related genes and downregulation of flagellar biosynthesis and virulence-related genes, thereby affecting bacterial adhesion, biofilm formation, flagellar assembly and virulence expression. Collectively, these findings suggest that ompR contributes to the adaptation of STm to food/food processing environments and virulence, and this gene may provide potential targets for future research to control STm contamination in food production systems.