Effect of rpoS on the survival and gene expression of Salmonella Enteritidis in low water activity foods

This study evaluated the effect of the transcriptional regulator RpoS on bacterial survival, tolerance to simulated gastrointestinal conditions, and the expression of stress resistance genes in wild-type (WT) and rpoS-deletion (Delta rpoS) Salmonella Enteritidis CICC 21482 in low water activity (A(w)) foods at 24 hr of drying and 7 days of storage. The Delta rpoS strain was produced by allelic exchange using a suicide plasmid. Bacteria were inoculated into ginger powder, nonfat milk powder, and chocolate powder in the presence of simulated gastric and intestinal fluids, and survival was measured. Gene expression was measured using real-time quantitative polymerase chain reaction. The Delta rpoS strain was less resistant to desiccation stress than the WT strain. The bacterial tolerance to gastrointestinal conditions varied depending on food composition, and high-fat and protein content enhanced the survival of S. Enteritidis in the gastrointestinal conditions. In addition, under low A(w) conditions, rpoS was weakly correlated with the expression of fadA but strongly correlated with the expression of grpE, otsA, otsB, dnaK, proV, rpoH, and sigDE in food substrates. This study elucidates the role of rpoS in desiccation stress and serves as the basis for preventing and controlling Salmonella contamination in low A(w) foods.