Temporal expression of the staphylococcal enterotoxin D gene under NaCl stress conditions

Staphylococcus aureus is one of the most osmotolerant food-borne pathogens. While its growth is repressed by competing bacteria, the organism exhibits a growth advantage at increased salt concentrations. Staphylococcal enterotoxin D leads to vomiting and diarrhea upon ingestion. To date, the effect of NaCl on both sed expression and its regulatory control are unclear. We determined the impact of NaCl stress on sed expression and the influence of agr, sarA and sigB on sed expression under NaCl stress. The temporal expression of sed in LB and LB with 4.5% NaCl was compared, as well as sed expression of wild-type (wt) strains and isogenic Δagr, ΔsarA and ΔsigB mutants. In general, NaCl stress led to decreased sed expression. However, one strain exhibited a trend towards increased sed expression under NaCl stress. No significant effect of agr on sed expression was detected and only one ΔsigB mutant showed a significant decrease in sed expression in the early stationary phase under NaCl stress. One ΔsarA mutant showed decreased sed expression in the early stationary and another increased sed expression in the stationary growth phase under NaCl stress. These findings suggest high strain-specific variation in sed expression and its regulation under NaCl stress.