Extended staphylococcal enterotoxin D expression in ham products

Staphylococcal enterotoxin D (SED) is one of the most frequently recovered enterotoxins in staphylococcal food poisoning (SFP) outbreaks. The expression and production of SED were investigated in three ham products, i.e. boiled ham, smoked ham and dry-cured Serrano ham incubated at room temperature for seven days. Staphylococcus aureus was also, as a reference, grown in cultivation broth during optimal growth conditions for seven days. In boiled and smoked ham, continuous sed expression was observed throughout the incubation period with a second increase in sed expression found after five days of incubation. In smoked ham, nine times less SED per colony-forming unit of S. aureus was detected than in boiled ham. In boiled ham, the SED levels unpredictably decreased after three days of incubation. In the Serrano ham, SED was detected after five days of incubation although S. aureus growth was poor and sed expression was too low to determine. After five days of incubation, all three products contained enough SED to cause SFP. These results show that the specific production levels of SED vary in the different ham products, and that toxin production was in part uncoupled from bacterial growth.

Research highlights
► Continuous sed expression was observed for smoked and boiled ham throughout the incubation period.
► Smoked ham contained nine times lower SED amounts per CFU of S. aureus than boiled ham.
► Growth of S. aureus was restrained in the Serrano ham and SED was not detected until five days of incubation and onwards.
► The sed expression pattern observed in the ham products indicates a complex behavior of the regulatory network controlling sed expression.
► Results show that the specific production levels of SED vary in the different ham products, and that toxin production was in part uncoupled from bacterial growth.