Production of staphylococcal enterotoxin D in foods by low-enterotoxin-producing staphylococci

The goal of this investigation was to determine whether staphylococcal strains producing enterotoxins at nanogram levels per milliliter in laboratory medium, not detectable by gel diffusion methods, could produce sufficient enterotoxin in foods to result in food poisoning. Three low-enterotoxin D (SED)-producing strains were selected for this research because this enterotoxin is produced in smaller amounts than the other enterotoxins. The foods used were cream pie and cooked ham, divided into two portions, sterile and non-sterile. Each portion was inoculated with known concentrations of the staphylococcal strains under study and incubated for 48 h at 25, 30, and 37 degrees C. Samples were taken after 24 and 48 h. Enterotoxin was detectable in both sterilized and unsterilized cream and ham after 24 h at 37 degrees C with an inoculum of 10(3)/g. Some strains produced detectable amounts of enterotoxin in the sterilized foods after 24 h at 30 degrees C and some produced detectable amounts of enterotoxin in the sterilized foods after 24 h at 25 degrees C with inocula of 10(4)/g. It can be concluded that staphylococcal strains producing enterotoxin at ng/ml levels in laboratory medium, not detectable by gel diffusion methods, can produce sufficient enterotoxin (ng/g) in foods to cause food poisoning.