Persistence of serological and biological activities of staphylococcal enterotoxin A in canned mushrooms

Outbreaks in 1989 of staphylococcal food poisoning linked to the consumption of imported canned mushrooms indicated that staphylococcal enterotoxins (SE) may survive a commercial retort process. To examine this possibility, fresh mushrooms were blanched in boiling water for 5 min and cooled 5 min in sterile water inoculated with enterotoxigenic type A Staphylococcus aureus strain 743, to yield approximately 1.3 × 10³ staphylococci per g. Inoculated mushrooms were incubated 20 h at 30°C to simulate time-temperature abuse prior to canning. Mushrooms were sealed in 211 × 212 cans and thermally processed in a still retort to F values of 7, 12, and 18 min at 121 and 127°C. Pre- and post-thermal process staphylococcal enterotoxin A(SEA) serological activity was estimated from a standard curve with purified SEA using a commercial enzyme-linked immunosorbent assay (ELISA) kit. SEA was chromatographically separated from 4-can composite extracts of each F value and temperature. A feline emetic assay was used to determine the biological activity. The dose, administered on a body-weight basis, was equivalent to approximately 0.5 servings of mushrooms and brine for humans. The presence of SEA in the samples was confirmed by Western blotting using anti-SEA immunoglobulin G (IgG). The pre-thermal-process concentration of SEA was about 58 ng/g of mushrooms. Serological and biological activities were detected after all sterilizing values tested at 121 and 127°C. The inactivation of serological activity occurred in two phases, with a rapid initial rate and a distinctly slower rate at higher F values. Attenuation of biological activity, noted by a reduction in the number of emetic episodes and an increase in time to an emetic response, was observed with increasing F values of the processes.