Growth of Staphylococcus aureus and Enterotoxin Production in Blanched Mushrooms Held in Salt Brines and the Possible Role of Chitin

摘要

Several food poisoning outbreaks in the U.S. linked to the consumption of imported canned mushrooms have prompted studies to examine potential causes of this problem. It was shown that the presence of staphylococcal enterotoxin (SE) in the mushrooms was responsible for the outbreaks and was most likely present in the mushrooms before thermal processing. Several potential scenarios that could permit growth of S. aureus to sufficient populations to produce SE prior to canning have been examined. Blanched mushrooms were inoculated with enterotoxigenic type A S. aureus 743, packed in 7%, 14%, and 21% salt brines, stored at 30℃ for 56 days, and subsequently analyzed for growth of natural microflora and S. aureus as well as the formation of SE. Enterotoxin was detected by the miniVIDAS®( fluorescent immunoassay system and SE concentration was, estimated by employing a standard curve generated using purified enterotoxin. Blanched mushrooms held in 7% and 14% NaCl brines both permitted growth and enterotoxin production of 5. aureus; the 14% NaCl considerably delayed SE production when compared to the 7% NaCl (7 days vs. 2 days, respectively). No growth or enterotoxin production was observed in the mushrooms held for up to 56 days in 21% NaCl. Addition of 14% NaCl to BHI broth also significantly inhibited growth and SE production of 5. aureus. Blanched mushrooms held in 14% NaCl were demonstrated to be a superior growth medium for 5. aureus than was BHI broth with 14% NaCl. These results demonstrate that some components in mushrooms apparently enhance growth and SE production of 5. aureus under stressful conditions. The last objective of the present study was to determine the effect of chitin and chitosan on S. aureus growth and SE production. Although chitin added to BHI broth enhanced SE production only during the logarithmic phase of growth, the effect of chitin was more dramatic when BHI broth contained 14% NaCl. SE production occurred at < 5 days with 0.5% added chitin whereas toxin was not produced within 8 days without added chitin. These results support evidence indicating that the presence of chitin in mushrooms may predispose mushrooms to growth of S. aureus and to staphylococcal food poisoning outbreaks.