Growth of enterotoxigenic Bacillus cereus on salmon (Oncorhynchus nerka)

We previously demonstrated the widespread presence of enterotoxigenic Bacillus cereus in marine foods. In view of the widespread consumption of raw fish, we sought to determine the ability of this organism to grow on the surface of wild Alaskan salmon at abusive temperatures (12, 16, and 20°C), using an isolate able to produce elevated levels of hemolysin BL enterotoxin and nonhemolytic enterotoxin. An incubation temperature of 37°C for colony formation was found to be selective for B. cereus grown on salmon held for up to 24 h at each temperature. A fivefold increase in log CFU per gram was observed after 26 and 22 h at 16 and 20°C, respectively, while a >4-log CFU/g increase occurred on salmon held at 12°C for 48 h. Generation times of 169.7, 53.5, and 45.6 min were observed at 12, 16, and 20°C. Nonhemolytic enterotoxin was detected when levels of B. cereus were in excess of 10(8) CFU/g. Nisin, at concentrations of 1 and 15 m g/g of salmon, reduced levels of B. cereus 2.5- and 25-fold, respectively. Our results indicate that fresh salmon can serve as an excellent substrate for enterotoxigenic B. cereus and that this organism can reach levels associated with foodborne illness following moderate temperature abuse.