Survival during cooking and growth from spores of diarrheal and emetic types of bacillus cereus in rice

Abstract

Bacillus cereus is a gram-positive, spore-forming, facultative anaerobe that is responsible for two types of gastrointestinal diseases: emesis and diarrhea. A significant difference in the D95°C-values of spores of the emetic and the diarrheal types was initially determined. A mixture of B. cereus spores of the diarrheal type was inoculated into cooked rice. At inoculation levels of 2.5 × 102 spores per g of rice, cell numbers of 6.64 log were detected after 22 h at 20°C and 6.81 log after 34 h at 17°C, whereas at 12°C the counts did not go above 4.0 log even after 48 h. When added to raw rice before cooking at inoculum levels of 103/g, the number of viable spores decreased by 2 log, and a < 1 -log increase in cell numbers occurred after holding at 20°C for 24 h. In contrast, the emetic spores survived and increased ∼20-fold. Nonhemolytic enterotoxin was not detected in cooked rice at cell numbers of 8.0 log. Results here provide evidence that the absence of foodborne illness caused by the B. cereus diarrheal biotype with rice as the vehicle is due to the inability of their spores to survive and grow following standard heat processing procedures. Copyright ©, International Association for Food Protection.