Growth and inhibition by spices of growth from spores of enterotoxigenic Bacillus cereus in cooked rice

Bacillus cereus is a pathogenic spore-forming bacterium implicated in cases of diarrheal and emetic type of foodborne illness. We previously found that enterotoxigenic B. cereus is widely present in retail spices. Here we analyzed the spore heat resistance of nine diarrheal strains isolated from spices. Seven had D-95 degrees C values ranging from-0.64 to 3.53 min while two emetic strains had D-95 degrees C values of 7.04 min and 6.64 min. The ability of selected strains to grow in cooked rice at temperatures 20 degrees C, 17 degrees C and 12 degrees C was determined as well as their toxin expression capability. After 48 h, B. cereus levels of 1.26 x 107 and 3.8 x 10(7) CFU/g were obtained in cooked rice maintained at 17 degrees C and 20 degrees C respectively. At 12 degrees C, counts did not reach 104 CFU/g even after 48 h of incubation. The increase in the aerobic, mesophilic bacterial population (APC) and B. cereus population naturally present in 0.1% crushed pepper added to cooked rice was determined over a period of 48 h at 20 degrees C and 17 degrees C. Levels of B. cereus in pepper/rice samples reached a maximum of 1600 MPN/g at 20 degrees C while the APC was 2.4 x 1(0)8/g at this temperature. When thyme, containing the same initial natural level of B. cereus, was added to rice in place of pepper, more than a five-fold greater level of B. cereus was detected at 20 degrees C. Since thyme contained initial APC of 2.5 log(10) less than pepper we conclude that the high APC functions in a competitive-exclusion manner, minimizing the growth of B. cereus and potentially other agents of foodborne illness. This is particularly relevant in instances of temperature abuse of foods and may explain the low incidence of foodborne illness due to B. cereus despite its widespread presence shown in previous surveys of market spices. (C) 2015 Elsevier Ltd. All rights reserved.