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Behavior of an Emetic Bacillus cereus Strain in Rice Food


Elise N’Guessan, Laurence Delbrassinne and Jacques Mahillon

Subject description: This article deals with the possible competitiveness of emetic B. cereus against nonemetics, likely to confer a development advantage to the first mentioned. The goal of this study was to evaluate the behavior of an emetic strain with respect to other B. cereus contaminants.

Method: A rifampicin-resistant mutant of the emetic strain k5975c was grown in Luria-Bertani. Broth overnight at 30°C, then inoculated in cooked rice or in naturally contaminated rice and incubated for 24 h at 23°C and 30°C. The emetic toxin and its toxicity were detected using the boar sperm motility inhibition bioassay and the liquid chromatography-ion trap mass spectrometry (LC-MS).

Results: With an inoculum level of ca. 6 log CFU g-1, the emetic strain showed unhampered growth in rice contaminated by other B. cereus at both incubation temperatures, although it did not inhibit the growth of B. cereus contaminants. When the inoculum level of the emetic strain was reduced to ca. 3 log CFU g-1, its development was unaffected when the background of resident B. cereus was below 5 log CFU g-1. However, above this level of resident B. cereus strains, the emetic strain developed modestly at both 23°C and 30°C.

The presence of the emetic toxin activity was detected when the final concentration of the emetic strain reached ca. 5.6 log CFU g-1 and above this level.

Conclusion: Our study mimicked naturally occurring emetic food poisonings. It relates to the reported severe forms of rice food poisoning caused by emetic B. cereus.

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