Analysis of Bacillus cereus cell viability, sublethal injury, and death induced by mild thermal treatment

Abstract

AbstractAs the most cost‐effective tool to ensure microbial safety, thermal processing induces a large portion of sublethal injury presenting a potential hazard to food safety. Thermal treatment at 63 °C for 1 min injured 2.22 log cfu/ml totally, half of which were sublethally injured using plate counting method. After 2 min, the inactivation log of Bacillus cereus reached 1.55 while the sublethally injured log even reached 2.16. As for the sublethally injured rate of the B. cereus, it was over 65% after 0.5 min and kept ever‐increase with time. In the end, the injured rate arrived at 99.30% after 2 min processing time. Partial esterase enzyme inactivation was found after 0.5 min heat treatment with flow cytometry combining carboxyfluorescein diacetate/propidium iodide (PI) double‐staining, but B. cereus was not dyed by PI at 63 °C. Comparing with the initial protein concentration of 0.57 ± 0.02 μg/ml, the leakage of the protein was not so notable though the general trend was increasing with duration of heat. Scanning and transmission electron microscopy analysis revealed that a portion of cells morphology and structure had changed after thermal process.Practical applications Bacillus cereus is an endospore forming pathogenic, causing foodborne illnesses and outbreaks all over the world. This research indicated that thermal treatment at 63 °C was a sublethal stress for B. cereus that a portion of cells were sublethally injured which could resume growth in suitable condition and might exist potential safety hazard if used for food pasteurization.