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Several members of Bacillus cereus (B. cereus) have been recognized as significant agents of food and health problems. Thermotolerance of B. cereus after pre-exposure to temperature shifts or other stress may influence its ability to grow in minimally processed foods. This study was proposed to determine the recovery rate of B. cereus group from dairy products and fast foods. Moreover, the responses of a recovered isolate to different stresses were evaluated to stand upon its thermotolerance against lethal temperature. In total, 200 different samples from milk and milk products (164) and fast food (36) were examined for the presence of B. cereus group. The effects of heat, cold, acid and salt stresses on B. cereus heat resistance were physiologically and molecularly assessed through examining its viability and measuring the gene expression of its heat induced proteins (GroEL and DnaJ) by reverse transcription quantitative polymerase chain reaction (RT-qPCR), respectively. Overall, 35 (17.5%) samples were positive for B. cereus group isolates. The higher B. cereus count (> 10 CFU/mL or g) was found in 7% of milk and fast food samples. Three B. cereus group species (B. cereus, B. thuringiensis, and B. pseudomycoides or B. mycoides) were identified. After time intervals of mild heat pre-exposure at 42°C or exposure to other stresses, B. cereus developed an increased thermotolerance at 50°C/20 min. This thermotolerance was pronounced after exposure of B. cereus cells to 42°C /1 h and 2.5 % salt/30°C/ 40 min with a 3.9 CFU log increase compared to those exposed to lethal treatment only. RT-qPCR results revealed up-regulation of dnaJ gene expression in pretreated cells compared to the lethal only treated cells. Overall, these results confirmed the heat resistance occurs in B.cereus during food processing, which results in its survival in the food products.

Physiological and Molecular Studies on Thermotolerance of Bacillus cereus Isolated from Some Dairy Products and Fast Foods