Reducing the Thermal Resistance of Bacillus cereus Spores

Introduction:   Bacillus cereus is a Gram positive, endospore forming foodborne pathogen, which can survive pasteurization regimes. Strains of B. cereus can germinate and grow in foods held at low temperatures and produce toxins, which cause diarrhea and/or vomiting when the food is consumed. The application of wet heat at 121°C for five minutes (retorting) will eliminate spores, but this is not a feasible option for all the food products as it can result in significant denaturation of protein structures, a loss of functionality, and/or produce undesirable texture, color, or flavor changes.

 Purpose:   The purpose of this work was to understand factors influencing the germination of B. cereus spores and to develop hurdle strategies to inactivate germinating spores and newly developed vegetative cells. Methods : The impact of chilled storage at 4°C for 6 days on B. cereus spores in phosphate buffer (pH 7.2, 50mM) was assessed based on their thermal resistance (D-values), structural changes (SEM), and viability. Viability was assessed by plating for total microbial number (TMN: vegetative cells plus spores) and spore number (SN). To determine SN, vegetative cells were inactivated by holding the suspension at 80°C for 15 min. Structural changes were compared with those during germination using SEM. Results:  TMN and SN remained constant over 6 days at 4°C, despite D-values at 88, 92, 96, and 100°C reducing by 13.9, 8.3, 0.2, and 0.1 min. SEM images suggested that structural changes were different from cortex lysis in germination but were contributing to the loss of heat resistance, indicating onset of germination. Significance: A better understanding of the mechanisms controlling spore survival and germination may help in the development of hurdle strategies designed to eliminate spores from heat sensitive food products.