Bacillus cereus is an anaerobic sporogenic bacterium that can grow in various environments, including soil, plants, and the intestinal tracts of insects and mammals. It exhibits a wide temperature range for growth, with a minimum of 4-5°C, a maximum of 48-50°C, and an optimum at 30°C. In terms of pH, it can grow within the range of 4.9-9.3, with the optimal pH being between 6.0-7.0. The vegetative forms of B. cereus are sensitive to heat treatment, while the heat resistance of its spores can vary. Most strains have average resistance to heat, while some strains exhibit high heat resistance.
The ability of B. cereus to produce spores and thrive at low temperatures indicates its potential presence in high populations in food products. Populations of B. cereus associated with the production of emetic or enteric toxins are typically equal to or greater than 105 colony-forming units per gram (cfu/g) of food. The majority of B. cereus strains do not grow at temperatures below 10°C and do not pose a direct risk to milk and dairy products. However, there are strains that can grow at temperatures as low as 5°C, which could potentially pose a risk to public health.
It is worth noting that there have been reports of an increased isolation of cold-tolerant B. cereus strains from refrigerated food, with some strains producing diarrheal enterotoxins.
B. cereus can cause spoilage in milk, resulting in a bitter taste or sweet coagulation. The bitter taste occurs when thermophilic endospores germinate and produce spoilage enzymes (e.g., proteases, lipases, and phospholipases) in pasteurized milk. Sweet coagulation is caused by the metabolic activity of B. cereus cells when their population is high.
Studies have indicated that alterations associated with bitter taste and sweet coagulation are commonly observed in mildly and/or strongly heat-treated milk and dairy products, often resulting in the formation of “sweet curd.”
Considering the spoilage issues (such as sweet curdling) and the safety concerns (toxin production) associated with the presence of B. cereus in milk, this bacterium is of particular interest to the dairy industry.
