| Description |
Background Bacillus cereus is a typical food contaminant and one of the most important spoilage microorganisms in the dairy environment. B. cereus infection may lead to emetic and diarrheal diseases in humans. The optimal temperature range for B. cereus growth is reported to range from 30 °C to 40 °C. The effects of the carbohydrate substrates on the growth of B. cereus is, however, not known. This study aimed: i) to verify the optimal temperature for the growth of a particular B. cereus strain isolated from the environmental sample to be able to recommend proper storage conditions of products possibly containing this strain and ii) to investigate whether the growth of this bacterium is affected by the presence of specific carbohydrates, which might have implications for treatment of patients infected with this strain. Methods B. cereus isolated from improperly stored breast milk was statically cultured in Tryptone Soy Broth (TSB) media with or without carbohydrate substrate – glucose, fructose, galactose, lactose, sucrose, and xylitol (0.5%, w/v) – at 15 °C, 20 °C, 25 °C, 30 °C, and 37 °C (simulating physiological conditions of the human body) for 12 hours. The optical density (OD600) was measured at 600 nm to monitor cell growth. Carbohydrate substrate consumption and generation of its metabolic products at 37 °C were determined by capillary electrophoresis (CE). Results The temperature of 30 °C was optimal for the growth of the isolated strain while the growth was completely inhibited at the temperature of 15 °C. At all other temperatures, the minimum growth of B. cereus was observed after 12 hours of cultivation in substrates supplemented with lactose and galactose. The bacterial growth under these conditions was even lower than in a TSB medium without a carbohydrate substrate. On the other hand, these bacteria grew most efficiently with glucose. The CE analysis confirmed the kinetic results. Glucose consumption was 49.6 %, resulting in the production of lactate (18.4 mM), acetate (11.6 mM), and formate (1.26 mM). On the contrary, lactose and galactose consumption was only 6.47 % and 3.45 %, with acetate as the main product (10.4 mM and 12.1 mM, respectively). Conclusions To avoid the contamination of foods with the strain B. cereus used in this study, a temperature lower than 15 °C for food storage is sufficient. The optimal temperature range for B. cereus growth was confirmed, cultivation temperature 30 °C and/or eventually 37 °C are the best conditions for B. cereus growth. Moreover, we have shown that the growth of B. cereus highly depends on carbohydrate supplementation. The B. cereus growth was lowest with lactose and highest with glucose, which was also associated with the highest lactate production. These results imply that dietary supplementation with lactose might benefit patients suffering from B. cereus infection, which, however, needs to be verified in further (clinical) studies.
|
