Bacillus cereus is a foodborne pathogen which causes gastroenteritis. Fatal and severe cases have been reported within the last few years. Virulence of B. cereus is only partly understood. Most strains are presumably innocuous or mildly pathogenic whereas few are presumably highly virulent. No methods exist to distinguish virulent from non virulent strains. The project proposes to investigate virulence mechanisms of B. cereus in order to identify determinants specific of highly virulent strains. From these results, methods to detect virulent strains will be designed and dose response curve will be improved. The diversity of virulence within B. cereus and the ecology of virulent strains will be investigated in order to identify the contamination routes of foods with virulent strains and to estimate the risk for their presence and development in foods.
Bacillus cereus produces several highly active toxins, it is common in foods, it forms spores and therefore resists most food processing treatments, and it can multiply actively during storage of many food products. However, virulence is very diverse among strains of B. cereus, ranging from innocuous strains to strains which caused fatal foodborne poisoning. Therefore, B. cereus should not be considered as a whole. Highly virulent strains should be treated much more stringently.
The objectives of the project are to acquire the knowledge necessary:
- to identify highly virulent food poisoning strains of B. cereus;
- to propose methods and tools to reduce their incidence in foods.
To improve our knowledge of the virulence of B. cereus, the toxic activity of all the toxins identified so far, will be determined, taking into account possible synergies between toxins and their polymorphism within B. cereus. Then, mechanisms of the expression of the toxin genes will be investigated to explain the difference between high and low toxin producing strains. It should be possible to predict the toxicity of a strain (activity of the toxin x amount of toxin produced) from molecular determinants. Lastly, factors other than toxins which could increase virulence, such as adhesion to epithelial cells would be investigated. New toxic factors may also be identified. Rapid detection methods will be targeted on the virulence determinants. Both PCR methods (detection of genes) and immunochemical methods (detection of molecules) will be designed. Virulence of B. cereus is extremely diverse. For this reason, the project will be based on a selection of strains representative of the diversity of B. cereus, including strains from foodborne outbreaks, from various foods and from the environment. Strains from foods and from the environment will be collected in order to cover a wide diversity of products and to include several steps of the food chain, from the primary production to the final product. Representative strains will be characterised for virulence and for features necessary to predict their fate in foods (growth parameters and heat resistance). Results obtained on virulence and its diversity will permit a better hazard characterisation (how dangerous are the various types of B. cereus) and will be integrated in a model to improve the dose-response relation for B. cereus. Diversity for virulence will be combined with diversity for growth and survival parameters in another model to improve exposure assessment (probability for the presence and growth for virulent B. cereus as a function of food processing parameters).