Emetic and diarrhoeal pathotypes of Bacillus cereus

B. cereus is an important foodborne pathogen responsible for two types of syndromes: the emetic and the diarrhoeal. The emetic disease is caused by a small group of B. cereus able to produce the cereulide toxin, while the bona fide enterotoxins responsible for the diarrhoea have not clearly been identified. The aim of this study was to gather more insights into the relevant characteristics of both B. cereus pathotypes, in order to better understand the emergence of these outbreaks. First, the present work was focussed on the emetic pathotype. The genetic variability of cereulide-producing strains was assessed by pulsed-field gel electrophoresis (PFGE) and revealed that the diversity among emetic B. cereus is more important than previously thought. This study improved the knowledge on the spatio-temporal distribution of these strains and also confirmed that PFGE is the most suitable tool for epidemiological studies on B. cereus emetic isolates. Second, the contribution of several putative enterotoxins and virulence factors to the diarrhoeal syndrome was investigated. It appeared that the cytK genes were not more frequent in foodborne isolates, suggesting the negligible role of this toxin in diarrhoeal pathotype. In order to evaluate the role of other virulence factors, the enterotoxicity of B. cereus supernatants was then assessed in vitro on Caco-2 (entero-epithelial) cells. For each isolate, the genetic determinants of enterotoxin candidates were screened by PCR with the aim to correlate their presence with a specific cytotoxicity. Unsuccessfully however, no consistent correlation could be demonstrated between these genes and B. cereus enterotoxicity. The study also revealed that the mucus does not protect the Caco-2 cells from toxic B. cereus. Finally, this thesis suggests that the virulence factors already proposed in the literature are probably not the bona fide enterotoxins and further studies should focus on new candidates.