Bacillus cytotoxicus is the thermotolerant member of Bacillus cereus group. It was initially recovered from a fatal diarrhoeal outbreak that occurred in a nursing house in 1998 in France. Forming a remote cluster from the other B. cereus, B. cytotoxicus reliably harbours the cytK-1, a gene encoding the most cytotoxic variant of Cytotoxin K. This thesis aimed at exploring the ecological niche of B. cytotoxicus, assessing its diversity and clarifying the role of CytK-1 toxin in B. cytotoxicus cytotoxicity.

Our results support that B. cytotoxicus is rare in nature. Overall, 39 out of 339 food and environment samples were positive to B. cytotoxicus, with a high prevalence in potato products. From Malian foods though, B. cytotoxicus was also found in Millet flour for children and fish powder for the first time ever. A study case in a potato processing food-industry revealed that B. cytotoxicus and other thermotolerant B. cereus were positively selected by the industrial process. RAPD and plasmid profiling of 57 strains showed six RAPD patterns and eleven plasmids profiles. SNPs computing revealed that strains with RAPD pattern A and C fitted in genomic clade C and D, respectively. Members of RAPD patterns E and F did not fit in any genomic clade. Inositol degradation and acetoin anabolism operons, and lactose uptake and utilisation coding genes were first ever reported in B. cytotoxicus. Prophages sequences and plasmid content played a role in genomic diversity the species. Finally, a mutant lacking the cytK-1 gene showed significantly decreased in cytotoxicity on human cell lines. This suggests that CytK-1 is the main toxin responsible of B. cytotoxicus toxicity