The major toxins of Bacillus cereus : current knowledge on distribution, regulation and mechanism of action

The ubiquitous soil bacterium Bacillus cereus is one of the major pathogens causing toxin-based foodborne diseases, manifesting as diarrhoeal or emetic syndrome. Main symptoms of the former are diarrhoea and abdominal pain, caused by the proteinaceous enterotoxins haemolysin BL (Hbl), non-haemolytic enterotoxin (Nhe) and cytotoxin K (CytK). Responsible for the latter is the cyclic dodecadepsipeptide cereulide, leading to nausea and vomiting, and, in severe and rare cases, also to organ failure and death. This review first addresses the complex taxonomy of the B. cereus group as well as the distribution of toxin genes within this group. Further, recent developments in studying the multifaceted intrinsic and extrinsic factors regulating enterotoxin and ces gene expression and toxin secretion are summarised. Special emphasis is placed on the latest findings regarding the mode of action of the pore-forming toxins, as well as on specific target structures and receptors, and the activation of cellular signalling pathways and inflammatory/apoptotic responses upon toxin exposure. Production and toxicity of the relatively new discovered cereulide isoforms are also discussed. A profound understanding of the fundamental processes of toxin formation and action is essential for accurate risk assessment of B. cereus isolates and for improving diagnostic procedures to increase food safety.