Emetic strains of Bacillus cereus and Bacillus weihenstephanensis

Aims Cereulide, the emetic toxin of Bacillus cereus, is an important foodborne toxin implicated in lethal poisoning. It is structurally related to valinomycin, an antibiotic produced by Streptomyces sp. Both compounds are cyclododecadepsipeptides produced via non ribosomal peeptide synthesis. The aim of this study was to gather more insights into the ecology of the emetic strains of B. cereus, including their diversity and the actual "raison d'être" of cereulide in environmental niches. Methods and results A detailed study on phenotypic, genetic and genomic diversity revealed that cereulide is produced by two distinct clusters: a mesophilic group of B. cereus strains and a cluster of psychrotrophic strains displaying features of B. weihenstephanensis. Althought the cereulide genetic determinants(ces) are mostly plasmid-borne, they are chromosomally located in some B. weihenstephanensis. Using plamid-cured derivatives and cesKO mutants as negative controls, a fungistatic activity of cereulide-producing strains was observed towards nine fungal species. The antifungal activity of cereulide was shown to be similar to that valinomycin in term of active concetration, but distinct for its spectra. Conclusions In the environment, B. cereus shares its niches with various fungi. It is reasonabe to assume that cereulide helps the emetic strains to settle more efficiently in these environments thanks to their antifungal activity. Signifiance of study This study opens new perspectives in elucidation of cereulide ecological rol