Cereulide production capacities and genetic properties of 31 emetic Bacillus cereus group strains

The highly potent toxin cereulide is a frequent cause of foodborne intoxications. This extremely resistant toxin is produced by Bacillus cereus group strains carrying the plasmid encoded cesHPTABCD gene cluster. It is known that the capacities to produce cereulide vary greatly between different strains but the genetic background of these variations is not clear. In this study, cereulide production capacities were associated with genetic characteristics. For this, cereulide levels in cultures of 31 strains were determined after incubation in tryptic soy broth for 24 h at 24 °C, 30 °C and 37 °C. Whole genome sequencing based data were used for an in-depth characterization of gene sequences related to cereulide production. The taxonomy, population structure and phylogenetic relationships of the strains were evaluated based on average nucleotide identity, multi-locus sequence typing (MLST), core genome MLST and single nucleotide polymorphism analyses. Despite a limited strain number, the approach of a genome wide association study (GWAS) was tested to link genetic variation with cereulide quantities. Our study confirms strain-dependent differences in cereulide production. For most strains, these differences were not explainable by sequence variations in the cesHPTABCD gene cluster or the regulatory genes abrB, spo0A, codY and pagRBc. Likewise, the population structure and phylogeny of the tested strains did not comprehensively reflect the cereulide production capacities. GWAS yielded first hints for associated proteins, while their possible effect on cereulide synthesis remains to be further investigated.