Characterization of Bacillus cereus groups from food sources: phenotypic, toxigenic and 16S rDNA profiles

Bacillus cereus represents a group of genetically related species, some of which are frequently associated with diarrheal and emetic foodborne illness. Food isolates are often classified as Bacillus cereus sensu lato (s.l.) due to limited information for precise species assignment. Since not all members of this group are pathogenic to humans, characterization is essential to understand their diversity and pathogenic potential. This study evaluated the phenotypic and toxigenic profiles, as well as potential genetic divergence among thirty-three isolates, including two type strains. Isolates were analyzed for motility, hemolysis, and growth at 5°C and 43°C, and further screened using primers targeting diarrheal and emetic toxin genes (nheA, hblD, cytK2, cytK1, and ces), along with 16S rDNA primers. All isolates were motile and produced lecithinase enzyme, but none were able to hydrolyze mannitol. Most isolates, with the exception of two, exhibited hemolytic activity. Genotypic screening revealed that all isolates (100%) carried the nheA and hblD genes; 85% harbored cytK2 gene, while only 18% and 15.2% possessed the cytK1 and ces genes, respectively. Each isolate carried at least two toxin genes, and notably, one isolate harbored all toxin genes. 16S rDNA sequence analysis revealed 98– 100% similarity among isolates. Growth tests and sequence signatures confirmed that all strains belong to the mesophilic group of B. cereus, regardless of their diverse origins. These findings highlight the widespread presence of toxin genes among food isolates and emphasize that any presumptive B. cereus may pose a potential food safety risk.