Development of a real-time PCR assay for detection and quantification of enterotoxigenic members of Bacillus cereus group in food samples

A highly sensitive real-time PCR (qPCR) procedure, targeting the phosphatidylcholine-specific phospholipase C gene (pc-plc), was developed for specific detection and quantification of strains belonging to Bacillus cereus group. The target region was selected based on the enterotoxigenic profiles of 75 Bacillus strains. The inclusivity and exclusivity of the RTi-PCR assay were assessed with 59 isolates of the B. cereus group, 16 other Bacillus spp., and 4 non-Bacillus strains. The assay was also used to construct calibration curves for different food matrices, and it had a wide quantification range of 6 log units using both serial dilutions of purified DNA and calibrated cell suspensions of B. cereus CECT 148^T. The detection limit for B. cereus in artificially contaminated liquid egg and reconstituted infant formula was about 3 CFU per reaction or 60 CFU/ml of food, with a relative accuracy of 86.27% to 116.12% in artificially contaminated liquid egg. Naturally contaminated food samples were tested for the presence of B. cereus with the standard method, a conventional PCR and the new developed RTi-PCR assay. Results showed that the new developed RTi-PCR assay is very suitable for detection and quantification of strains of B. cereus group in food samples without an enrichment step.