Detection and quantification of spoilage and pathogenic Bacillus cereus, Bacillus subtilis and Bacillus licheniformis by real-time PCR

A new primer-probe set for the detection and quantification of Bacillus cereus, Bacillus licheniformis and Bacillus subtilis by real-time PCR (Rti-PCR) was developed. For it, forty-eight strains belonging to these species were considered. The DNA of these strains was isolated and a fragment of the 16S rRNA gene amplified. The amplicons were sequenced and the obtained sequences were aligned with reference sequences from the GenBank. For the development of the Real-Time PCR (RTi-PCR) methodology based on TaqMan probes, a primer pair and probe, specific for the studied Bacillus spp., were designed. To establish the quantification method, two RTi-PCR standard curves were constructed; one with DNA extracted from a serially-diluted B. cereus culture and a second curve with DNA extracted from a sterilised food product inoculated with serial dilutions of B. cereus. The curves exhibited R2 values of 0.9969 and 0.9958 respectively. Linear correlations between the log10 input DNA concentration and the threshold cycle (Ct) values were observed with a magnitude of linearity in the range of 1.65 × 101 CFU/mL to 1.65 × 106 CFU/mL for both standard curves. The specificity of the designed primers and probe was tested with DNA extracted from B. cereus, B. licheniformis and B. subtilis strains, which gave Ct values between 14 and 15, whereas non-specific amplifications of the DNA from other microbial species of food interest exhibited a Ct value above 28.5. To our knowledge, this method represents the first study about the quantification of spoilage and/or pathogenic B. cereus, B. licheniformis and B. subtilis in food products, with the aim to prevent the presence of these undesirable species in the food chain.