Growth and toxin profiles of Bacillus cereus isolated from different food sources

Eleven strains of Bacillus cereus isolated from milk and meat products have been used to study growth and sporulation profiles in detail. Polymerase chain reaction (PCR) using primers detecting cold shock protein A gene signatures ( cspA), showed that none of the strains were the newly suggested species in the B. cereus group, B. weihenstephanensis, comprising psychrotolerant cereus strains, although one of the strains grew at 4°C, two at 6°C and seven grew at 7°C. One of the two strains that grew at 6°C had a maximum growth temperature of 42°C, while the remaining 10 strains all grew at temperature of 43°C or higher. Only three strains grew at 48°C. At 42°C, the generation time varied between 11 and 34 min. Spore germination was much faster for the two strains that grew at 6°C than for the other nine strains in milk at 7°C and 10°C. All strains were cytotoxic and contained the non-haemolytic enterotoxin gene ( nhe), 10 strains contained the enterotoxin T gene ( bceT), and only six had the gene ( hbl) encoding haemolytic enterotoxin. Two strains showed some microheterogeneity in the nhe operon, but contained all three genes. We can conclude that true B. cereus strains can have growth profiles as expected for B. weihenstephanensis, and that nhe and bceT were not correlated with growth profiles. However, the two psychrotolerant strains with minimal growth temperature of 4°C and 6°C did not contain hbl, as judged from our PCR results.