Identification of contamination sources of Bacillus cereus in pasteurized milk

In order to determine the sources of Bacillus cereus in pasteurized milk, a total of 232 milk samples from various sampling points along milk processing lines and 122 environmental swabs were collected in two dairy plants between March and September, 1996. The incidence of B. cereus vegetative cells in raw milk from the plants was low (≤10%). However, the incidence and the average counts of B. cereus spores in the raw milk were very high and similar to those of B. cereus vegetative cells in pasteurized milk or final products after enrichment (>80% and 1.1×105 cfu ml−1, respectively). The incidence and average count of both vegetative cells and spores of B. cereus in environmental swabs was low. Using the microbial idendification system (MIDI), a library of B. cereus fatty acid profiles comprising 229 B. cereus isolates from milk samples and environmental swabs was constructed using a critical Euclidian distance of 6.0 units as the cut-off value. Using this library, the relationship between 546 B. cereus isolates from the different sampling points along the milk processing lines and the environmental swabs was determined. Most B. cereus isolates obtained from the pasteurized milk and final products belonged to the same sub-groups as the B. cereus strains germinated from spores in raw milk. Furthermore, specific sub-groups were found in pasteurized milk, different dairy plants and at different sampling times. The results suggested that B. cereus spores in raw milk were the major source of B. cereus in pasteurized milk and that post-pasteurization contamination along the milk processing lines was possibly a minor source of B. cereus in pasteurized milk.