Isolation and characterisation of Bacillus cereus from pasteurised milk in household refrigerators in the Netherlands

The incidence and some characteristics (carbohydrate metabolism, growth profiles, haemolysin production and enterotoxin production) of Bacillus cereus, in pasteurised, low-fat (1.5%) milk, in household refrigerators in the Netherlands, was investigated. In 247 (74%) of the 334 milk samples analyzed, the mesophilic aerobic counts were between 50 and 5000 per millilitre. B. cereus could be isolated from 133 (40%) of the samples. In general, the B. cereus counts were low; numbers of less than five per millilitre were observed in 258 (77%) of the samples. As expected, both the mesophilic aerobic counts and levels of B. cereus increased with increasing storage temperatures in the refrigerator and prolonged storage times. In total, 143 presumptive B. cereus colonies were isolated. According to the ISO confirmation tests and the carbohydrate patterns (API 50 CHB) 134 (94%) of these isolates were confirmed to be B. cereus. Of these 134 isolates 20% fermented lactose and 53% of the 106 strains tested were able to grow at 7 °C. These percentages are much higher than expected for strains isolated from non-dairy products, suggesting that strains can adapt to environmental conditions in milk. All 106 strains tested, produced haemolysin, 27% showed the discontinuous haemolytic pattern characteristic for haemolysin BL, possibly a virulence factor. Of the 37 B. cereus isolates tested for enterotoxin production 27 (73%), 28 (76%) and 26 (70%) were found to be enterotoxigenic (as determined by the Western immunoblot technique, polymerase chain reaction (PCR) and Vero cell assays, respectively). Isolates unable to ferment lactose, produced less enterotoxin in comparison with those able to utilize lactose. Although only a few outbreaks of food poisoning caused by B. cereus in milk (products) have been reported, most strains isolated from these products are able to produce enterotoxins and may represent a health hazard.