OCCURRENCE AND CHARACTERIZATION OF (PSYCHROTROPHIC) BACILLUS-CEREUS ON FARMS IN THE NETHERLANDS

In order to determine the major contamination sources of raw milk with (psychrotrophic) Bacillus cereus, the incidence of B. cereus spores and vegetative cells on farms in the Netherlands was investigated. Samples were taken from air, soil, grass, bedding (used and unused), feed (hay, silage, concentrate, beet), drinking water, faeces, udders and milk. Some biochemical and growth characteristics of the strains isolated were examined. In total 847 presumptive B. cereus colonies were isolated from all sources examined. The levels present in the various samples ranged from < 10 up to 10(7) per g or ml for vegetative cells and < 10 to 10(5) per g or ml for spores. According to the ISO confirmation tests and/or the carbohydrate patterns (API 50 CHB), 766 (90%) of these isolates were confirmed to be B. cereus. The carbohydrate patterns revealed more than 30 different B. cereus types. No distinct relation between the biochemical characteristics and the contamination source was observed. The major contamination sources of B. cereus were soil and faeces. In winter when cows are housed, used bedding probably also participates in this contamination route. The udder will be contaminated, finally resulting in the presence of these organisms in raw milk. About 40% of the isolated strains showed growth at 7 degrees C and were considered to be psychrotrophic. Of the strains isolated from raw milk 30% were able to grow at 7 degrees C. The presence of psychrotrophic spores in raw milk will lead to their presence in pasteurized milk due to heat-resistance properties. Since at low storage temperatures these psychrotrophs will germinate, grow and subsequently spoil the product faster than mesophilic types, the psychrotrophic strains will have a more significant impact on the keeping quality of milk.