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 a nd vegetative cells on farms in the Netherlands was investigated. Samp les were taken from air, soil, grass, bedding (used and unused), feed (hay, silage, concentrate, beet), drinking water, faeces, udders and m ilk. Some biochemical and growth characteristics of the strains isolat ed were examined. In total 847 presumptive B. cereus colonies were iso lated from all sources examined. The levels present in the various sam ples 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 confirmati on tests and/or the carbohydrate patterns (API 50 CHB), 766 (90%) of t hese isolates were confirmed to be B. cereus. The carbohydrate pattern s revealed more than 30 different B. cereus types. No distinct relatio n 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 contamina ted, finally resulting in the presence of these organisms in raw milk. About 40% of the isolated strains showed growth at 7 degrees C and we re considered to be psychrotrophic. Of the strains isolated from raw m ilk 30% were able to grow at 7 degrees C. The presence of psychrotroph ic spores in raw milk will lead to their presence in pasteurized milk due to heat-resistance properties. Since at low storage temperatures t hese psychrotrophs will germinate, grow and subsequently spoil the pro duct faster than mesophilic types, the psychrotrophic strains will hav e a more significant impact on the keeping quality of milk.