Characteristics of Bacillus cereus related to safe food production

Thirty Bacillus cereus strains, isolated from different sources, were characterized in relation to safe food production. The minimal growth temperatures of the strains varied from < or = 5 degrees C to 11 degrees C. Generation times at 7 degrees C of strains capable of growing at temperatures < or = 5 degrees C were approximately 8.2 h. The D90 degrees C-values of spores of strains with a minimal growth temperature of 11 degrees C determined in phosphate buffer at pH 7.0 ranged from 4.8 to > 200 min. Strains with the capacity to grow at temperatures < or = 9 degrees C, had a D90 degrees C value ranging from 4.6 to 14 min. Addition of either nisin (250 micrograms/ml) or diacetyl (1500 micrograms/ml) to the heating menstruem at the single concentrations investigated seemed not influence the thermal destruction of spores. Germination of spores of almost all strains occurred in all three media tested (Brain Heart Infusion, rice extract and milk) even at temperatures below the minimal growth temperature. All B. cereus strains tested yielded positive results with a commercial test kit for diarrhoeal enterotoxin. The results indicate that strains with the capacity to grow at temperatures < or = 7 degrees C are not essentially different from those with minimal growth temperatures of > 10 degrees C.