Characteristics of some psychrotrophic Bacillus cereus isolates

Twelve strains of Bacillus cereus isolated from different food products and foodborne disease outbreaks, and able to grow at temperatures < 7 degrees C, were characterised. Generation times at 7 degrees C varied from 9.4 h up to 75 h. Lag phase of the vegetative cells at 7 degrees C was strongly influenced by the previous temperature history of the cells. Preincubation at 37 degrees C increased the duration of the lag phase drastically. The heat resistance at 90 degrees C (D90 degrees C-values in min) for spores produced at 30 degrees C varied from 2.2 to 9.2 min for 11 strains. One strain, however, showed a D90 degrees C-value of > 100 min. Germination of spores in milk was delayed compared to those grown in brain heart infusion broth (BHI). All strains showed production of the diarrheal type enterotoxin in BHI. Addition of 50 IU of nisin to skim milk resulted in a decrease of numbers for 9 of the 12 strains tested. At a nisin concentration of 250 IU, a decrease in bacterial numbers was observed for all strains tested.