Combined effects of pH, nisin, and temperature on growth and survival of psychrotrophic Bacillus cereus

Growth of vegetative cells and outgrowth of spores of enterotoxigenic psychrotrophic Bacillus cereus in refrigerated minimally processed food products is a public health concern. A study was undertaken to determine the combined effects of pH, nisin, and temperature on growth and survival of 20 strains of B. cereus. The minimum growth temperatures in tryptic soy broth (pH 7.3) and brain heart infusion broth (BHI broth, pH 7.4) were 5 degrees C for two strains and 8 degrees C for five other strains. Vegetative cells of four of eight strains grew at 8 degrees C in BHI broth (pH 6.01 and 6.57) containing 10 mu g of nisin per mi. At 15 degrees C, all strains grew at pH 5.53 to 6.57; three strains tolerated nisin at 50 mu g/ml (pH 6.57), whereas two other strains had a maximum tolerance of 10 mu g of nisin per mi. Tolerance of vegetative cells of B. cereus to nisin increased as the pH of the broth was increased from 5.53 to 6.01 and again to pH 6.57. Outgrowth of spores (six of six strains tested) was inhibited by 5 and 50 mu g of nisin per mi at 8 and 15 degrees C, respectively. At 15 degrees C, outgrowth of spores of two strains occurred at pH 6.52 in BHI broth containing 10 mu g of nisin per mi. The effectiveness of nisin in controlling the growth of psychrotrophic strains of B. cereus capable of causing human illness was more pronounced at 8 degrees C than at 15 degrees C and as the pH was decreased from 6.57 to 5.53. Studies to determine the effectiveness of nisin in controlling growth of psychrotrophic B. cereus in nonpasteurized foods held at refrigeration temperatures are warranted.