Characteristics of Selected Strains of Bacillus cereus

Identification techniques for Bacillus cereus are unsettled even though there is an increased awareness of the organism's potential public health implications. Biochemical and morphological characteristics of 17 strains of B. cereus , including 10 isolated from confirmed foodborne outbreaks, were studied by routine methods. Of the characteristics attributed to B. cereus , two strains were negative for the Voges-Proskauer reaction and nitrate reduction; three did not utilize salicin; and five exhibited rhizoidal growth on nutrient agar. Heat resistance of the strains was determined using the serum bottle technique. In demineralized water D- values at 100°C ranged from 0.6 to 27 min, with z-values from 7.4 to 14.5°C. Mean growth constant (k/h) determined turbidimetrically in nutrient broth at 15, 21, 25, 35, 40, 45 and 50°C was 0.15, 0.39, 0.89, 1.54, 1.99, 2.54 and 2.08, respectively. No single feature typified pathogenic strains. At 7 or 11°C, sixteen strains produced hemolysin on blood agar plates, whereas at 45°C, only two strains were hemolytic. Phospholipase activity measured on egg yolk agar plates was evident for three strains at 7°C, for all strains at 35°C, and for only two strains at 45°C.