Thermal Injury and Recovery of Bacillus cereus

A system to measure thermal injury to vegetative cells of Bacillus cereus B4ac was developed. After heating in 0.1 M potassium phosphate buffer, pH 6.0 for 5 min at 47 C, cells became both pH-and NaCl-sensitive. The cell population showed decreased viability at pH values less than 6.3 or greater than 7.3, and at concentrations of 2% NaCl or greater in plating media. Cells did not become sensitive to polymyxin B sulfate and/or phenol red at concentrations in which these substances are present in media routinely used to enumerate B. cereus in foods. Injury was normally detected as differential plating ability on Plate Count Agar (PCA) vs. PCA plus 2.5% NaCl. Injured cells could partially recover in 0.1% peptone, Brain Heart Infusion broth (pH 7.3), a mixture of 20 amino acids (10 or 50 μg/ml each), 1% glucose, and 0.1% peptone plus a protein synthesis inhibitor (chloramphenicol, 2.5 μg/ml) or 0.1% peptone plus a DNA replication inhibitor (nalidixic acid, 5 μg/ml). Cells recovered equally well at 20, 32 and 37 C. Recovery did not occur in 0.1% peptone plus an RNA synthesis inhibitor (rifampicin, 7.5 ng/ml) or in 0.1 M potassium phosphate buffer, pH 6.0 or 7.0.