Thermal Activation and Dry‐heat Inactivation of Spores of Bacillus subtilis MD2 and Bacillus subtilis var. niger

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Abstract
Spores of Bacillus subtilis MD2 and Bacillus subtilis var. niger were heat activated for different times at 60° and 80°C. Strain MD2 required considerable heat activation while B. subtilis var. niger did not. Maximum germination rates increased with heat activation dose and declined subsequently without loss of germinability. Germination rates and percentages were considerably greater in tryptone glucose extract (TGE) than in nutrient broth. The addition of 2°° dimethyl sulphoxide did not increase germination in nutrient broth. The spores of var. niger are more resistant to dry‐heat than MD2 although they are less resistant to moist heat. Survivor curves in the dry‐heat range 140°‐170°C gave D‐values from 4–123 to 0.106 min for MD2 and 5.679 to 0.233 min for var. niger recovered on TGE agar. D‐values were lower on poorer media. The z‐values for MD2 and var. niger on TGE were 18.7°C and 21.25d̀C respectively.