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Summary. Increasing concentrations (2, 4 and 8% w/v) of NaCl in the heating medium reduced the heat resistance of spores of Bacillus stearothermophilus but had no effect on the heat or radiation resistances of B. pumilus, B. subtilis and B. subtilis var. niger or on the radiation resistance of B. stearothermophilus. Increasing heat damage rendered all the spores increasingly sensitive to NaCl in the recovery medium. The presence of NaCl during heating reduced the subsequent NaCl sensitivity of B. pumilus, B. subtilis and B. subtilis var. niger but increased that of B. stearothermophilus. Increasing doses of γ radiation rendered the spores of B. stearothermophilus increasingly sensitive to NaCl in the recovery medium. The effect was less pronounced with B. pumilus, whilst B. subtilis and B. subtilis var. niger showed little change in their post‐irradiation sensitivity to NaCl. The presence of NaCl during γ irradiation had no effect on the subsequent pattern of postirradiation NaCl sensitivity.

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Summary. The resistance of the spores of 6 species of Bacillus to 5% (w/v) of phenol at 37°, heat and gamma radiation has been determined. Two, and with heat treatment three, different shapes of log survivor time curves were observed with each lethal agent. In relation to the conditions employed in accepted sterilization procedures, all of the strains were highly resistant to phenol. Bacillus stearothermophilus, B. licheniformis and B. subtilis were resistant to gamma radiation, all three having a D value of 0.22 Mrad. Only B. stearothermophilus was heat resistant having a D value of 22.6 min at 115°. When the relative order of resistance to each agent was considered, with the exception of B. stearothermophilus, the spores showed little evidence of any relationship between their resistances to phenol, heat and gamma radiation.