The Effect of Freezing on the Radiation Sensitivity of Vegetative Bacteria

S ummary : Five strains of bacteria were irradiated, suspended in heart infusion broth or in phosphate buffer, in aerated or anoxic conditions, at temperatures of 10–13° or -79°. Survivors under the different conditions were enumerated by plate counts on heart infusion agar.
Exponential survivor-dose curves were obtained with a Pseudomonas strain and with Escherichia coli B/r when irradiated at room temperature with aeration, whereas an Alcaligenes strain and 2 strains of Streptococcus faecium gave sigmoid curves. The decreased radiosensitivity in the frozen state was measured by comparing the D₁₀ values for exponential curves, or for the exponential portion of sigmoid curves, with that observed for irradiation at room temperature with aeration. This 'D₁₀ ratio' svaried between 2°5 and 8·5. For the Alcaligenes strain it was about 4, whether the frozen irradiation took place in the presence or absence of oxygen. With the Pseudomonas irradiated in the frozen state in the absence of oxygen the 'D₁₀ ratio' was usually about 1·5 times higher than when oxygen was present. The highest ratio (8·5) was obtained for anoxic irradiation of the Pseudomonas strain.
In general, the shapes of survival curves for frozen irradiation differed from those obtained at room temperature. The sigmoid curves for the Alcaligenes strain irradiated under aerobic conditions when frozen showed a marked decrease in extrapolation numbers. E. coli B/r when frozen in heart infusion broth gave a double exponential curve with a shallow slope initially, followed by a steeper slope. The most radiation resistant strain, Strep. faecium R53, gave sigmoid curves with a D₁₀ value of 300 Krads when frozen, and was then of similar resistance to Clostridium botulinum spores.