The Effect of Freezing on the Radiation Sensitivity of Vegetative Bacteria

Abstract

Summary: 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.

In vivo damage and recA-dependent repair of plasmid and chromosomal DNA in the radiation-resistant bacterium Deinococcus radiodurans

“…To determine whether the small amount of residual open circular DNA was the result of repair during irradiation (78 min on wet ice; -0°C), the cells were frozen on dry ice during irradiation (-70°C) and then rapidly melted and fixed by immersion in 100% ethanol. At the D37 under these frozen irradiation conditions (D37 = 3.0 megarads at -70°C; freezing of any bacteria, including D. radiodurans, increases radioresistance because of diminished DNA oxygen effects [26,31]), the extent of both chromosomal and plasmid breakage was identical to that seen at the D37 under the usual (-0°C) irradiation conditions (data not shown). Since no repair is likely to occur while the cells were frozen, we concluded that the presence of the same amount of residual open circular plasmid form seen following irradiation at -0°C is not the result of repair.…”

Sensitivity of bacteria to irradiation on poultry meat under various atmospheres

“…The D,, values for each organism are shown in The presence of oxygen might be expected to enhance the lethal effect of irradiaton, due to oxygen-radical formation. This phenomenon has been observed when organisms in buffer solutions were found to be more sensitive when irradiated under aerobic rather than anaerobic conditions (Matsuyama et al 1964). However, from the results obtained (Table 2) of D,, values in poultry mince, it is evident that atmosphere does influence the irradiation effect.…”

Radiation Resistance and Injury of Yersinia enterocolitica

“…Effect of frozen state on the radiation resistance of Y. enteroolitica. The radiation resistance of vegetative bacteria is usually two to five times greater in the frozen state than in the nonfrozen state (1, 4, 10,19,23 Effect of the suspending medium on the radiation resistance of Y. enterocolitica (strain IP107) at 25°C. A straight line was fitted as in Fig.…”