Thermorestoration of Radiation Damage in Dry Bacterial Spores

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Abstract
Spores of Bacillus megaterium mounted on membrane filters were handled according to technics previously described. Heating of these dry spores after anaerobic irradiation without an intervening exposure to air results in a decrease in radiation sensitivity (slope of the dose-survival curve) as much as 50%. This "thermorestoration" appears to be caused by annealment of long-lived X-ray-induced states which damage the spore through a reaction with O2. If the spores are exposed to air after irradiation and before heating no restoration is observed. Kinetics of annealment are indicated by changes in the slope of the dose-survival curves under different heat treatments. Annealment proceeds in a simple exponential fashion with time at any given temperature over the range studied (1[degree]C to 99[degree]C). The rate is strongly dependent on temperature; the time required to produce one-half of the maximum restoration varies from about 1 min. at 99oC to about 110 min. at 40[degree]C. The apparent energy of activation is 15,890 [plus or minus] 460 cal, which differentiates annealment from the small effect of temperature during irradiation in this system over the larger temperature range of -145[degree]C to 35[degree]C. The ratio of the slopes of the dose-survival curves at any 2 temperatures below 35[degree]C is not changed by thermorestoration, although the numerical values of the constants may be changed by as much as 50%. Also, the previously reported temperature below which radiation sensitivity is independent of temperature during irradiation, at about -145[degree]C, is not significantly changed by thermorestoration.
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