Role of O₂ and N₂ in Thermorestoration of Hydrated Bacterial Spores

Fully hydrated spores of the highly radioresistant organism, Clostridium botulinum 33A exposed to 0.8 Mrad⁶⁰ Co showed a characteristic pattern of survival associated with thermorestoration. When temperature during radiation was controlled at 0-100°C, the lowest viable cell count was at 0°C. Above 0°C radiosurvival gradually increased by 1 to 1 1/2 log cycles, reaching a peak at 75-80°C. Above 80°C, a rapid drop in survival occurred presumably caused by heat itself. The general features of the above survival pattern were observed in water saturated with N₂ as well as in water saturated with O₂. Radio-survival of spores in oxic water was at all temperatures approximately 1 log cycle lower than that in <tex-math>${\rm N}_{2}\text{-saturated}$</tex-math> water although the general features of the survival curves were approximately the same. An analysis of the shapes of survival curves in N₂ and O₂ suggested that the gradual radioprotection in the temperature range between 0°C and 80°C was due to annealment of lethal radicals in anoxic water, i.e. probably ·OH. The lethal radicals in oxic water (probably <tex-math>$\cdot {\rm HO}_{2}$</tex-math>) appeared not to be annealed appreciably.