Effects of ionizing radiation on recovery from ultraviolet inactivation in Escherichia coli B/r

When ultraviolet (UV)-irradiated E. coli B/r cells were subjected to liquid holding and then irradiated with high energy electrons, the survival increased with the holding time. On the other hand, such an increase in survival was not observed when UV plus electron-irradiated cells were subjected to liquid holding. When photoreactivating light was further illuminated after these treatments, photoreactivating effect was completely lost within 3 hr of liquid holding in the former case, but was observed significantly after 3 hr of liquid holding in the latter case. When liquid holding was performed in the presence of caffeine, such an increase in survival and such a loss of photoreactivating effcct were inhibited. They suggest that synergistic killing action of UV and ionizing radiations is partially caused by inhibition of excision-repair mechanism.