Bacterial spores: biophysical aspects of recovery from radiation injury

Differences in radiation resistance of 14 strains of Clostridium botulinum spores could be correlated by computer analysis with differences in the lag or shoulder portion (L) of their respective radiation survival curves. The exponential decline portions (E) of the survival curves were nearly identical for all 14 strains. Autoradiographic and diphenylamine assays indicated that strains 33A, 62A, and 51B contained 1, 2, and 2 genomes per spore, respectively. However, no relationship could be detected between number of genomes and radiation resistance of the spores. Alkaline sucrose gradient sedimentation of 3H-DNA indicated that L was characterized by production of DNA single-strand breaks (SSB). Radiation resistant strain 33A rejoined 50 to 90% of the initial SSB during irradiation or shortly thereafter, i.e., while the spores were still in the cryptobiotic dormant state. Rejoining of SSB seems to be due to high DNA-ligase activity in strain 33A. On the other hand, the radiation sensitive strain 51B showed no shoulder (L) and very little or no SSB rejoining. The exponential decline portion (E) seems to be associated with those lesions which cannot be repaired during irradiation or germination. It is thought that repair of these lesions is attempted after germination and initiation of metabolism and may involve DNA excision-resynthesis and recombination. These repair mechanisms are error-prone and thus frequently result in cell death characteristic of the E portion of the survival curve