Influence of postirradiation incubation temperature on recovery of radiation injured Clostridium botulinum 62A spores

Abstract

The number of colonies formed by unirradiated Clostridium botulinum 62A spores was independent of temperature, in the range from 20 to 45°C (in 5°C increments); no colonies developed at 50°C. Spores irradiated at 1.2 or 1.4 Mrads produced more macrocolonies at 40°C than at higher or lower temperatures. Apparently, radiation injured spores were more capable of repair at 40°C than at the other temperatures studied. More than 99% of the radiation (1.2 Mrads) survivors were injured and were unable to form macrocolonies in the presence of 5% NaCl. The germinated radiation injured spores were also sensitive to dilution, resulting in the loss of viability of 77 to 79% of the radiation survivors. At 30 and 40°C, the irradiated spores did not differ significantly in the extent of germination (>99% at both 30 and 40°C), emergence (64% at 30°C and 67% at 40°C), and in the maximum number of emerged cells that started to elongate (69% at 30°C and 79% at 40°C). However, elongation was remarkably more extensive at 40°C than at 30°C. Many elongated cells lysed within 48 h at 30°C, indicating an impaired repair mechanism. If the radiation injured spores were incubated at 40°C in the recovery (repair) medium for 8 to 10 h, they germinated, emerged, and elongated extensively and were capable of repair. If, after 8 to 10 h at 40°C, these cultures were shifted to 30°C, the recovery at 30°C increased by more than eightfold, resulting in similar colony counts at 30 and 40°C. Thus repair appeared to be associated with outgrowth. Repair did not occur in the presence of chloramphenicol at 40°C, whereas penicillin had no effect, suggesting that the repair involved protein synthesis but did not require multiplication.