PHYSIOLOGICAL AND GENETICS STUDIES OF HIGHLY RADIATION-RESISTANT BACTERIA

The phenomenon of radiation resistance was studied using micrococci and Moraxella-Acinetobacter capable of surviving very high doses of gamma radiation which were isolated from foods. Chemical mutagenesis and indirect selection by the replica plating technique were employed to obtain presumptively radiation-sensitive mutants of highly resistant strains. Physiological age, or growth phase, was found to be an important factor in making comparisons of radiation-resistance among different bacteria and their mutants. Moraxella Isolate 4 exhibited a different pattern of growth-related variation in radiation resistance than had been previously observed for other bacteria, and C-3-6, a radiation-sensitive micrococcal mutant, was found to have lost the radiation-resistance variation in growth phase observed in its highly resistant parent strain. Radiation-induced injury, as measured by NaCl sensitivity, was also studied to determine the effect of growth phase on extent of injury and rate of recovery in E. coli and Isolate 4. From the results of these studies, it was concluded that experimenters should determine that growth phase-related resistance characteristics of pure cultures, and control culture conditions to obtain the most resistant physiological state of the cell population before performing dose-response studies to be used for setting irradiation dose levels to be used in irradiation of foods. Radiation-resistant bacteria were highly resistant to the lethal effect of nitrosoguanidine used for mutagenesis, with Isolate 4 surviving prolonged exposure to (GREATERTHEQ)100 (mu)g/ml. Studies of relative resistance of radiation-resistant bacteria, radiation-sensitive mutants, and nonradiation-resistant bacteria to killing by different chemical mutagens did not reveal a correlation between the traits of radiation resistance and mutagen resistance among different strains. Moreover, no patterns of resistance could be established in resistance to different mutagens, classified on the basis of types of mutational lesions produced, among radiation-resistant and radiation-sensistive strains. Resistance of Isolate 4 to ultraviolet (UV) radiation increased significantly when cultures were exposed to UV radiation and allowed to incubate 15 min in plate count broth prior to an UV challenge dose. This inducible response was inhibited by exposing cells to chloramphenicol either before or after the preexposure dose, indicating the need for protein synthesis for expression of this response. The inducible survival response of Isolate 4 differed from the error-prone inducible resistance response of E. coli and other bacteria because mutation frequencies to antibiotics (streptomycin, sulfanilamide, and trimethoprim) were not increased when the resistance response was induced prior to mutagenesis treatment. Comparisons of plasmid profiles of radiation-resistant bacteria and selected radiation-sensitive mutants suggested the possibility that plasmids may carry genes involved in radiation resistance