STUDIES ON THE IRRADIATION OF MICROORGANISMS IN RELATION TO FOOD PRESERVATION: II. IRRADIATION RESISTANT MUTANTS

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Abstract
Using a Co60source, repeated irradiation of survivors of bacterial cultures at a level initially destroying a high proportion of cells gave rise to strains which were more resistant to irradiation than the original cultures. Development of resistance was shown in single strains of Escherichia coli, Streptococcus faecalis, and Clostridium botulinum type A, and in one of three strains of Staphylococcus aureus, but not in two strains of C. botulinum type E nor in Salmonella gallinarum. Changes were noted in phage patterns of the staphylococci, biochemical characteristics of E. coli, and toxin production of C. botulinum in response to repeated irradiation.Irradiation of parent and resistant cultures at −78 °C did not change their comparative resistance, though each was afforded some protection at this temperature. These findings suggest that the induced resistance is an expression of resistance to the primary effect of irradiation and not to possible toxic substances formed by free-radical interaction.

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