Using a Co60 source, 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.

The limits of tolerance to gamma irradiation have been determined for 21 cultures representative of the following bacterial species of public health significance in foods: Staphylococcus aureus, Escherichia coli (types I and II), Aerobacler aerogenes, A. cloacae, Streptococcus faecalis, Salmonella gallinarum, S. tennessee, Mycobacterium tuberculosis, Clostridium botulinum (types A and E). A several-fold difference in radiation sensitivity has been demonstrated, specific sensitivity being influenced by the nature of the suspending medium during irradiation.Spores of C. botulinum, type A, were more resistant than those of type E, while six strains of staphylococci of differing phage pattern showed closely similar tolerances. The most sensitive species among those tested was M. tuberculosis. The coliform organisms were more sensitive than the salmonellae, which, in turn, were more sensitive than the staphylococci and S. faecalis. Destruction of coliform bacteria in irradiated foods cannot, therefore, be used as an index of adequate pasteurization.Staphylococcal emetic toxin present in pork sausage and suspended in culture filtrates was destroyed by irradiation at comparatively low levels in contradistinction to the findings for the toxin of C. botulinum.

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