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.

Summary The development of botulinum toxin in smoked fish and of staphylococcal enterotoxin in packed bacon has been demonstrated. It is to be expected that these toxins may be produced in contaminated commercial products stored under conditions which permit substantial growth of the organisms, though formation of enterotoxin is not a predictable sequel to multiplication of staphylococci.

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.

Microbiological standards are proposed for ground beef sold in Canada. The proposal is based on a national survey conducted in 1974–75. The proposed standards are: aerobic colony count (35 C), ⩽ 107 for non-frozen and ⩽106 for frozen products; Escherichia coli, ⩽102; Staphylococcus aureus, ⩽102 per g; and Salmonella, absent in 25-g portions from each of five subsamples. To accomodate the variable distribution of bacteria between packages in a single lot of product, three-class plan based on a format suggested by the International Commission on Microbiological Specifications for Foods is used.

The Health Protection Branch, Health and Welfare Canada is considering proposals for microbiological standards for cheese. These proposals are based on a 2-year study (1974–1976) carried out by the Branch. The proposed standards per gram are: total coliforms m = 500, M = 1500, fecal coliforms m = 100, M = 500, and Staphylococcus aureus m = 100, M = 1000, for cheeses made from pasteurized milk; total coliforms m = 5000, M = 50,000, fecal coliforms m = 500, M = 1000, and S. aureus m = 1,000, M = 10,000 for cheeses made from heat treated or unpasteurized milk. The type of standard proposed will be based on a three-class acceptance plan as developed by the International Commission on Microbiological Specifications for Foods. Use of this plan in interpretation of the analytical results allows for the normal variation between analytical samples.