SUMMARY: Many strains of carry phages capable of lysing other strains. Diphtheria phages are type-specific and do not lyse strains which are serologically unrelated but susceptibility to a phage may vary among different members of the same serological type and this can be used to differentiate subtypes of different biological behaviour. Phage-resistant mutants are readily selected in diphtheria cultures and the resistant mutants frequently have characteristics quite distinct from those of the parent strain, e.g. virulent organisms have been isolated from a number of avirulent strains. It is suggested that the possibility of converting avirulent strains to virulent ones may be of importance epidemio-logically and that the phage selection of mutants may account for the wide variety of serological types isolated from patients. Some strains found to be resistant to type-specific phages have been shown to be lysogenic and when subjected to ultraviolet irradiation liberate phage particles which are capable of lysing susceptible strains of the same serological type. The diphtheria phages investigated are thermolabile. difficult to filter, of fairly large size and give rise to small plaques of lysis with susceptible strains.


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