Fifty-six toxigenic mitis and mitis-like strains isolated in the western part of the United States and Canada were examined for lysogeny and for the relationship of their phages to conversion to toxigenicity. Twenty-five strains (46%) were demonstrably lysogenic. Twenty-two of the phages from the toxigenic strains were able to convert a sensitive non-toxigenic strain to toxigenicity. Due to technical limitations three phages could not be tested in a similar manner. On the basis of serological relationships, cross-immunizing ability and host range all 22 converting phages proved to be very closely related. They were also related to β- and γ-phages previously studied in connexion with the phenomenon of conversion to toxigenicity by bacteriophage.


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