Mutation of streptomycin-dependent Mycobacterium ranae selected from a sulfathiazole-resistant variant.

The recent extensive use of antibacterial agents has been instrumental in developing new procedures in the study of bacterial inheritance. Much has been written on the genetic aspects of bacterial resistance to penicillin, streptomycin, and other chemotherapeutic agents, and it is now conceded that these resistant variants arise by spontaneous mutation. In the case of streptomycin there has also been described a variant that requires this agent for growth (Miller and Bohnhoff, 1947; Paine and Finland, 1948; Yegian and Budd, 1948). These streptomycin-dependent strains provide a practical method of studying reverse mutation, for by plating large populations on medium free from streptomycin it is possible to select nondependent variants for analysis. This report describes the variants obtained in this manner from a streptomycindependent strain of Mycobacterium ranae isolated directly from the parent streptomycin-sensitive culture and those obtained from a streptomycin-dependent strain selected from a sulfathiazole-resistant variant of this microorganism. In the latter case two "factors" are involved, and this is reflected in the variety of morphological and biological variants recovered.