A general method of determining the nature of the genotypes in mutants of transformable bacteria with similar phenotypes is discussed. The method is used to identify the genotypic patterns of three mutants of Hemophilus influenzae which are resistant to different levels of streptomycin. A mutant resistant to 700 µg per ml of the antibiotic was found to be made up of two unlinked, independent loci—presumably on different molecules of transforming DNA. These loci, when in separate cells, render them resistant to maximum levels of 10 and 100 µg per ml streptomycin respectively and are therefore designated as Sm10 and Sm100. When they enter the same cell they produce a resistance up to 700 µg per ml streptomycin, so the cells are noted as Sm700. This multiplicative action is more easily visualized as due to two independent processes of combating the antibiotic which enhance each other rather than two identical processes.

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