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SUMMARY: Streptomycin resistant strains of Bacterium coli were obtained by plating numerous 1 or 1.5 ml. volume cultures of a sensitive strain on plates containing 100 μg. streptomycin/ml. The growth rate of these strains was measured by mixing them with the sensitive strain and measuring the fall in the numbers of resistant organisms during several periods of daily subculture in streptomycin free broth. Their resistance to streptomycin and stability during successive subcultures in broth were also studied. They could be divided into three main types. All type I strains had identical slow growth rates, were completely resistant, though growing poorly at high streptomycin concentrations and were quickly replaced by rapidly growing slightly resistant variants when subcultured in broth. All type II strains had identical slightly more rapid growth rates, were partially resistant and gradually decreased their resistance in a stepwise manner during subculture. Type III strains had growth rates slightly less than that of the sensitive strain and of a degree of variability similar to those found in clones of the sensitive strain. They were completely resistant and were stable during subculture. They could be subdivided on the basis of one characteristic. Type III a strains grew as well in the presence or absence of streptomycin but type III b strains grew much more slowly in the presence of 1600 μg. ml. streptomycin or over. Since all the strains from any one plate were usually of the same type they were considered to have arisen from a single mutant cell. Since each mutation occurred several times in independent cultures and the number of tubes in which multiple mutations occurred was not greater than would be expected by chance, each type is considered to have arisen by an independent mutation.

The Occurrence of Independent Mutations to Different Types of Streptomycin Resistance in Bacterium coli