MODIFIED DEGREES OF STREPTOMYCIN DEPENDENCE AND RESISTANCE IN ESCHERICHIA COLI.

Abstract

SUMMARY: For three streptomycin-dependent (S-dependent) strains of Escherichia coli, the streptomycin concentration necessary for optimal growth during incubation for 24 hr in a nutrient broth medium was about 10 μm./ml. Less than 5 μg./ml. was sufficient for fairly heavy growth and the minimal streptomycin concentration permitting appreciable growth was 0·40/μg./ml. Division of S-dependent bacteria was inhibited at streptomycin concentrations greater than 20 μg./ml. and small inocula gave no visible growth in 24 hr at more than 30 μg. streptomycin/ml. Following addition of any of several salts to the growth medium at 0·05m growth occurred over a wide range of streptomycin concentrations and the optimum was increased from twofold to as much as 1000-fold. Maximal concentration of streptomycin in which growth of S-dependent Escherichia coli was possible increased to as high as 20,000 μg. streptomycin/ml. in some instances, and the minimal concentration which supported growth was increased in the presence of several of the salts. Salts also increased the degree of resistance of a streptomycin-resistant E. coli mutant from 20 to as much as 10,000μg. streptomycin/ml. In 0·10m phosphate-buffered nutrient broth, maximal and optimal concentrations of streptomycin increased with increasing acidity; at pH 5·8 heavy growth of an S-dependent strain of E. coli occurred at 200,000 μg. streptomycin/ml.