PLUNKETT, GEOFFREY E. (Biochemical Research Foundation, Newark, Del.). A temperature-sensitive streptomycin requirement in an Escherichia coli mutant. J. Bacteriol. 84:275-277. 1962.-A strain of Escherichia coli, streptomycin-dependent at 37.5 C, has been found to be capable of continuing cell division in the absence of streptomycin when incubated at 31 C. This property appeared to be heritable and persisted after repeated single-colony isolations from streptomycin-containing agar maintained at 37.5 C, or from streptomycin-free agar maintained at 31 C. The cells were stored under refrigeration on streptomycin-free agar slants, and retained their requirement for streptomycin
SUMMARYFor 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 lopg./ml. Less than 5,ug./ml. was sufficient for fairly heavy growth and the minimal streptomycin concentration permitting appreciable growth was 0-40pg./ml. Division of S-dependent bacteria was inhibited at streptomycin concentrations greater than 20pg./ml. and small inocula gave no visible growth in 24 hr at more than 30pg. streptomycin/ml.Following addition of any of several salts to the growth medium a t 0 . 0 5~ 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,ug. streptomycinlml. 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 10,000,ug. streptomycinlml. In 0 . 1 0~ 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 a t 200,OOOpg. streptomycin/ml.
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