Development of resistance to streptomycin by Bact . lactis aerogenes ( Aerobacter aerogenes ) I. The role of mutation and of physiological adaptation

Abstract

Streptomycin at a concentration below 0.1 unit/ml. has no detectable inhibitory effect on cultures of Bact . lactis aerogenes in a salt-glucose medium. At higher concentrations of the drug only resistant mutants, occurring in a proportion of about 1 in 4 x 10 8 survive. These ‘first-stage’ mutants have been isolated directly by the selective action of 1 unit/ml. streptomycin and indirectly by replica plating (1 unit (u) ≡ 1 μg streptomycin base). Mutation to resistance has also been induced by suitable irradiation with ultra-violet light. The mutants give no lag in streptomycin media but at high concentrations grow at a greatly retarded rate which can be progressively improved by subculture in the presence of the drug. It has been demonstrated that at this stage the majority of cells undergo adaptation and that further selection at the high drug concentrations does not occur. Examination of colony diameter histograms for successive subcultures on streptomycin plates reveals a progressive increase in the average size reached in a standard time, although the width of the distribution about the mean value remains essentially constant. Reconstruction experiments with mixtures of untrained cells and those trained to 1000 u/ml. streptomycin have provided further evidence of an adaptive response. It is concluded that full resistance to higher concentrations of streptomycin results from a combination of first-stage mutation and physiological adaptation of these mutants.