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The effect of streptomycin on the induction of Penicillinase in Bacillus cereus Mitchell, Constance Ann Lorna


The effect of dihydrostreptomycin on the growth and induction of the enzyme, penicillinase, in Bacillus cereus has been studied. Two B. cereus variants were used: a sensitive culture, the growth of which was arrested by approximately 0.3 units of dihydrostreptomycin per milliliter of medium; and a resistant type which would grow in the presence of 2,000 units per milliliter of dihydrostreptomycin. This resistant strain was developed from the parent sensitive organism by successive transferring and plating techniques. The enzyme, penicillinase, was induced with penicillin and assayed manometrically. In the antibiotic-sensitive B. cereus, it was found that the formation of penicillinase, and not penicillinase action, was inhibited by dihydrostreptomycin. Further, total inhibition of penicillinase induction occurred with a concentration of antibiotic that inhibited growth of the organism. This inhibition of penicillinase formation was found to fit the mass law equation, xy = C, where x is the dihydrostreptomycin concentration, y is the fractional enzyme synthesis, and C is a constant. In the antibiotic-resistant B. cereus, neither growth nor penicillinase formation was inhibited by much higher concentrations of dihydrostreptomycin. A slight "partial dependence" on the antibiotic was noted. When antibiotic-resistant cultures, which had been grown in the absence of dihydrostreptomycin, were induced with penicillin in the absence and in the presence of streptomycin, there was, in the absence of the antibiotic, a longer lag period for the formation of penicillinase. That is, the resistant organism showed a slight dependence on streptomycin in the early stages of growth and enzyme induction. It was found that short periods of sonic treatment of suspensions of B. cereus produced an increase in the rate of penicillinase induction. Longer periods of sonic treatment, however, decreased the rate of enzyme induction. The results of this study - that streptomycin inhibits the formation of penicillinase in sensitive B. cereus but does not inhibit the action of this enzyme -were speculatively correlated with the known synergism between streptomycin and penicillin.

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