Abstract
In a previous study(1) it was shown that the development of resistance to penicillin, streptomycin or erythromycin in vitro may be delayed and depressed by the use of a combination of erythromycin with either penicillin or streptomycin. the organisms used in that study were originally susceptible to small or moderate concentrations of each of these antibiotics, and when they were repeatedly subcultured in the combination of 2 antibiotics, these were present in increasing concentrations, but always in a constant ratio. In the present study an attempt was amde to determine whether the presence of a moderately high concentration of an antibiotic to which bacteria are originally resistant would also serve to delay or depress the development of resistance to another antibiotic following repeated exposures to increasing concentrations of the latter.
Materials and methods. The 8 strains of Staphylococcus aureus used in the present study had been isolated from infected materials during a recent survey(2); they were each inhibited by 25 μg/ml of streptomycin and 0.2 μg/ml of erythromycin but grew well in the presence of 1000 μg/ml of penicillin. They were all hemolytic, coagulase-positive and produced penicillianse. Each of the strains was subcultured on 3 parallel series of blood agar plates: one contained no antibiotic, the second contained penicillin 200 μg/ml plus graded (2-fold) concentrations of streptomycin [as in the plate-dilution test for sensitivity (3)], while the third contained the same amounts of penicillin and similarly graded concentrations of erythromycin. the inoculum for the antibiotic-containing plates was obtained in each instance by scraping a 3 mm loopful of the surface growth from the plate containing the maximum concentration of antibiotic on which nearly full growth occurred after 48 hours incubation, suspending this in 0.2 ml of broth and streaking a 1 mm loopful of the suspension on a similar series of plates.
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