Production of Staphylococcus Strains Resistant to Various Concentrations of Penicillin

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Abstract
Strains of Staph. aureus resistant to penicillin were developed by successively isolating the survivors from increasing concs. of penicillin. Though there were no survivors of the original strain at a conc. of 0.15 Oxford units of penicillin per ml. of agar medium, some survivors could be isolated from strains on an 0.125 conc. and they were resistant to 0.25 units. By repeating the step 4 times, a strain was isolated that was not affected by 250 units per ml. That the resistance was inherited is indicated by the fact that it was not affected by 20 broth transfers. It is argued that expts. which showed wide variability in the number of resistant bacteria from samples of independent cultures and low variability in samples from a single culture support the theory that resistance is an inherited mutant with penicillin acting as a selective agent. The building up of resistance, increasingly rapidly with each selection step, is interpreted as the summation of effects of several independent genetic mutant factors for resistance in one individual.

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