Variation in penicillin resistance in single-cell cultures of Staphylococcus aureus.

Using staphylococci grown from single-cell isolations made at various stages of selection on penicillin media, it was observed that the 18 hr. progeny show variations in penicillin sensitivity, as tested by increase in resistance. The range of variation is about the same at any stage in selection; variation appears to be continuous in any one population of the microbes, since the number showing increased resistance depends upon the closeness of the steps in penicillin concentration used in testing the resistance.