SUMMARY: Four strains of Staphylococcus aureus (coagulase-positive) were each subjected to three or more series of transfers on penicillin ditch plates. The plates were examined for colonial variants and four quite distinct types of penicillin-resistant variant, one of which was penicillin-dependent, were isolated. These types often occurred in association, and sometimes as many as three were isolated from a single culture plate. The main characteristics of the different types are described.Studies on the incidence of penicillin-resistant bacterial variants began soon after the isolation of the antibiotic and have continued in increasing numbers (see Abraham et al. 1941; Todd, Turner & Drew, 1945; Demerec, 1945, 1948; Bellamy & Klimek, 1948; Gale & Rodwell, 1948; Hughes, 1952). In most studies of this description bacteria have been subcultured in fluid medium containing increasing concentrations of penicillin. In the present investigation solid medium was used to determine whether different types of variant could be isolated from the same culture.
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