SUMMARY: A methicillin-resistant strain of (no. 5982) contained cocci with differing intrinsic resistances to methicillin. Most of the population was slightly more resistant than a typical methicillin-sensitive staphylococcal isolate, but a few were mutants with ability to grow at markedly increased concentrations of methicillin (500 μg./ml. or more). Penicillinase production was not essential for methicillin resistance since a penicillinase-negative variant of strain 5982 was able to produce penicillinase-negative strains with high degrees of methicillin resistance. Methicillin-sensitive strains also gave rise to mutants with increased intrinsic resistance to methicillin, but, in the mutants from methicillin-resistant strains, the increase in resistance was much greater. The mutants were characteristically slower growing than the wild type. Many were small colony variants which reverted to the parent type when passed in liquid media. With 5% NaCl-methicillin plates strain 5982 exhibited a phenotypic increase in resistance. On such plates a few L-type colonies were produced at high concentrations of methicillin.


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