SUMMARY: Penicillin-sensitive and penicillin-resistant colonies of , belonging to the same phage pattern, were found in six out of 200 apparently resistant cultures isolated from infective processes. The instability of peni-cillinase-producing strains of was confirmed. It was found that growth with or in the presence of other antibiotics did not greatly influence the natural development of sensitive variants. This process, however, tended to be accelerated by growth at 44°, although some strains did not yield any sensitive variants in spite of prolonged incubation. Some inherited factor is considered to be primarily responsible for the production of penicillinase.


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