SUMMARY: Penicillinase-producing strains of isolated from human infections and kept under various conditions were plated out and fifty colonies from each plate tested for penicillin-sensitivity to determine the permanence of their resistance to penicillin.

Of 32 strains kept in Lemco broth and tested once, 5-12 months after isolation, two yielded only penicillin-sensitive colonies and fifteen others yielded a proportion of such.

Of six strains preserved by the gelatin-ascorbic acid drying process and kept for over a year, only one yielded penicillin-sensitive colonies.

Of six strains kept in Lemco broth with and without regular subculture and tested at intervals for 9 months, all gave rise to at least one penicillin-sensitive variant colony and more than half the colonies of one strain became penicillin-sensitive.

The natural tendency of these strains to yield penicillin-sensitive variant colonies was not appreciably accelerated by treatment with X-rays or by growing them with other organisms.


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