According to Schmidt and his associates1 of Christ Hospital, Cincinnati, penicillin resistant strains of bacteria are developed as a result of unsuccessful penicillin therapy in experimental animals. Four groups of 10 white mice were injected intraperitoneally by the Cincinnati investigators using 10-6 cc. doses of a twelve to fourteen hour broth culture of type I or type III pneumococcus. In the control or untreated group there was a 100 per cent type III mortality within twenty-eight hours. There was a 100 per cent survival in the parallel group given repeated 2,000 Oxford units (10 mg.) of penicillin by stomach tube. There was only a 50 per cent survival with half this adequate therapeutic dose (5 mg.) and but a 10 per cent survival following a quarter dose (2.5 mg.).
Pooled cultures were made from the heart bloods of mice which died from these inadequate doses and injected intraperitoneally
PENICILLIN FAST BACTERIA. JAMA. 1943;122(11):748. doi:10.1001/jama.1943.02840280032014