Abstract
Summary
1. Thirteen strains of Group A streptococci were transferred serially on medium containing penicillin in an effort to induce resistance. Two strains developed a 17-fold increase in resistance. Eleven strains either remained unchanged or showed an increase up to only 6-fold at the end of 60 subcultures.
2. No increased resistance was induced in the same organism by serial transfers on control medium.
3. Strains which had acquired resistance were restored to their original level of penicillin sensitivity by serial subculture on penicillin-free medium or by serial intracerebral passages in mice.
4. A marked loss of virulence for mice was shown by all resistant strains of streptococci. The virulence was restored in 2 strains by passage in normal mice.
5. In about one-half of the resistant strains the group specific antigen could not be demonstrated.
6. Transient changes in the colonial appearance, and changes in hemolysis from beta to alpha or gamma were shown by most strains when growing on maximal concentrations of penicillin.
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