Antibiotic Studies on Beta Hemolytic Streptococci: III. Penicillin Resistance Acquired by Group G Organisms

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Abstract
5 strains of group C streptococci were subcultured serially on medium containing penicillin in an effort to induce resistance. 3 of the 5 strains developed a 14- to 16-fold increase after 60 transfers. 2 strains demonstrated only a 2- to 4-fold increase after similar subcultures. No increased resistance was induced by serial transfers on control medium. Organisms maintained their acquired resistance on serial subcultures on blood agar medium or on serial passages through mice. Decreased mouse virulence was shown by all resistant strains of streptococci. Relatively little change appeared in the control organisms. Lost virulence was partially restored in 2 strains by passage in normal mice. The group-specific pre-cipitinogen was demonstrable in only 2 of the 5 resistant strains, but was present in 4 control organisms. Transient changes in the colonial appearance, and changes in hemolysis from [beta] to [alpha] or [gamma] were demonstrated by all strains when grown on max. concns. of penicillin. These colonies reverted to the parent type when subcultured on blood agar.

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