Antibiotic Studies on Beta Hemolytic Streptococci: II. Penicillin Resistance Acquired by Group B Organisms

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Abstract
Ten strains of group B streptococci were transferred serially on media containing increasing quantities of penicillin in an effort to produce resistance. All 10 organisms showed a significant increase in resistance varying from 11- to 190-fold over the parent strains. No increased resistance was induced by serial transfers on control medium. In contrast to group A organisms, group B streptococci did not lose resistance on sub-culture on blood agar or on serial passage through mice. Virulence decreased in all resistant strains of streptococci. The pathogenicity was partially restored in 1 organism but remained unchanged in 2 after passage in normal mice. In only 2 resistant strains was the group specificity lost. Transient alterations 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.