Antibiotic Studies on Beta Hemolytic Streptococci.V. Streptomycin Resistance Acquired by Group A, B, and C Organisms.

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Abstract
Twenty-three strains of beta hemolytic streptococci of groups A, B, and C were subcultured serially on media containing streptomycin in an effort to induce resistance. The min. increase in resistance after 40 transfers was 40-fold and the max., 3000-fold. Most organisms, however, of all 3 groups showed increases of 100-400 fold. The streptomycin sensitivity range for recently isolated strains of beta hemolytic streptococci is 10-50 micrograms/ml. for 275 strains of group A, 30-400 micrograms/ml. for 20 strains of group B, and 10-50 for 47 strains of group C organisms. Organisms which had acquired streptomycin resistance maintained it throughout sub-cultivations on blood agar medium. Partial loss of resistance was observed in 5 of 6 strains on serial passages through mice. Mouse virulence was decreased in 8 of 9 resistant organisms studied. Virulence was partially restored by passage through mice. Group specificity was maintained by all resistant organisms. Reduction in streptokinase production occurred in 2 of 23 resistant organisms and streptolysin S in all of 5 group A and C strains. Transient changes in the colonial appearance and changes in hemolysis from beta to alpha or gamma types of colonies were demonstrated by all strains when growing on max. concns. of streptomycin. These colonies reverted to their original type when subcultured onto blood agar.