Antibiotic Studies on Beta Hemolytic Streptococci: VI. Acquired In vitro and In vivo Resistance to Aureomycin.

Abstract

The aureomycin sensitivity of 274 recently isolated groups A, B, and C beta hemolytic streptococci ranged from 0.02 to 0.5 [mu]g/ml. Aureomycin resistance was induced in 23 strains of groups A, B, and C beta hemolytic streptococci by 40 serial transfers on aureomycin-containing medium. A min. increase in resistance of 2-fold in a group A organism, and a max. increase of 60-fold in a group C organism were observed. Serial transfer on control medium or passage through normal mice partially restored aureomycin sensitivity to resistant variants. Mouse virulence was decreased in only the 2 group C organisms which had shown a max. aureomycin resistance. In only 1 group B-resistant variant was group specificity lost. Growth on max. concn. of aureomycin stimulated transient conversion from clear to green hemolysis in organisms normally demonstrating only a clear beta zone. Reduction in streptokinase activity in 2 of 5 group A- and C -resistant strains and diminished ribonuclease activity in 2 of 7 group A-resistant strains were noted. No change in streptolysin S or proteinase activity was observed. The C203 MV strain of group A streptococcus failed to acquire aureomycin resistance after repeated exposures to sublethal amts. of the antibiotic, either in mice or in embryonated hens'' eggs.