Abstract
Summary
1. The aureomycin semitivity range of 274 recently isolated group A, B, and C beta hemolytic streptococci was 0.02-0.5 μg/ml.
2. Aureomycin resistance was induced in 23 strains of group A, B, and C beta hemolyticstreptococci by 40 serial subcultivations on aureomycin medlium. The minimum increase in resistance was 2-fold in a group A organism, and maximum increase was 60-fold in a group C organism.
3. Serial transfer on control medium or passage through normal mice partially restored aurmmycin sensitivity to resistant variants.
4. Mouse virulence was decreased in only the 2 group C organisms which had shown the highest degree of aureomycin resistance.
5. In only 1 group B resistant variant was the group specificity lost after the develop ment of aureomycin resistance.
6. Growth on maximum concentrations of aureomycin stimulated transient conversion from clear to green hemdysis in organisms normally producing only a dear beta zone. Conversely, group C organisms with normal green hemolytic characteristics lost their green hemolytic zones when gmwn on maximum aureumycin concentrations.
8. Reduction in streptokinase production occurred in 2 of 5 group A and C resistant strains itnd diminished ribnuclease activity in 2 of 7 group A resistant variants. No change in stmptolysin S or pmteinase activity was observed.
9. The C203 MV strain of group A streptococcus failed to acquire aureomycin resistance after repeated exposures to sublethal amounts of the antibiotic either in mice or embryonated hens' eggs.
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