Abstract
Summary
1. The bacitracin sensitivity range of 276 strains of groups A, B and C hemolytic streptococci was 0.003 to 1 unit/ml. 2. Bacitracin resistance was induced in 23 strains of groups A, B, and C streptococci by 40 serial transfers on bacitracin-agar. The increase in resistance ranged from 8-fold in a group A to 3,750-fold in a group B organism. The median fold increase in resistance was comparable for all 3 groups. 3. Serial transfers on control media, passages through normal mice and storage at 4°C partially or completely restored the bacitracin sensitivity to all resistant variants. 4. Mouse virulence was decreased in 7 of 9 strains. It was restored in 6 of the 7 strains by subsequent passages in normal mice. 5. Group specificity was lost in 3 of 7 group B strains after induced resistance. 6. Growth on sub-inhibitory concentrations of bacitracin stimulated transient conversion from beta to alpha hemolysis in most strains studied. 7. The streptolysin “S” titre was decreased in 2 of 3 resistant variants. Ribo- and desoxyribohuclease activity and the proteinase-streptokinase ratios remained unchanged after acquired resistance.
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