Antibiotic Studies on Beta Hemolytic Streptococci: VII. Acquired in vitro Resistance to Bacitracin

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Abstract
The bacitracin sensitivity range of 276 strains of groups A, B, and C hemolytic streptococci were 0.003-1 U./ml. 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 3750-fold in a group B organism. The median-fold increase in resistance was comparable for all 3 groups. Serial transfers on control media, passages through normal mice and storage at 4[degree]C partially or completely restored the bacitracin sensitivity to all resistant variants. Mouse virulence was decreased in 7 of 9 strains. It was restored in 6 of the 7 strains by subsequent passages in normal mice. Group specificity was lost in 3 of 7 group B strains after induced resistance. Growth on sub-inhibitory concns. of bacitracin stimulated transient conversion from beta to alpha hemolysis in most strains studied. The streptolysin "S" titer was decreased in 2 of 3 resistant variants. Ribo- and desoxyribonuclease activity and the proteinase-streptokinase ratios remained unchanged after acquired resistance.