[Effect of subinhibitory concentrations of penicillin on antigens of group G streptococci]

Correlation between growth in antibiotic-medium and hemolytic activity of group C and G streptococci

The effect of a subminimal inhibitory concentration of penicillin on the production of bound and free hemolysins by streptococci was examined using sheep red blood cells. A marked decrease of a group C cell-free and bound activities was observed with penicillin at a concentration of 1/3 of the MIC whereas an increase was observed with those of a group G strain. Potassium ferricyanide and anti-streptolysin O (group A streptococcus) were strongly inhibitory for the free activities of both strains. The cell-bound activities were stimulated by addition of RNA during bacterial growth in control cultures and also in drug-containing media.

Penicillin and clindamycin alter some group A streptococcal products

The effects of subMICs of penicillin and clindamycin on group A streptococcal products were analyzed. Penicillin caused an increase in the expression of the group carbohydrate for the three strains tested; in contrast, clindamycin inhibited the expression in two strains. The M and T proteins were not affected by the drugs. Clindamycin caused an increase in the hyaluronidase activity and both antimicrobial agents inhibited the activity of streptolysin at 1/2 MIC.