The Combined Action of Penicillin with Streptomycin or Chloromycetin on Enterococci in Vitro

Abstract

The effect of penicillin, streptomycin, and chloromycetin on 9 strains of enterococci was studied by making bacterial counts in broth or agar cultures at intervals after massive inoculation. Penicillin, 6 [mu]g./ml., caused a rapid reduction in the no. of bacteria, reaching a minimum in 48 hrs.; after this the population increased, reaching a level slightly below that of drug-free controls within 5 days, and maintaining this level for at least 10 days. Streptomycin, 25-50 [mu]g./ml., completely failed to inhibit bacteria, but when these amts. of streptomycin were added to the penicillin-containing medium, the rate of bactericidal action was greater than with penicillin alone and there was no recovery, the medium being completely sterilized within 3-5 days. Chloromycetin, 10 [mu]g./ml., had no significant effect on the bacterial population. However, when this concentration of chloromycetin was mixed with 6 [mu]g./ml. of penicillin, the rate of bactericidal action was less than with penicillin alone. The low value for no. of viable bacteria attained with penicillin alone in 24 hrs. was not reached with chloromycetin-penicillin mixture until the 6th or 12th day. Penicillin and streptomycin are synergistic, while penicillin and chloromycetin are antagonistic.