Interference of Aureomycin and of Terramycin with Action of Penicillin in Vitro.

Abstract

Penicillin (0.006 [mu]g./ml.) was bactericidal to Streptococcus pyogenes as detd. by counts of viable bacteria; aureomycin and terramycin (10 [mu]g./ml.) were bacteriostatic. Bactericidal rates were slower with mixtures of penicillin and either of the other drugs during 7-12 hrs. at 37[degree]C than with penicillin alone, interference increasing directly with the concn. of aureomycin or terramycin. Each drug (10 [mu]g./ml.) was bactericidal to Klebsiella pneumoniae. Bacteriostatic concn. (0.1 [mu]g./ml.) of aureomycin or terramycin mixed with bactericidal concn. of penicillin reduced the death rate below that with penicillin alone, interference increasing inversely with the concn. of aureomycin or terramycin. With either organism, interference with the early bactericidal action of penicillin was most marked with bacteriostatic but not actively bactericidal concn. of the interfering drug. Ultimately, mixtures of drugs were more likely to destroy all bacteria than were single antibiotics.