SUMMARY: Inactive analogues and degradation products of active antibiotics failed to participate in either synergistic or antagonistic antimicrobial action in combination with active antibiotics. Streptobiosamine hydrochloride contaminated with 2–3% (w/w) streptomycin was synergistic with other antibiotics because of the active streptomycin present. The ratio of the smallest amount of antibiotic participating in synergism to that having independent antimicrobial activity varied with the drug and the micro-organism. Synergism with another antibiotic usually required from 1/20 to 1/3 of the minimal inhibitory concentration of an antibiotic when acting alone. Streptomycin, however, in 1/1000 of the minimal inhibitory concentration was synergistic with penicillin in its action on a strain of Streptococcus faecalis.
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