COMPARISON OF THE POST‐ANTIBIOTIC EFFECT OF STREPTOCOCCUS FAECALIS AND STREPTOCOCCUS FAECIUM WITH AMPICILLIN ALONE OR COMBINED WITH STREPTOMYCIN: STUDIES ON A NOVEL TYPE OF ANTIMICROBIAL INTERACTION

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Abstract
Determination of post-antibiotic effect (PAE) and time-kill studies were made with twelve strains of Streptococcus faecalis and nine strains of Streptococcus faecium, comparing the effect of ampicillin alone with a combination of ampicillin and streptomycin at achievable serum concentrations. Bactericidal synergism (.gtoreq. one log10 decrease in viable counts) and prolongation in PAE (.gtoreq. 0.5 h) were demonstrated in all streptomycin-susceptible strains (Minimum inhibitory Concentration < 2000 mcg/ml), whereas only one of five highly streptomycin-ressistant strains exhibited a synergistic effect. A significant correlation between the magnitude of increased killing and the increase in recovery period by the combinations of ampicillin and streptomycin was demonstrated.