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.

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 (greater than or equal to one log10 decrease in viable counts) and prolongation in PAE (greater than or equal to 0.5 h) were demonstrated in all streptomycin-susceptible strains (Minimum inhibitory Concentration less than 2000 mcg/ml), whereas only one of five highly streptomycin-resistant 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.