Evaluation of the combination effects of ampicillin or vancomycin combined with streptomycin, gentamicin, tobramycin or netilmicin against enterococci.

Seven strains of Streptococcus faecalis, of which two possessed high-level resistance to streptomycin (MIC greater than or equal to 2000 mcg/ml), and two strains of Streptococcus faecium were evaluated with respect to killing-effect and duration of post-antibiotic effect (PAE) of ampicillin and vancomycin in combination with streptomycin, gentamicin, tobramycin and netilmicin. No synergistic combination effects were seen with the two strains highly-resistant to streptomycin or with the two Streptococcus faecium strains to netilmicin and tobramycin. When these strains were excluded, no significant difference in average killing could be detected between the four aminoglycosides. The mean prolongation in recovery period of susceptible strains was significantly longer with combinations of ampicillin and netilmicin or streptomycin as compared with either gentamicin or tobramycin. A similar relationship was seen for combinations of vancomycin with the four aminoglycosides.