In vitro activity of daptomycin-metronidazole combinations against mixed bacterial cultures: Reduced activity of metronidazole againstBacteroides species in the presence ofEnterococcus faecalis

The in vitro activity of daptomycin-metronidazole combinations against mixed cultures of gram-positive facultative cocci and strains of theBacteroides fragilis group was investigated. Metronidazole did not influence the high activity of daptomycin against strains ofStaphylococcus aureus, Staphylococcus epidermidis andEnterococcus faecalis in the absence or presence of the co-culturedBacteroides strains. In contrast, theEnterococcus faecalis isolates protected the co-culturedBacteroides strains against the killing effect of metronidazole, even at a concentration four- or eightfold the MIC of metronidazole. Killing curve experiments confirmed this protective effect of different isolates ofEnterococcus faecalis.