In-vitro synergy testing of triple antibiotic combinations against Staphylococcus epidermidis isolates from patients with endocarditis.

In-vitro synergy testing was performed against ten blood or valve isolates of Staphylococcus epidermidis taken from patients with endocarditis. A three-dimensional microtitre checkerboard method was used for evaluation of vancomycin-rifampicin-gentamicin. The triple combination of vancomycin plus rifampicin plus gentamicin was found to be synergistic in 70% of the isolates. Vancomycin plus rifampicin was not synergistic. Oral agents including dicloxacillin, rifampicin, and fusidic acid were also evaluated. Four methicillin-resistant isolates were relatively resistant to dicloxacillin (MIC greater than 0.79 mg/l) but all four isolates were susceptible to fusidic acid alone and rifampicin alone. The triple combination of dicloxacillin plus fusidic acid plus rifampicin was found to be synergistic in 50% of the isolates and generally superior to any two-drug combination raising the possibility of an effective oral combination of antibiotics.