Fusidic acid alone or in combination with vancomycin for therapy of experimental endocarditis due to methicillin-resistant Staphylococcus aureus

The usefulness of fusidic acid, alone or combined with vancomycin, was investigated for the therapy of experimental endocarditis caused in rabbits by a methicillin-resistant strain of Staphylococcus aureus. In vitro killing curves showed an indifferent interaction between the two antibiotics. In vivo, vancomycin alone was as effective as a vancomycin-fusidic acid combination (P < 0.05 versus control animals). No resistance to fusidic acid emerged during combination therapy. Fusidic acid alone was not effective. Resistance emerged in 5 of 12 animals treated with fusidic acid alone and was responsible for antibacterial failure. Fusidic acid alone was effective (P < 0.001) and did not select resistant strains if therapy was started when animals retained a smaller inoculum. We concluded that the vancomycin-fusidic acid combination exhibited no advantage over vancomycin alone in this model.