Triple-combination penicillin-vancomycin-gentamicin for experimental endocarditis caused by a moderately penicillin- and highly glycopeptide-resistant isolate of Enterococcus faecium.

An in vitro bacteriostatic synergy between beta-lactam and glycopeptide antibiotics has been recently described against isolates of Enterococcus faecium moderately resistant to penicillin and highly resistant to vancomycin. The relevance of this synergy in a rabbit endocarditis model was evaluated. Penicillin was tested at low- (LoD) and high-dose (HiD) regimens, alone or combined with vancomycin and/or gentamicin. Compared with controls, after a 5-day treatment: LoD penicillin, vancomycin, gentamicin, LoD penicillin plus gentamicin or vancomycin, and vancomycin-gentamicin were not effective; LoD penicillin-vancomycin caused a small reduction of bacterial titers in vegetations that was strongly enhanced by adding gentamicin; HiD penicillin-gentamicin, the most effective regimen, was not significantly better than LoD penicillin-vancomycin-gentamicin. These results suggest that the relative in vivo inefficacy of penicillin-vancomycin might be related to the fact that this combination was poorly bactericidal, and the triple combination of LoD penicillin-vancomycin-gentamicin or the combination of HiD penicillin-gentamicin should be considered in the treatment of serious infections due to beta-lactam- and glycopeptide-resistant enterococci.