THE BACTERICIDAL ACTIVITY OF AMPICILLIN, DAPTOMYCIN, AND VANCOMYCIN AGAINST AMPICILLIN-RESISTANT ENTEROCOCCUS-FAECIUM

Ampicillin, daptomycin, and vancomycin, alone and in combination with gentamicin, were examined for bactericidal effects on ampicillin-resistant Enterococcus faecium using broth dilution minimum inhibitory concentrations (MICs) and time-kill studies. We tested 12 ampicillin-resistant isolates and demonstrated the following MICs and MBCs, respectively: ampicillin, greater-than-or-equal-to 32-mu-g/ml and > 256-mu-g/ml; daptomycin, less-than-or-equal-to 4-mu-g/ml and less-than-or-equal-to 16-mu-g/ml; and vancomycin, less-than-or-equal-to 4-mu-g/ml and > 64-mu-g/ml. Time-kill studies demonstrated that daptomycin alone had marked activity against the ampicillin-resistant E. faecium and that the addition of gentamicin resulted in synergistic killing. In addition, ampicillin and vancomycin were not bactericidal for the ampicillin-resistant isolates without the addition of gentamicin. The present study supports the consideration of daptomycin alone or in combination with an aminoglycoside as an alternative therapy for ampicillin-resistant enterococci, although additional clinical experience is now necessary.