The in vitro activity of daptomycin against Staphylococcus aureus and Enterococcus species

Objective: The purpose of this study was to examine the in vitro activity of daptomycin using an optimal calcium (Ca2+) concentration (50 mg/L) against a diverse collection of enterococcal and Staphylococcus aureus clinical isolates, including glycopeptide-resistant enterococci (GRE) and methicillin-resistant S. aureus (MRSA). Methods: The activity of daptomycin was compared with the activities of seven other agents against 1483 enterococcal and S. aureus clinical isolates, including 303 GRE and 193 methicillin-resistant S. aureus (MRSA) strains. Susceptibility testing was performed by the NCCLS broth microdilution method, with one exception: Mueller-Hinton (MH) broth was supplemented to a physiological level of 50 mg/L Ca2+ when testing daptomycin. Daptomycin zone diameters were determined by disc diffusion with MH agar plates containing Ca2+ 50 mg/L. Results: All staphylococcal isolates tested, and the majority of enterococcal isolates (96.5%), would be considered susceptible to daptomycin if the breakpoint previously proposed of less than or equal to2 mg/L was applied. The activity of daptomycin against MRSA and methicillin-susceptible S. aureus was essentially equal. Daptomycin also had similar activity against GRE and glycopeptide-susceptible enterococci. Every S. aureus isolate had a daptomycin zone diameter greater than or equal to20 mm, and all of the enterococcal isolates had daptomycin zone diameters greater than or equal to17 mm. Conclusions: Overall, daptomycin showed potent activity against S. aureus and enterococcal isolates, comparable to quinupristin-dalfopristin and linezolid.