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 </=2 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 > or = 20 mm, and all of the enterococcal isolates had daptomycin zone diameters > or = 17 mm.

CONCLUSIONS

Overall, daptomycin showed potent activity against S. aureus and enterococcal isolates, comparable to quinupristin-dalfopristin and linezolid.