Antimicrobial activity of daptomycin tested against Staphylococcus aureus with vancomycin MIC of 2 microg/mL isolated in the United States and European hospitals (2006-2008)

Abstract

We evaluated the activity of daptomycin (minimum inhibitory [MIC] and bactericidal [MBC] concentration) against Staphylococcus aureus strains with elevated (2 microg/mL) vancomycin MIC values. A total of 410 contemporary clinical S. aureus isolates (282 from the United States and 128 from Europe) with vancomycin MIC values of 2 microg/mL were tested by reference broth microdilution method. Vancomycin MBC and the presence of vancomycin-heteroresistant population (heterogeneous vancomycin-intermediate S. aureus [hVISA]) were evaluated in 31 randomly selected strains. Overall, 97.3% of isolates were susceptible to daptomycin (MIC(90), 0.5 microg/mL). Daptomycin exhibited potent bactericidal activity with MBC values at the MIC concentration (74.2%) or 1 log(2) dilution above the MIC (25.8%). In contrast, vancomycin MBC was > or = 32 microg/mL in 12.9% of strains tolerance, and 25.8% of strains tested positive for hVISA (AB BIODISK GRD Etest, Solna, Sweden). In conclusion, S. aureus strains with vancomycin MIC of 2 microg/mL showed high rates of hVISA and vancomycin tolerance. Daptomycin retained potent bactericidal activity against S. aureus with decreased susceptibility to vancomycin.