LAUSR

Background Emergence of reduced susceptibility to vancomycin warrants the development of new antimicrobial agents for the treatment of MRSA. We evaluated the activity of dalbavancin, a novel lipoglycopeptide antibiotic, both alone and combined with β-lactams, in combination MIC testing and time-kill assays against resistant phenotypes of Staphylococcus aureus. Methods S. aureus isolates included 50 organisms with varying susceptibility patterns. Dalbavancin was tested alone and in combination with five β-lactams: cefazolin, cefepime, ceftaroline, ertapenem and oxacillin. MIC values of the antibiotics were determined for all isolates. After initial MIC testing, dalbavancin MICs were determined in the presence of 0.5 × MIC of each β-lactam to determine the effect of each β-lactam on dalbavancin MIC. Time-kill assays were performed with dalbavancin and β-lactams tested at 0.5 × MIC for randomly selected organisms representing each MRSA phenotype. Time-kill curves were generated by plotting mean colony counts (log10 cfu/mL) versus time. Results Dalbavancin MIC50 was 0.0313 mg/L and MIC90 was 0.0625 mg/L. Dalbavancin MICs decreased by zero to greater than five 2-fold dilutions in combination with each β-lactam. In time-kill assays, dalbavancin was synergistic with cefazolin, cefepime and ertapenem against all strains and the combination of dalbavancin and ceftaroline was synergistic against all but one. The combination of dalbavancin and oxacillin was synergistic against 5/8 strains. Conclusions Dalbavancin was active against all MRSA strains tested, including heteroresistant vancomycin-intermediate S. aureus, vancomycin-intermediate S. aureus, daptomycin-non-susceptible and linezolid-resistant isolates. The synergy demonstrated against these organisms supports the use of dalbavancin in combination with β-lactams against resistant phenotypes of S. aureus. Further evaluation is warranted.