In Vitro Activity of Daptomycin Against Methicillin-Resistant Staphylococcus aureus Strains Isolated from Blood Cultures

The aim of this study was to detect the in vitro activity of daptomycin against methicillin-resistant Staphylococcus aureus (MRSA) strains isolated from blood cultures at Ataturk Training and Research Hospital, Izmir, Turkey between 2006-2010. A total of 64 MRSA clinical isolates were included in the study, and daptomycin susceptibility were investigated by E-test (AB bioMerieux, Sweden). The identification of the MRSA isolates was based on conventional microbiological methods and an additional automated identification system (Phoenix 100, BD Diagnostic Systems, USA). E-test strips were applied to the surface of Mueller-Hinton agar plates and incubated at 35 C in ambient air for 18 to 24 hours. Strains with a MIC value of <= 1 mu g/ml were accepted as susceptible to daptomycin. In our study all of the 64 MRSA isolates were found susceptible to daptomycin (MIC <= 1 mu g/ml). The MIC50, MIC90 and MIC ranges were detected as 0.125, 0.5 and 0.125-0.5 mu g/ml, respectively. Only a single isolate yielded MIC value of 1 mu g/ml. As a result daptomycin was found to be very active against MRSA strains in vitro. Our findings suggested that daptomycin might be a suitable alternative agent for treating bacteremia caused by MRSA. However, further large-scaled studies and clinical trials are necessary to support these in vitro data.