In Vitro Synergistic Effect of Vancomycin Combined with Daptomycin Against Vancomycin-Resistant Enterococci

New antimicrobial agents are being designed to treat infections of multidrug-resistant pathogens and have been introduced in the past few years, but there has been a worldwide increase in the incidence of vancomycin-resistant enterococci (VRE) infections, and treatment options are still limited. In this study, it was aimed to investigate the in vitro activity of vancomycin combined with daptomycin against 30 VRE strains. The minimum inhibitory concentration (MIC) values of antibiotics were determined using broth microdilution assay as described by the Clinical and Laboratory Standards Institute (CLSI). The activities of antibiotics in combination were assessed using a broth microcheckerboard method. In addition, 4 of the 30 strains were randomly selected to determine the time-kill curves at 1 × MIC concentrations and 2 of 4 strains at ½ × MIC concentrations. Viable counts were determined at 0-, 4-, 8-, 12-, 24-, 48-, and 72-hr intervals. This was accomplished by subculturing 0.1 mL from repetitive serial dilutions in Eppendorf tubes, followed by subculturing 0.1 mL from tubes onto Tryptic Soy Agar plates. All plates were incubated at 35°C for 24 hr. The synergistic effect was found 100% using the broth microcheckerboard method, and in strains tested using the time-kill method at both 1 × MIC and ½ × MIC concentrations. The results of this study suggest that this combination could be effective in the treatment of the VRE-associated infection.