[In vitro activities of quinupristin/dalfopristin in combination with vancomycin and gatifloxacin against Staphylococcus aureus and Enterococcus faecium clinical isolates].

In this study, it was aimed to investigate the effects of quinupristin/dalfopristin in combination with vancomycin and gatifloxacin against Staphylococcus aureus and Enterococcus faecium isolates. A total 17 gram-positive bacterial isolates, composed of 4 methicillin-susceptible S. aureus (MSSA), 5 methicillin-resistant S. aureus (MRSA), 3 vancomycin-susceptible E. faecium (VSEF) and 5 vancomycin-resistant E. faecium (VREF) isolates, recovered from several clinical specimens in Ege University Faculty of Medicine, Turkey, were enrolled in this study. Antibiotic susceptibilities and interactions between antibiotics were determined by E-test (AB Biodisk, Sweden) method and fractional inhibitory concentration (FIC) indices were calculated for each combination. Synergistic activity was detected in only one MSSA isolate with the combination of quinupristin/dalfopristin and vancomycin (sigma FIC= 0.5). While the combination of quinupristin/dalfopristin and gatifloxacin yielded synergistic interaction in two MRSA and one MSSA isolate (sigma FIC= 0.37, 0.36 and 0.28, respectively) and additive interaction in one MSSA isolate (sigma FIC= 0.75), synergic activity was detected in one of the VREF isolate (sigma FIC= 0.29) and additive activity in two isolates (sigma FIC= 0.75 and 0.91, respectively). In this study, it was observed that the combination of quinupristin/dalfopristin and gatifloxacin was superior to the combination of quinupristin/dalfopristin and vancomycin especially in MRSA and VREF isolates. These in vitro results should be supported by in vivo studies which will guide the use of antibiotic combinations especially in the treatment of multi-resistant gram-positive bacterial infections.