Characterization of Vancomycin-Resistant Enterococcus faecium Isolates from the United States and Their Susceptibility In Vitro to Dalfopristin-Quinupristin

In the course of clinical studies with the investigational streptogramin antimicrobial dalfopristin-quinupristin, isolates of vancomycin-resistant Enterococcus faecium were referred to our laboratory from across the United States. Seventy-two percent of the strains were of the VanA type, phenotypically and genotypically, while 28% were of the VanB type. High-level resistance to streptomycin or gentamicin was observed in 86 and 81%, respectively, of the VanA strains but in only 69 and 66%, respectively, of the VanB strains. These enterococci were resistant to ampicillin (MIC for 50% of the isolates tested [MIC(50)] and MIC(90), 128 and 256 μg/ml, respectively) and to the other approved agents tested, with the exception of chloramphenicol (MIC(90), 8 μg/ml) and novobiocin (MIC(90), 1 μg/ml). Considering all of the isolates submitted, dalfopristin-quinupristin inhibited 86.4% of them at concentrations of ≤1 μg/ml and 95.1% of them at ≤2 μg/ml. However, for the data set comprised of only the first isolate submitted for each patient, 94.3% of the strains were inhibited at concentrations of ≤1 μg/ml and 98.9% were inhibited at concentrations of ≤2 μg/ml. Multiple drug resistance was very common among these isolates of vancomycin-resistant E. faecium, while dalfopristin-quinupristin inhibited the majority at concentrations that are likely to be clinically relevant.