Disk diffusion-based method aids in the detection of vanM-positive Enterococcus faecium with low vancomycin minimum inhibitory concentrations.

We first reported the vanM vancomycin resistance gene in enterococci in Shanghai, China in 2006 and later found it to be the predominant van gene in vancomycin-resistant enterococci (VRE). In this study, we successively collected 1292 Enterococcus faecium and Enterococcus faecalis strains from in- and outpatients at Huashan Hospital, Fudan University and found that nearly all of the isolates (1290/1292) were vancomycin-sensitive determined by the VITEK 2 system. However, using a modified macromethod-based disk diffusion test, 10 E. faecium isolates that were previously determined to be vancomycin-sensitive by the VITEK 2 system were found to have colonies in the vancomycin disk inhibition zone. Pulse-field gel electrophoresis results showed that each randomly selected colony in the inhibition zone belonged to the same clone as the original strain. All 10 isolates were later found to be vanM-positive. The disk diffusion-based method may aid in the detection of vanM-positive E. faecium with low vancomycin minimum inhibitory concentrations and prevent missing the detection of vancomycin sensitivity-variable enterococci.