Vancomycin-variable Enterococcus faecium: in vivo emergence of vancomycin resistance in a vancomycin-susceptible isolate.

We report the emergence of vancomycin resistance in a patient colonized with a vanA-containing, vanRS-negative isolate of Enterococcus faecium which was initially vancomycin susceptible. This is a previously undescribed mechanism of drug resistance with diagnostic and therapeutic implications.