Outbreak of vancomycin-resistant Enterococcus faecium of the phenotype VanB in a hospital in Warsaw, Poland: probable transmission of the resistance determinants into an endemic vancomycin-susceptible strain.

The first outbreak caused by vancomycin-resistant enterococci of the VanB phenotype in Poland was analyzed. It occurred in a single ward of a Warsaw hospital which is a specialized center for the treatment of hematological disorders. Between July 1999 and February 2000, 11 patients in the ward were found to be infected and/or colonized by Enterococcus faecium that was resistant in vitro to vancomycin and susceptible to teicoplanin. PCR analysis confirmed that the vancomycin-resistant E. faecium (VREM) isolates carried the vanB gene, which is responsible for the VanB phenotype. Pulsed-field gel electrophoresis (PFGE) typing revealed that the isolates belonged to four distinct PFGE types and that one of these was clearly predominant, including isolates collected from seven different patients. The isolates contained one or more copies of the vanB gene cluster of the identical, unique DraI/PagI (BspHI) restriction fragment length polymorphism type, which resided in either the same or different plasmid molecules or chromosomal regions. All this data suggested that the outbreak was due to both clonal spread of a single strain and horizontal transfer of resistance genes among nonrelated strains, which could be mediated by plasmids and/or by vanB gene cluster-containing transposons. The comparative analysis of vancomycin-susceptible E. faecium (VSEM) isolates collected from infections in the same ward at the time of the VREM outbreak has led to identification of a widespread VSEM strain that was possibly related to the major VREM clone. It is very likely that this endemic VSEM strain has acquired vancomycin-resistance determinants and that the acquisition occurred more than once during the outbreak.