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 p henotype in Poland was analyzed. It occurred in a single ward of a Warsaw h ospital, which is a specialized center for the treatment of hematological d isorders. Between July 1999 and February 2000, 11 patients in the ward were found to be infected and/or colonized by Enterococcus faecium that was res istant 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 ge l electrophoresis (PFGE) typing revealed that the isolates belonged to four distinct PFGE types and that one of these was clearly predominant, includi ng 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 i n either the same or different plasmid molecules or chromosomal regions. Al l 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-con taining transposons. The comparative analysis of vancomycin-susceptible E. faecium (VSEM) isolates collected from infections in the same ward at the l ime of the VREM outbreak has led to identification of a widespread VSEM str ain that was possibly related to the major VREM clone. It is very likely th at this endemic VSEM strain has acquired vancumycin-resistance determinants and that the acquisition occurred more than once during the outbreak.