Clustering of polyclonal VanB-type vancomycin-resistant Enterococcus faecium in a low-endemic area was associated with CC17-genogroup strains harbouring transferable vanB2-Tn5382 and pRUM-like repA containing plasmids with axe-txe plasmid addiction system

VanB-type vancomycin-resistant Enterococcus faecium isolates (n = 17) from 15 patients at the Orebro University hospital in Sweden during a span of 18 months was characterized. All patients had underlying disorders and received broad-spectrum antimicrobial therapy. Pulsed-field gel electrophoresis (PFGE) grouped 14 isolates in three PFGE types and three isolates in unique PFGE patterns. All isolates had multi-locus sequence types [ST17 (n = 5); ST18 (n = 3); ST125 (n = 7); ST262 (n = 1); ST460 (n = 1)] belonging to the successful hospital-adapted clonal complex 17 (CC17), harboured CC17-associated virulence genes, were vanB2-positive and expressed diverse vancomycin minimum inhibitory concentration (MICs; 8 to > 256 mg/L). Isolate 1 had a unique PFGE type and a chromosomal transferable vanB2-Tn5382 element. Interestingly, the other five PFGE types had Tn5382 located on plasmids containing pRUM-like repA and a plasmid addiction system (axe-txe) shown by co-hybridization analysis of PFGE-separated S1-nuclease digested total DNA. The resistance plasmids were mainly of 120-kb and supported intraspecies vanB transfer. Two strains were isolated from patient 6 and we observed a possible transfer of the vanB2-resistance genes from PFGE type III ST460 to a more successful PFGE type I ST125. This latter PFGE type I ST125 became the predominant type afterwards. Our observations support the notion that vanB-type vancomycin-resistant Enterococcus faecium can persist in a low-endemic area through successful clones and plasmids with stability functions in hospital patients with known risk factors.