High-level gentamicin-resistant Enterococcus faecium strains isolated from bone marrow transplant patients: accumulation of antibiotic resistance genes, large plasmids and clonal strain dissemination

Twenty-six high-level gentamicin-resistant (HLGR) Enterococcus faecium strains colonising neutropenic bone marrow transplant patients were studied. Polymerase chain reaction analysis showed that high-level gentamicin resistance was mediated by the aac(6′)-Ia-aph(2″)-Ie gene; the aph(2″)-Id gene responsible for gentamicin resistance was also detected in 16 strains. Multiple antibiotic resistance was related to the presence of aph(3′)-IIIa, ant(6)-Ia, erm(B), erm (A) and tet(M) genes. Strains clustered into 18 groups according to their plasmid content as well as 16 pulsed-field gel electrophoresis (PFGE) patterns. Although the majority of PFGE patterns were single isolates, three microclones were identified. Hybridisation showed that in the majority of the strains the aac(6′)-aph(2″) gene resided on a large plasmid of ca. 96 kb detected only on PFGE gels. Based on these findings, colonisation by HLGR E. faecium strains was a result of either possibly related plasmid spread or strain dissemination.