Increased high-level gentamicin resistance in invasive Enterococcus faecium is associated with aac(6')Ie-aph(2″)Ia-encoding transferable megaplasmids hosted by major hospital-adapted lineages.

Gentamicin is important in synergistic bactericidal therapy with cell wall agents for severe enterococcal infections. During 2003-2008, a 10-fold increase in the prevalence of high-level gentamicin resistance (HLGR), to above 50%, in blood culture isolates of Enterococcus faecium, was reported by the Norwegian Surveillance System for Antimicrobial Resistance. A representative national collection of invasive E. faecium isolates (n = 99) from 2008 was examined by a multilevel approach. Genotyping revealed a polyclonal population dominated by major hospital-associated lineages (mainly ST203, ST17, ST18, ST202 and ST192). The presence of aac(6')-Ie-aph(2″)-Ia, encoding the bi-functional aminoglycoside-modifying enzyme, was found in 98% of HLGR isolates (56/57). Furthermore, a significantly higher prevalence of potential virulence genes, toxin-antitoxin loci as well as pRE25 and pRUM type replicons was demonstrated in isolates belonging to major hospital-associated lineages compared to other sequence types. Megaplasmids of pLG1 replicon type (200-330 kb) were present in 90% of the isolates. Co-hybridization analyses revealed genetic linkage of aac(6')-Ie-aph(2″)-Ia to this replicon type. Transfer of HLGR-encoding plasmids was restricted to E. faecium. In conclusion, the increased prevalence of HLGR in invasive E. faecium in Norway is associated with hospital-adapted genetic lineages carrying aac(6')-Ie-aph(2″)-Ia-encoding transferable megaplasmids of the pLG1 replicon type.