Possible origin of a vanB-type Enterococcus faecium causing a multicentre outbreak in Sweden

Abstract

Dissemination of vancomycin-resistant Enterococcus faecium (VREfm) with similar pulsed-field gel electrophoresis patterns in three Swedish hospitals between 2007 and 2011 prompted molecular characterization to reveal the possible origins and features of the strain. A representative subset of collected isolates (VREfm (n=18) and vancomycin-susceptible Enterococcus faecium (VSEfm) (n=2)) reflecting the spread in time and location was subjected to Multi Locus Sequence Typing, antibiotic resistance testing, virulence gene screening, characterization of mobile genetic units carrying the resistance gene and their ability to transfer. In addition, 3 outbreak strains and 1 isolate collected prior to the outbreak was whole-genome sequenced. The isolates were predominantly ST192, considered to belong within a high-risk lineage, and concordantly harbored at least eight virulence genes associated with high-risk genotypes, as well as were geno- and phenotypically resistant to ampicillin, gentamicin, ciprofloxacin and vancomycin, with susceptibility to teicoplanin. The vancomycin resistance was of vanB2-type, and this gene cluster was part of the conjugative transposon Tn1549/Tn5382. PFGE analysis with S1 nuclease restriction as well as filter mating experiments indicated that vanB2-Tn1549/Tn5382 was placed in a 70 kb sized pRUM replicon, which readily transferred between E. faecium. The plasmid also contained an axe-txe toxin-antitoxin stability module capable of securing persistence within the bacterial host. The two VSEfm were similar by PFGE and MLST and harbored a 30 kb smaller pRUM plasmid lacking the vanB2-Tn1549/Tn5382. In conclusion, the obtained results indicate introduction of vanB2-Tn1549/Tn5382 into a pRUM plasmid harbored in a pre-existing high-risk clone. Afterwards, the resulting VRE containing the pRUM-vanB2-Tn1549/Tn5382-axe-txe plasmid successfully disseminated in the three hospitals.