Identification of chromosomal mobile element conferring high-level vancomycin resistance in Enterococcus faecium.

A clinical isolate of Enterococcus faecium that contains a chromosomally encoded vanA gene cluster, Tn1546::IS1251, transferred vancomycin resistance to the plasmid-free strain Enterococcus faecalis JH2-2 during filter matings. Hybridization of a vanHAXY probe to SmaI restriction-digested genomic DNA separated by pulsed-field gel electrophoresis showed that the vanA gene cluster was located on a 40-kb fragment in the original donor strain and on fragments of different sizes (150 to 450 kb) in the transconjugants. No hybridization to vanA gene cluster probes was obtained with plasmid DNA preparations from the donor or transconjugants. These results suggested that in each case, the van genes had integrated into the recipient chromosome. The transconjugants in turn could act as donors of vancomycin resistance, and resistance was transferable to a Rec- recipient. The results of restriction analyses and DNA hybridizations of genomic DNA from the donor and transconjugants were consistent with the transfer of a mobile element that includes the 12.3-kb Tn1546::IS1251 gene cluster and at least 13 kb of additional DNA. This element has been tentatively designated Tn5482. DNA sequence analysis of a fragment predicted to contain the left end of Tn5482 revealed two insertion sequence-like elements: IS1216V and an apparently truncated IS3-like element. Restriction mapping and DNA hybridization patterns of the van gene clusters of three additional clinical isolates from New York City showed an element similar to Tn5482. Transfer of Tn5482 and related elements may be involved in dissemination of vancomycin resistance.