Multiple copies of functional, Tet(M)-encoding Tn 916-like elements in a clinical Enterococcus faecium isolate

Abstract

Tn 916 and similar elements are very common in clinical enterococcal isolates, and are responsible for transmission of a variety of resistance determinants. It is commonly assumed that clinical strains carrying Tn 916 have a single copy, although the actual number of copies in clinical isolates has never been systematically studied. We report a clinical isolate of Enterococcus faecium in which three distinct and excision-proficient copies of Tn 916-like elements are present in the genome. All of the elements contain tet(M) genes, at least one of which confers resistance to tetracycline and minocycline. Two elements (Tn 6085a, Tn 6085b) are indistinguishable, containing an inserted 2758 bp Group II intron at the start of open reading frame Tn 916ORF_06. The third (Tn 6084) also contains the intron, but also has an IS Efa11 integrated upstream of tet(M). All three copies are able to excise from plasmid vectors when cloned in E. coli, and at least two of the elements can transfer to an E. faecium recipient strain. These data indicate that nearly identical Tn 916-like elements encoding Tet(M)-mediated tetracycline/minocycline resistance can coexist in clinical E. faecium isolates.