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Multiple copies of functional, Tet(M)-encoding Tn916-like elements in a clinical Enterococcus faecium isolate
Louis B Rice1, Lenore L Carias, Susan Rudin
1Medicine and Research Services, Louis Stokes Cleveland VA Medical Center, Case Western Reserve University, Cleveland, OH 44106, USA. louis.rice@va.gov
Abstract:
Tn916 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 Tn916 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 Tn916-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 (Tn6085a, Tn6085b) are indistinguishable, containing an inserted 2758bp Group II intron at the start of open reading frame Tn916ORF_06. The third (Tn6084) also contains the intron, but also has an ISEfa11 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 Tn916-like elements encoding Tet(M)-mediated tetracycline/minocycline resistance can coexist in clinical E. faecium isolates.
Insights
Clinical Enterococcus faecium isolates can harbor multiple copies of Tn916-like elements. These elements confer tetracycline and minocycline resistance, challenging assumptions about single-copy Tn916 in clinical settings.
Area of Science:
- Microbiology
- Genetics
- Antimicrobial Resistance
Background:
- Tn916 and related elements are prevalent in clinical enterococci, facilitating the spread of resistance genes.
- The number of Tn916 copies in clinical isolates is often assumed to be one, but this has not been systematically investigated.
Purpose of the Study:
- To investigate the copy number and characteristics of Tn916-like elements in a clinical Enterococcus faecium isolate.
- To determine the functional capabilities (excision and transfer) of these elements.
Main Methods:
- Genomic analysis of a clinical Enterococcus faecium isolate.
- Cloning of Tn916-like elements into plasmid vectors.
- Transformation and conjugation experiments in E. coli and E. faecium.
Main Results:
- Identification of three distinct, excision-proficient Tn916-like elements in the E. faecium genome.
- All elements contain tet(M) genes conferring tetracycline and minocycline resistance.
- Two elements (Tn6085a, Tn6085b) are identical and contain a Group II intron; the third (Tn6084) also contains the intron and an ISEfa11 insertion.
- Demonstration of excision from plasmids and transfer to recipient strains.
Conclusions:
- Clinical E. faecium isolates can harbor multiple, nearly identical Tn916-like elements.
- These elements encode Tet(M)-mediated resistance and retain mobility, contributing to antimicrobial resistance spread.
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