Plasmid-mediated Resistance to Gentamicin in Staphylococcus Aureus: the Involvement of a Transposon

Resistance to gentamicin, tobramycin and kanamycin (GmrTmrKmr) in strains of Staphylococcus aureus isolated from clinical sources in Australia is mediated by a 4.7 kb transposable element, designated Tn4001. A 2.5 kb HindIII fragment which maps symmetrically within Tn4001, and encompasses the aminoglycoside-resistance coding region, has been shown to hybridise with fragments of identical size in HindIII digests of three different GmrTmrKmr plasmids, two of which were self-transmissible, from strains of S. aureus isolated in the USA. Examination by electronmicroscopy of self-annealed molecules of the North American GmrTmrKmr plasmids revealed the presence of stem and loop structures similar to those produced by Tn4001, but with shorter inverted repeats. These results suggest that GmrTmrKmr in strains of S. aureus isolated in the USA is, or once was, transposable, and that transposable elements analogous to Tn4001 may be found in isolates of GmrTmrKmr S. aureus worldwide.