Molecular Analysis of a Gentamicin Resistance Transposonlike Element on Plasmids Isolated from North American Staphylococcus Aureus Strains

Plasmid-encoded resistance to the aminoglycosides gentamicin (Gm), tobramycin (Tm), and kanamycin (Km) (GmTmKmr) in strains of Staphylococcus aureus isolated in Australia and North America appears to be mediated by one resistance determinant. In Australian isolates, this determinant is flanked by inverted copies of a 1.3-kb insertion sequence, IS256, thereby forming a composite transposon, Tn4001. Analysis of two conjugative plasmids and a related nonconjugative plasmid from strains of S. aureus isolated in North America showed that the GmTmKmr determinant on these plasmids is also flanked by inverted repeats. In the nonconjugative plasmid, these repeats include only 425 bp of IS256 immediately adjacent to the GmTmKmr region and identical to that on Tn4001. This truncated Tn4001 element is flanked by copies of the insertion element IS257, and together these elements form a truncated Tn4001-IS257 hybrid transposonlike structure. A third copy of IS257 was located 418 bp from the hybrid structure. The truncated Tn4001 and three repeats of IS257 were present at a conserved site on the plasmids studied. Four additional copies of IS257 were identified on the two conjugative plasmids. These elements flank determinants for resistance to the aminoglycosides neomycin and paromomycin and to ethidium bromide and quaternary ammonium compounds, as well as the region involved in conjugative plasmid transfer.