Diversity of structures carrying the high-level gentamicin resistance gene (aac6-aph2) in Enterococcus faecalis...

Of 24 high-level gentamicin-resistant clinical isolates of Enterococcus faecalis, 20 carried gentamicin resistance (Gmr) plasmids. The plasmids ranged from 65.0 to 80.0 kb in size. Three of these plasmids were nonconjugative, and 17 transferred by conjugation to an E. faecalis recipient at low frequency (10(-5) to 10(-6) transconjugants per donor). The remaining four strains had a nonconjugative chromosomal Gmr determinant. On the basis of restriction enzyme and DNA-DNA hybridization profiles, Tn4001-like alpha elements were located on the chromosome and three types of Tn4001-truncated structures, I, II, and III, were found to be carried by the Gmr plasmids. Structure I lacked IS256 in the right-hand flanking extremity of Tn4001. Structure II was the same as structure I except that it also had a partial deletion of IS256 in the left-hand flanking extremity of Tn4001. Structure III lacked both the right- and left-hand flanking extremities of Tn4001. One of the wild-type strains carried the Gmr determinant both on the chromosome, as a Tn4001-like alpha element, and on a conjugative plasmid, as a Tn4001-truncated type I structure.