Location of antibiotic resistance markers in clinical isolates of Enterococcus faecalis with similar antibiotypes.

Eight wild-type strains of Enterococcus faecalis, resistant to chloramphenicol (Cmr), erythromycin (Emr), tetracycline (Tcr), and minocycline (Mnr), were examined for the genetic basis of their antibiotic resistance, Five of the strains transferred all of their antibiotic resistance markers by conjugation, while the other three strains transferred only Tcr and Mnr. Cmr and Emr determinants were localized by DNA-DNA hybridization experiments, in which the Cmr gene of plasmid pIP501, of group B Streptococcus origin, and the Emr gene of transposon Tn917, of E. faecalis origin, served as probes. A chromosomal location was found for the nonconjugative Cmr and Emr markers of one wild-type strain. In two strains these markers were carried by nonconjugative plasmids, and in the other strains they were carried by plasmids that transferred by conjugation. Plasmids isolated from three transconjugants resistant to tetracycline but susceptible to minocycline bore nucleotide sequences homologous to the tetL gene. Nucleotide sequences homologous to conjugative transposon Tn916, of E. faecalis origin, were detected by hybridization in the tetracycline-minocycline-resistant transconjugants. Three of these transconjugants were plasmid free, while four harbored conjugative cryptic plasmids. Sequences homologous to Tn916 were also found on two conjugative plasmids, one of which appeared to be a conjugative cryptic plasmid that had acquired chromosomal Tcr Mnr markers during transfer.