Plasmid-mediated quinolone resistance in clinical isolates of Escherichia coli

Objective To investigate the importance of recently discovered plasmid-mediated quinolone resistance in the develop- ment of quinolone resistance in clinical isolates of Escherichia coli.Methods Seventy-eight unique ciprofloxacin-resistant clinical isolates of E.coli from 5 Shanghai hospitals were screened for the qnr gene by colony blotting and Southern hybridization of plasmid DNA.Conjugation experiments were done with azide-resistant E.coli J53 as a recipient,with selection for plasmid-en- coded antimicrobial resistance(chloramphenicol,gentamicin,or tetracycline),and azide counterselection,qnr genes were se- quenced,and the structure of plasmid DNA adjacent to qnr was analyzed by primer walking.Results qnr was detected in six (7.7%) of 78 strains.Quinolone resistance was transferred from all the six probe-positive strains.Transconjugants had 16-to 250-fold increases in MICs of eiprofloxacin relative to that of the recipient.All six strains contained qnr with a nucleotide se- quence identical to that originally reported,qnr was located in complex In4 family class 1 integrons.Conclusions Transferable plasmid mediated quinolone resistance associated with qnr existed in quinolone-resistant E.coli strains in Shanghai hospitals and may contribute to the rapid increase of bacterial resistance to quinolones in China.