The prevalence of the plasmid-mediated quinolone resistance genes qnr and aac(6')-Ib-cr was investigated among clinical isolates of Escherichia coli and Klebsiella spp. selected from 2 collections of consecutive isolates collected in 2004 to 2005 in Norway (n = 2479) and Sweden (n = 2980) and 1 group of extended-spectrum beta-lactamase (ESBL)-producing isolates collected in 2003 in Norway (n = 71). A total of 414 isolates was selected for screening based on resistance to nalidixic acid and/or reduced susceptibility/resistance to ciprofloxacin. The prevalence of both qnr and aac(6')-Ib-cr was higher among the ESBL producers (9.1% and 52.3%, respectively) than in the consecutive isolates (1.1% and 3.2%, respectively). qnrS1 was detected in 6 isolates, whereas qnrB1 and qnrB7 were detected in 2 isolates. The genetic structure surrounding qnrS1 was similar to previously described structures. In 2 isolates, qnrS1 was located on conjugative IncN-type plasmids of approximately 140 kb.
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