Antimicrobial-modifying resistance enzymes have traditionally been class specific, having coevolved with the antibiotics they inactivate. Fluoroquinolones, antimicrobial agents used extensively in medicine and agriculture, are synthetic and have been considered safe from naturally occurring antimicrobial-modifying enzymes. We describe reduced susceptibility to ciprofloxacin in clinical bacterial isolates conferred by a variant of the gene encoding aminoglycoside acetyltransferase AAC(6')-Ib. This enzyme reduces the activity of ciprofloxacin by N-acetylation at the amino nitrogen on its piperazinyl substituent. Although approximately 30 variants of this gene have been reported since 1986, the two base-pair changes responsible for the ciprofloxacin modification phenotype are unique to this variant, first reported in 2003 and now widely disseminated. An intense increase in the medical use of ciprofloxacin seems to have been accompanied by a notable development: a single-function resistance enzyme has crossed class boundaries, and is now capable of enzymatically undermining two unrelated antimicrobial agents, one of them fully synthetic.

Among 313Enterobacteriaceae from the United States with a ciprofloxacin MIC of >0.25 g/ml and reduced susceptibility to ceftazidime, aac(6)-Ib was present in 50.5% of isolates, and of these, 28% carried the cr variant responsible for low-level ciprofloxacin resistance. aac(6)-Ib-cr was geographically widespread, stable over time, most common in Escherichia coli, equally prevalent in ciprofloxacin-susceptible and -resistant strains, and not associated with qnr genes.Quinolone resistance is traditionally mediated by chromosomal mutations in bacterial topoisomerase genes, genes regulating expression of efflux pumps, or both (4, 5). In addition, qnr genes responsible for plasmid-borne quinolone resistance have been found in clinical isolates of Enterobacteriaceae (7). These genes encode pentapeptide repeat proteins that block the action of ciprofloxacin on bacterial DNA gyrase and topoisomerase IV (11). Recently, a new mechanism of transferable quinolone resistance was reported: enzymatic inactivation of certain quinolones. The cr variant of aac(6Ј)-Ib encodes an aminoglycoside acetyltransferase that confers reduced susceptibility to ciprofloxacin by N-acetylation of its piperazinyl amine (9). Aac(6Ј)-Ib-cr has two amino acid changes, Trp102Arg and Asp179Tyr, which together are necessary and sufficient for the enzyme's ability to acetylate ciprofloxacin. When both qnrA and aac(6Ј)-Ib-cr are present in the same cell, the level of resistance is increased fourfold more than that conferred by qnrA alone, with an MIC of ciprofloxacin of 1.0 g/ml, a value near the clinical breakpoint for susceptibility. In addition, the presence of aac(6Ј)-Ib-cr alone increased substantially the frequency of selection of chromosomal mutants upon exposure to ciprofloxacin (9).The three known qnr genes, qnrA, qnrB (6), and qnrS ( Between January and March of each study year, participating clinical microbiology laboratories from each of the nine continental U.S. census regions provided 6,979 nonrepeat clinical isolates of requested enterobacterial genera without regard to antibiotic resistance phenotype. All Klebsiella pneumoniae, Enterobacter, and E. coli isolates with a ceftazidime MIC of Ն16 g/ml and a ciprofloxacin MIC of Ն0.25 g/ml were selected from this collection for study. Of 323 such isolates, 313 (97%) were available. Thirty-one (29%) of 106 K. pneumoniae isolates, 54 (34%) of 160 Enterobacter isolates, and none of 47 (0%) E. coli isolates were ciprofloxacin susceptible (MIC Յ 1.0 g/ml).aac(6Ј)-Ib was amplified by PCR with primers 5Ј-TTGCGA TGCTCTATGAGTGGCTA and 5Ј-CTCGAATGCCTGGC GTGTTT to produce a 482-bp product. Primers were chosen to amplify all known aac(6Ј)-Ib variants. PCR conditions were 94°C for 45 s, 55°C for 45 s, and 72°C for 45 s for 34 cycles. Strains positive and negative for aac(6Ј)-Ib were included as

Recently, several plasmid-mediated quinolone resistance (PMQR) genes conferring low levels of quinolone resistance have been discovered. To evaluate the temporal change in the prevalence of PMQR genes over a decade in a tertiary hospital in the Republic of Korea, we selected every fifth isolate of Escherichia coli and Klebsiella pneumoniae and every third isolate of Enterobacter cloacae between 1998 and 2001 and between 2005 and 2006 from a collection of blood isolates. Six PMQR genes [qnrA, qnrB, qnrC, qnrS, aac(6)-Ib-cr, and qepA] were screened by multiplex PCR and then confirmed by direct sequencing, and the aac(6)-Ib-positive PCR products were digested with BtsCI to identify the aac(6)-Ib-cr variant. Of 461 isolates, 37 (8%) had one of the six PMQR genes; 13 (5%) of 261 E. coli strains, 13 (10%) of 135 K. pneumoniae strains, and 11 (17%) of 65 E. cloacae strains. qnrB was the most common PMQR gene and was found as early as 1998, whereas qnrS, aac(6)-Ib-cr, and qepA emerged after 2000. None of the isolates carried qnrA or qnrC. Ciprofloxacin resistance increased over time (P < 0.001), and the overall prevalence of PMQR genes tended to increase (P ‫؍‬ 0.20). PMQR-positive isolates had significantly higher ciprofloxacin resistance and multidrug resistance rates (P ‫؍‬ 0.005 and P < 0.001, respectively). The increasing frequency of ciprofloxacin resistance in Enterobacteriaceae was associated with an increasing prevalence of PMQR genes, and this change involved an increase in the diversity of the PMQR genes and also an increase in the prevalence of the mutations in gyrA, parC, or both in PMQR-positive strains but not PMQR-negative strains.

Plasmid-mediated quinolone resistance appears to be widely distributed, though it is still uncommon in non-Typhi Salmonella isolates from the United States, including strains that are quinolone susceptible by the criteria of the Clinical and Laboratory Standards Institute (formerly the National Committee for Clinical Laboratory Standards). The presence of this gene in non-Typhi Salmonella that causes infection in humans suggests potential for spread through the food supply, which is a public health concern.