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Category: Clinical Microbiology
Plasmid-Mediated Quinolone Resistance, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555818982/9781555818975_Chap25-1.gif /docserver/preview/fulltext/10.1128/9781555818982/9781555818975_Chap25-2.gifAbstract:
Plasmid-mediated quinolone resistance (PMQR) was late in being discovered. Nalidixic acid, the first quinolone to be used clinically, was introduced in 1967 for urinary tract infections. Resistance was soon observed and could also be readily selected in the laboratory. It was produced by amino acid substitutions in the cellular targets of quinolone action: DNA gyrase and topoisomerase IV ( 1 – 3 ). Later, decreased quinolone accumulation due to pump activation and porin loss was added as an additional resistance mechanism. The search for transferable nalidixic acid resistance in over 500 Gram-negative strains in the 1970s was unrevealing ( 4 ). In the 1980s fluoroquinolones became available that were more potent and broader in spectrum. Quinolone usage increased, with subsequent parallel increases in quinolone resistance ( 5 , 6 ). In 1987 PMQR was reported to be present in a nalidixic acid-resistant isolate of Shigella dysenteriae from Bangladesh ( 7 ), but this claim was later withdrawn ( 8 ). True PMQR was reported in 1998 in a multiresistant urinary Klebsiella pneumoniae isolate at the University of Alabama that could transfer low-level resistance to nalidixic acid, ciprofloxacin, and other quinolones to a variety of Gram-negative recipients ( 9 ). The responsible gene was termed qnr, later amended to qnrA as additional qnr alleles were discovered. Investigation of a qnrA plasmid from Shanghai that provided more than the expected level of ciprofloxacin resistance led to the discovery in 2006 of a second mechanism for PMQR: modification of certain quinolones by a particular aminoglycoside acetyltransferase, AAC(6′)-Ib-cr ( 10 ). A third mechanism for PMQR was added in 2007 with the discovery of plasmid-mediated quinolone efflux pumps QepA ( 11 , 12 ) and OqxAB ( 13 ). A multiplex PCR assay for eight PMQR genes (lacking only qnrVC) has recently been perfected ( 14 ). In the past decade these genes have been found in bacterial isolates from around the world. They reduce the susceptibility of bacteria to quinolones, usually not to the level of nonsusceptibility but facilitating the selection of more quinolone-resistant mutants and treatment failure. PMQR has been frequently reviewed ( 15 – 20 ).
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The rod-like structure of the QnrB1 dimer is shown above, with the sequence of the monomer below. The sequence is divided into four columns representing the four faces of the right-handed quadrilateral β-helix. Face names and color are shown at the top along with the naming convention for the five residues of the pentapeptide repeats. Loops A and B are indicated by one and two asterisks, respectively, with their sequences indicated below and the loops shown as black traces on the diagram. The N-terminal α-helix is colored pink. The molecular 2-fold symmetry is indicated with a black diamond. Type II turn-containing faces are shown as spheres, and type IV-containing faces as strands ( 235 ). Adapted from the Journal of Biological Chemistry ( 44 ), copyright 2011, the American Society for Biochemistry and Molecular Biology.
QnrB1 protection of DNA gyrase from ciprofloxacin inhibition of supercoiling. Reaction mixtures of 30 µl were analyzed by agarose gel electrophoresis. Reaction mixtures contained 0.2 µg relaxed pBR322 DNA (lanes 1 to 14), 6.7 nM gyrase (lanes 2 to 14), 2 µg/ml ciprofloxacin (lanes 3 to 14), and QnrB-His6 fusion protein at 25 µM (lane 4), 5 µM (lane 5), 2.5 µM (lane 6), 0.5 µM (lane 7), 50 nM (lane 8), 5 nM (lane 9), 0.5 nM (lane 10), 50 pM (lane 11), 5 pM (lane 12), or 0.5 pM (lane 13). Reprinted from reference 27 .
Genetic environment of qnr alleles.
Genetic environment of qnr alleles.
Genetic environment of qnr alleles.
Survival at increasing fluoroquinolone concentrations for E. coli J53 and J53 pMG252. A large inoculum (1010 colony forming units) and appropriate dilutions were applied to Mueller-Hinton agar plates containing the indicated concentration of ciprofloxacin, and surviving colonies were counted after incubation for 72 h at 37°C.