What do fluoroquinolones inhibit

Close mobile search navigation Article Navigation. Volume Article Contents Abstract. Efflux Mechanisms of Resistance.

Selection of Resistance Mutations by Newer Fluoroquinolones. Hooper David C. Reprints or correspondence: Dr. Oxford Academic. Cite Cite David C. Select Format Select format.

Permissions Icon Permissions. Open in new tab Download slide. Google Scholar PubMed. Google Scholar Crossref. Search ADS. Bacterial topoisomerases, anti-topoisomerases, and anti-topoisomerase resistance. The role of topoisomerase IV in partitioning bacterial replicons and the structure of catenated intermediates in DNA replication. Topoisomerase IV, not gyrase, decatenates products of site-specific recombination in Escherichia coli.

DNA strand cleavage is required for replication fork arrest by a frozen topoisomerase-quinolone-DNA ternary complex. Escherichia coli K mutants resistant to nalidixic acid: genetic mapping and dominance studies. Genetic and biochemical characterization of norfloxacin resistance in Escherichia coli. Quinolone resistance locus nfxD of Escherichia coli is a mutant allele of parE gene encoding a subunit of topoisomerase IV.

Analysis of gyrA and grlA mutations in stepwise-selected ciprofloxacin-resistant mutants of Staphylococcus aureus. Quinolone resistance mutations in topoisomerase IV: relationship of the flqA locus and genetic evidence that topoisomerase IV is the primary target and DNA gyrase the secondary target of fluoroquinolones in Staphylococcus aureus. DNA gyrase and topoisomerase IV are dual targets of clinafloxacin action in Streptococcus pneumoniae.

Activities of fluoroquinolones against Streptococcus pneumoniae type II topoisomerases purified as recombinant proteins. Mechanisms and frequency of resistance to gatifloxacin in comparison with ciprofloxacin in Staphylococcus aureus.

Streptococcus pneumoniae DNA gyrase and topoisomerase IV: overexpression, purification, and differential inhibition by fluoroquinolones. ParC and GyrA may be interchangeable initial targets of some fluoroquinolones in Streptococcus pneumoniae. Relationship between mutations in the coding and promoter regions of the norA genes in 42 unrelated clinical isolates of Staphylococcus aureus and the MICs of norfloxacin for these strains.

Quinolone resistance mediated by norA: physiologic characterization and relationship to flqB, a quinolone resistance locus on the Staphylococcus aureus chromosome. Efflux-mediated fluoroquinolone resistance in Staphylococcus aureus. Efflux pump-mediated quinolone resistance in Staphylococcus aureus strains wild type for gyrA, gyrB, grlA, and norA. Antibacterial activity of gatifloxacin AM, CG, BMS , a newly developed fluoroquinolone, against sequentially acquired quinolone-resistant mutants and the norA transformant of Staphylococcus aureus.

Activities of new fluoroquinolones against fluoroquinolone-resistant pathogens of the lower respiratory tract. Identification of an efflux pump gene, pmrA, associated with fluoroquinolone resistance in Streptococcus pneumoniae. Nucleotide sequence and characterization of the Staphylococcus aureus norA gene, which confers resistance to quinolones.

Hydrophilicity of quinolones is not an exclusive factor for decreased activity in efflux-mediated resistant mutants of Staphylococcus aureus. Issue Section:. Download all slides. Comments 0. Add comment Close comment form modal. I agree to the terms and conditions. You must accept the terms and conditions. Add comment Cancel.

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Your comment will be reviewed and published at the journal's discretion. Please check for further notifications by email. View Metrics. Resistance is likely to become increasingly important, since three types of plasmid-borne resistance have been reported. Jekyll into Mr. Hyde Curr. Bentham Science Publishers. A review with refs. Quinolones are a very important family of antibacterial agents that are widely prescribed for the treatment of infections in humans.

Although the founding members of this drug class had little clin. In contrast to most other anti-infective drugs, quinolones do not kill bacteria by inhibiting a crit. Rather, they corrupt the activities of two essential enzymes, DNA gyrase and topoisomerase IV, and induce them to kill cells by generating high levels of double-stranded DNA breaks.

A second unique aspect of quinolones is their differential ability to target these two enzymes in different bacteria. Depending upon the bacterial species and quinolone employed, either DNA gyrase or topoisomerase IV serves as the primary cytotoxic target of drug action.

While this unusual feature initially stymied development of quinolones with high activity against Gram-pos. In addn. These antineoplastic quinolones represent a potentially important source of new anticancer agents and provide an opportunity to examine drug mechanism across divergent species. Because of the clin. Agents Chemother. American Society for Microbiology.

In the present review we consider cell death through a two-part "poison" hypothesis in which quinolones form reversible drug-topoisomerase-DNA complexes that subsequently lead to several types of irreversible lethal damage. Topics discussed include: quinolone-topoisomerase-DNA complexes; inhibition of DNA replication; SOS response and cell filamentation; toxin-antitoxin modules and programmed cell death; chromosome fragmentation; and destabilization of cleaved complexes.

Wohlkonig, Alexandre; Chan, Pan F. Nature Publishing Group. Quinolone antibacterials have been used to treat bacterial infections for over 40 years. A crystal structure of moxifloxacin in complex with Acinetobacter baumannii topoisomerase IV now shows the wedge-shaped quinolone stacking between base pairs at the DNA cleavage site and binding conserved residues in the DNA cleavage domain through chelation of a noncatalytic magnesium ion.

This provides a mol. Aldred, Katie J. Bacillus anthracis, the causative agent of anthrax, is considered a serious threat as a bioweapon. The drugs most commonly used to treat anthrax are quinolones, which act by increasing the levels of DNA cleavage mediated by topoisomerase IV and gyrase.

Quinolone resistance most often is assocd. Therefore, to det. The three enzymes displayed similar catalytic activities in the absence of drugs.

However, the resistance mutations decreased the affinity of topoisomerase IV for ciprofloxacin and other quinolones, diminished quinolone-induced inhibition of DNA religation, and reduced the stability of the enzyme-quinolone-DNA ternary complex. Wild-type DNA cleavage levels were generated by mutant enzymes at high quinolone concns. Moreover, it maintained high potency and efficacy against the mutant enzymes, effectively inhibited DNA religation, and formed stable ternary complexes.

Our findings provide an underlying biochem. Although quinolones are the most commonly prescribed antibacterials, their use is threatened by an increasing prevalence of resistance. The most common causes of quinolone resistance are mutations of a specific serine or acidic residue in the A subunit of gyrase or topoisomerase IV.

These amino acids are proposed to serve as a crit. Individual mutation of Ser81 or Glu85 partially disrupted bridge function, whereas simultaneous mutation of both residues abrogated protein-quinolone interactions.

Results provide functional evidence for the existence of the water-metal ion bridge, confirm that the serine and glutamic acid residues anchor the bridge, demonstrate that the bridge is the primary conduit for interactions between clin. A review with 36 refs.

Broad use of fluoroquinolones has been followed by emergence of resistance, which has been due mainly to chromosomal mutations in genes encoding the subunits of the drugs' target enzymes, DNA gyrase and topoisomerase IV, and in genes that affect the expression of diffusion channels in the outer membrane and multidrug-resistance efflux systems. Resistance emerged first in species in which single mutations were sufficient to cause clin. Subsequently, however, resistance has emerged in bacteria such as Campylobacter jejuni, Escherichia coli, and Neisseria gonorrhoeae, in which multiple mutations are required to generate clin.

In these circumstances, the addnl. Resistance in Streptococcus pneumoniae, which is currently low, will require close monitoring as fluoroquinolones are used more extensively for treating respiratory tract infections.

Sage Publications Ltd. Quinolones are a group of antimicrobial agents that were serendipitously discovered as byproducts of the synthesis of chloroquine. The efficacy and consequent widespread use of quinolones and fluoroquinolones has led to a steady global increase in resistance, mediated via gene mutations, alterations in efflux or cell membranes and plasmid-conferred resistance.

The first plasmid-mediated quinolone resistance gene, qnrA1, was detected in Since then, many other genes have been identified and the underlying mechanisms of resistance have been elucidated.

This review provides an overview of quinolone resistance, with particular emphasis on plasmid-mediated resistance. A new class of chemotherapeutic agents J. Journal of medicinal and pharmaceutical chemistry , 91 , ISSN: Libertas Academica. Completing its initial phases of drug development in the mid s as the one of the first fluoroquinolones that could be used with confidence to treat respiratory tract infections, levofloxacin went on to become one of the most widely prescribed antibiotics in the world.

Available in both oral po and i. Results of clin. Regimens of levofloxacin, initially involving total daily doses of mg to mg, but more recently regimens involving mg doses, have been shown to be safe and effective. Nearly a decade and a half of clin. As resistance to currently available fluoroquinolones has emerged, the clin. However, consistently high rates of susceptibility of clin.

Google Scholar There is no corresponding record for this reference. Acta , 29 — 43 Google Scholar There is no corresponding record for this reference. Annual Reviews Inc. DNA topoisomerases solve the topol. In recent years, the crystal structures of a no. These structures provide remarkable insights into the mechanisms of these enzymes and complement previous conclusions based on biochem.

Surprisingly, despite little or no sequence homol. The type IB enzymes are structurally distinct from all other known topoisomerases but are similar to a class of enzymes referred to as tyrosine recombinases. For the type II topoisomerases, the binding and hydrolysis of ATP further modulate conformational changes in the enzymes to effect changes in DNA topol.

Elsevier B. DNA topoisomerases are essential for DNA replication, transcription, recombination, as well as for chromosome compaction and segregation. They may have appeared early during the formation of the modern DNA world. Several families and subfamilies of the 2 types of DNA topoisomerases I and II have been described in the 3 cellular domains of life Archaea, Bacteria and Eukarya , as well as in viruses infecting eukaryotes or bacteria. However, some of them share homologous modules or subunits that were probably recruited independently to produce different topoisomerase activities.

The puzzling phylogenetic distribution of the various DNA topoisomerase families and subfamilies cannot be easily reconciled with classical models of early evolution describing the relations between the 3 cellular domains. Different families of DNA topoisomerases some of them possibly of viral origin would then have been independently introduced in the different cellular domains. Here, the authors review the main characteristics of the different families and subfamilies of DNA topoisomerases in a historical and evolutionary perspective, with the hope to stimulate further works and discussions on the origin and evolution of these fascinating enzymes.

Cell Press. DNA topoisomerases are the targets of important anticancer and antibacterial drugs. Camptothecins and novel noncamptothecins in clin. Bacterial type II topoisomerases gyrase and Topo IV are the targets of quinolones and aminocoumarin antibiotics. This review focuses on the mol.

We also discuss the common mechanism of action of topoisomerase poisons by interfacial inhibition and trapping of topoisomerase cleavage complexes.

Schmidt, Bryan H. Type II topoisomerases are required for the management of DNA tangles and supercoils, and are targets of clin. These enzymes catalyze the ATP-dependent passage of one DNA duplex the transport or T-segment through a transient, double-stranded break in another the gate or G-segment , navigating DNA through the protein using a set of dissociable internal interfaces, or gates'. For more than 20 years, it has been established that a pair of dimer-related tyrosines, together with divalent cations, catalyze G-segment cleavage.

Recent efforts have proposed that strand scission relies on a "two-metal mechanism", a ubiquitous biochem. Here we present the structure of the DNA-binding and cleavage core of Saccharomyces cerevisiae topoisomerase II covalently linked to DNA through its active-site tyrosine at 2. Unexpectedly, metal-soaking expts. Comparative analyses extend this scheme to explain how distantly-related type IA topoisomerases cleave single-stranded DNA, unifying the cleavage mechanisms for these two essential enzyme families.

The structure also highlights a hitherto undiscovered allosteric relay that actuates a mol. This connection illustrates how an indispensable chromosome-disentangling machine auto-regulates DNA breakage to prevent the aberrant formation of mutagenic and cytotoxic genomic lesions.

Royal Society of Chemistry. Type II topoisomerases are essential enzymes that regulate DNA under- and overwinding and remove knots and tangles from the genetic material. In order to carry out their crit. Consequently, while necessary for cell survival, type II topoisomerases also have the capacity to fragment the genome. This feature of the prokaryotic and eukaryotic enzymes, resp.

All type II topoisomerases require divalent metal ions for catalytic function. These metal ions function in two sep. ATPase activity is required for the strand passage process and utilizes the metal-dependent binding and hydrolysis of ATP to drive structural rearrangements in the protein.

This article will focus primarily on eukaryotic type II topoisomerases and the roles of metal ions in the catalytic functions of these enzymes. Zechiedrich, E. Lynn; Khodursky, Arkady B. American Society for Biochemistry and Molecular Biology. DNA supercoiling is essential for bacterial cell survival. We demonstrated that DNA topoisomerase IV, acting in concert with topoisomerase I and gyrase, makes an important contribution to the steady-state level of supercoiling in Escherichia coli.

Following inhibition of gyrase, topoisomerase IV alone relaxed plasmid DNA to a final supercoiling d. Inhibition of topoisomerase IV in wild-type cells increased supercoiling to approx.

The role of topoisomerase IV was revealed by two functional assays. A nonconserved acidic C-terminal tail modulates Escherichia coli gyrase activity J. Teaching a 'Knotty' subject. DNA is essentially an extremely long double-stranded rope in which the two strands are wound about one another. As a result, topol. Despite the importance of DNA topol. This article will familiarize the reader with the concept of DNA topol. Furthermore, it will discuss topoisomerases, the enzymes that regulate the topol.

These ubiquitous enzymes perform a no. During this catalytic event, topoisomerases maintain genomic stability by forming covalent phosphotyrosyl bonds between active site residues and the newly generated DNA termini. Topoisomerases are essential for cell survival. However, because they cleave the genetic material, these enzymes also have the potential to fragment the genome.

This latter feature of topoisomerases is exploited by some of the most widely prescribed anticancer and antibacterial drugs currently in clin. Finally, in addn. The nalA45 Ts ext. Thus, gyrase subunit A and the nalidixic acid target are one and the same protein, the nalA gene product. Shift of the nalA45 Ts mutant to a nonpermissive temp. The rates of incorporation of uridine-3H pulses and continuously administered uracil-3H were quickly reduced approx. The growth of phage T7 was strongly inhibited by nalidixic acid but essentially unaffected by the nalA43 Ts mutation.

The inhibition of T7 growth by nalidixic acid was, however, eliminated by temp. Therefore, nalidixic acid may block T7 growth by a corruption rather than a simple elimination of the nalidixic acid target.

Possible mechanisms for such a corruption are considered, and their relevance to the puzzling dominance of drug sensitivity is discussed.

Laponogov, Ivan; Sohi, Maninder K. Mark; Sanderson, Mark R. We present the structures of cleavage complexes formed by the Streptococcus pneumoniae ParC breakage-reunion and ParE TOPRIM domains of topoisomerase IV stabilized by moxifloxacin and clinafloxacin, two antipneumococcal fluoroquinolones.

These structures reveal two drug mols. They change DNA topology by forming a transient covalent cleavage complex with a gate-DNA duplex that allows transport of a second duplex though the gate. Despite its biological importance and targeting by anticancer and antibacterial drugs, cleavage complex formation and reversal is not understood for any type II enzyme.

To address the mechanism, we have used X-ray crystallography to study sequential states in the formation and reversal of a DNA cleavage complex by topoisomerase IV from Streptococcus pneumoniae, the bacterial type II enzyme involved in chromosome segregation. A high resolution structure of the complex captured by a novel antibacterial dione reveals two drug molecules intercalated at a cleaved B-form DNA gate and anchored by drug-specific protein contacts.

These structures, the first for putative reaction intermediates of a type II topoisomerase, suggest how a type II enzyme reseals DNA during its normal reaction cycle and illuminate aspects of drug arrest important for the development of new topoisomerase-targeting therapeutics. Nature , , ISSN:. Despite the success of genomics in identifying new essential bacterial genes, there is a lack of sustainable leads in antibacterial drug discovery to address increasing multidrug resistance.

Type IIA topoisomerases cleave and religate DNA to regulate DNA topology and are a major class of antibacterial and anticancer drug targets, yet there is no well developed structural basis for understanding drug action. Here we report the 2. The inhibitor 'bridges' the DNA and a transient non-catalytic pocket on the two-fold axis at the GyrA dimer interface, and is close to the active sites and fluoroquinolone binding sites. This work provides new insights into the mechanism of topoisomerase action and a platform for structure-based drug design of a new class of antibacterial agents against a clinically proven, but conformationally flexible, enzyme class.

The mechanistic basis of dione activity vis-a-vis quinolones is not understood. First, PD targets primarily gyrase in Streptococcus pneumoniae. This "dione resistance-detg. Third, dione-promoted DNA cleavage by gyrase occurred at a distinct repertoire of sites, implying that structural differences with quinolones are sensed at the DNA level. Agents 39 , — [ Crossref ], [ PubMed ], [ CAS ], Google Scholar 67 Curing bacteria of antibiotic resistance: reverse antibiotics, a novel class of antibiotics in nature.

By screening cultures of soil bacteria, the authors re-discovered an old antibiotic nybomycin as an antibiotic with a novel feature. Nybomycin is active against quinolone-resistant Staphylococcus aureus strains with mutated gyrA genes but not against those with intact gyrA genes against which quinolone antibiotics are effective.

Nybomycin-resistant mutant strains were generated from a quinolone-resistant, nybomycin-susceptible, vancomycin-intermediate S. Thus, the authors here describe nybomycin as the first member of a novel class of antibiotics designated reverse antibiotics.

Martinez-Freijo, P. Class I integrons are assocd. Expression of inserted resistance genes within these structures can be poor and, as such, the clin. Of unrelated Gram-neg. Integrons were detected in isolates from all hospitals with no particular geog. This assocn. As such, their widespread presence is a cause for concern. There was no assocn. Elsevier Ltd. Fluoroquinolone resistance is emerging in Gram-neg. The traditional understanding that quinolone resistance is acquired only through mutation and transmitted only vertically does not entirely account for the relative ease with which resistance develops in exquisitely susceptible organisms, or for the very strong assocn.

The recent discovery of plasmid-mediated horizontally transferable genes encoding quinolone resistance might shed light on these phenomena. The Qnr proteins, capable of protecting DNA gyrase from quinolones, have homologues in water-dwelling bacteria, and seem to have been in circulation for some time, having achieved global distribution in a variety of plasmid environments and bacterial genera.

AAC 6' -Ib-cr, a variant aminoglycoside acetyltransferase capable of modifying ciprofloxacin and reducing its activity, seems to have emerged more recently, but might be even more prevalent than the Qnr proteins. Both mechanisms provide low-level quinolone resistance that facilitates the emergence of higher-level resistance in the presence of quinolones at therapeutic levels. Much remains to be understood about these genes, but their insidious promotion of substantial resistance, their horizontal spread, and their co-selection with other resistance elements indicate that a more cautious approach to quinolone use and a reconsideration of clin.

Blackwell Publishing Ltd. Although resistance to quinolones is commonly chromosomally-encoded in Enterobacteriaceae, the emergence of plasmid-mediated quinolone resistance PMQR has also been reported, with at least three known resistance mechanisms to date, i. Qnr proteins protect target enzymes DNA gyrase and type IV topoisomerase from quinolone inhibition, the AAC 6' -Ib-cr enzyme acetylates norfloxacin and ciprofloxacin, and the QepA efflux pump extrudes hydrophilic fluoroquinolones.

A review on plasmid-mediated quinolone resistance. The main mechanism of quinolone resistance is the accumulation of mutations in the bacterial enzymes targeted by fluoroquinolones: DNA gyrase and DNA topoisomerase IV. Multiple mutations are generally required for clin. Because spontaneous double mutations are rare genetic events, the traditional understanding of quinolone resistance as a mutational phenomenon has not provided a fully satisfying explanation for the frequency with which this resistance has arisen.

Springer Japan. In , the first plasmid-mediated gene involved in quinolone resistance currently named qnrA1 was reported. Extra qnr-like plasmid-mediated genes qnrB, qnrS, qnrC, qnrD and their chromosomal homologues have also been characterized. These genes code for a pentapeptide repeat protein that protects type II topoisomerases from quinolones. Since then, there have been reports of two other plasmid-mediated resistance mechanisms: the modification of quinolones with a piperazinyl substituent by the acetyltransferase, Aac 6' -Ib-cr; and active efflux by QepA and OqxAB, pumps related to major facilitator superfamily MFS transporters.

These genes have a wide geog. Because of the difficulties of phenotypic detection of this type of resistance, its real prevalence is only partially known. One important point is that although these mechanisms cause only low-level resistance, they favor and complement the selection of other resistance mechanisms. Elsevier GmbH. Plasmids represent one of the most difficult challenge for counteracting the dissemination of antimicrobial resistance. They contribute to the spread of relevant resistance determinants, promoting horizontal gene transfer among unrelated bacteria.

Undistinguishable plasmids were identified in unrelated bacterial strains isolated at huge geog. These plasmids belong to families that are largely prevalent in naturally occurring bacteria, usually carry multiple phys.

Plasmids also harbor virulence factors and addiction systems, promoting their stability and maintenance in the bacterial host, in different environmental conditions.

Since the plasmid-mediated quinolone resistance gene qnr was first described in a ciprofloxacin-resistant strain of Klebsiella pneumoniae in , many addnl. To reduce the chaos that developed from new sequences being given the same QnrB no.

Antimicrob Agents Chemother on the basis of eight priority rules. Furthermore, to fundamentally and continuously prevent misleading designations of Qnr alleles, we most importantly need a central clearing website with bioinformatics capabilities to serve as a repository for all sequences and name designations for Qnr alleles.

Whether this system succeeds or not depends on the cooperation of each researcher and the voluntary participation of the keeper s of the website. Therefore, allele designations should be assigned upon application, before new Qnr alleles are submitted to the GenBank database. Here, we focus on the characterization of a new Qnr family QnrAS, the gene of which is located on the chromosome of Aliivibrio salmonicida , which can help researchers to designate correctly new Qnr alleles and make the website effective and successful.

To ensure that the Lahey website is effective and successful, the information GenBank accession no. Because there were some misleading data, information for Qnr alleles shown on the Lahey website was updated recently through personal communication between the current authors and Dr Jacoby. 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 clin. This enzyme reduces the activity of ciprofloxacin by N-acetylation at the amino nitrogen on its piperazinyl substituent. Although approx. 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 enzymically undermining two unrelated antimicrobial agents, one of them fully synthetic.

Drug Resist. Mary Ann Liebert, Inc. The quinoxaline olaquindox has been used extensively as a growth promoter for pigs. Recently, we isolated a plasmid pOLA52 conferring resistance to olaquindox from swine manure. It facilitates resistance to olaquindox as well as resistance to other antimicrobials like chloramphenicol. In this study, 10 of the 1. In nine of the ten strains, the oqxA gene was detected.

Sequencing of an internal fragment of oqxA from the oqxA-pos. All of the oqxA-pos. It was verified by Southern hybridization that the oqxAB operon was situated on plasmids in most, if not all, resistant strains.

Furthermore, horizontal transfer of olaquindox resistance from three olaquindox-resistant isolates was achieved using an olaquindox-sensitive E.

In addition, RamA overexpression in MAR strains overexpressing AcrAB has been justified by the presence of mutations within the encoding region of the repressor leading to its inactivation. In addition, a deletion in the putative RamR binding site upstream ramA prevents RamR binding and thus, its repressor effect over RamA. The regulatory effects of RamA in fluoroquinolone resistance acquisition may play a key role in strains lacking an altered level of any of the other regulators, such as MarA, SoxS or AcrR, suggesting that the role of these regulators is not as significant as in E.

However, other studies compromise these conclusions as they reveal that ramA inactivation in some S. However, an exception has been reported in K. Despite the proposal of all these mechanisms further investigation is needed for a complete explanation, for example, when: i fluoroquinolone resistance has at times been reported not to be linked to marOR , soxRS or acrR mutations, even when AcrAB is overproduced in E.

Furthermore, the outer membrane protein profile has also been studied in strains with a high level of fluoroquinolone resistance. It has been found that the major outer membrane proteins of E.

This deficiency in porins has been reported to achieve a significant effect only when mutations occur within the QRDR or efflux mechanisms appear simultaneously. Despite this general information, other efflux systems as well as particular situations affect bacteria individually:.

Fluoroquinolone efflux transporters characterized to date and their clinical implication. In microorganisms such as Pseudomonas aeruginosa and Acinetobacter baumannii , a single mutation in the gyrA gene is sufficient to cause clinically important levels of resistance to fluoroquinolones as these bacteria already show an intrinsic resistance to these antibacterial agents, likely due to low permeability or constitutive expression of some efflux pump s or the interplay between both.

Therefore, this decreased susceptibility low level of resistance can favour the acquisition of a mutation and increase the MIC of fluoroquinolones. Beside mutations in target genes, DNA modifications increasing the efflux of the antibiotics by overexpression of the efflux pump systems play an important role in triggering resistance. The most important group of efflux pumps found in P. Despite the lack of a final gene encoding for a porin, it does not mean that it is not needed.

All these operons have their own promoter in the upstream region, and in some cases the presence of a regulatory gene upstream from the promoter has been demonstrated.

Two new ORFs have recently been characterized as additional repressors of the pump. Interestingly, P. MexZ is a repressor protein that binds to the pump promoter and inhibits MexXY expression. The intrinsic resistance in A. The clinical implication of the overexpression of this pump was shown as strains recovered from two different outbreaks. It has not been possible to clone these genes in plasmids probably because expression is toxic. AbeM is another efflux pump characterized in A.

The substrates susceptible to be pumped out with this efflux pump are fluoroquinolones and gentamicin, whereas those with lower affinity include kanamycin, erythromycin, chloramphenicol and trimetoprim.

Microorganisms lacking Topo IV. Campylobacter jejuni , Corynebacterium spp. The main efflux pump found in C. However, its overexpression in C. The first mutation associated with fluoroquinolone resistance in S. The mechanisms by which norA is overexpressed in clinical isolates have been reported. The third option is the presence of a transcriptional regulator, MgrA formerly NorR , which has been shown to bind specifically to the norA promoter. Recently, another regulator, dubbed NorG, has been proposed.

This protein is able to bind to its own promoter, meaning that it can be autoregulated, and to norA promoter although it has not resulted in any increase in norA transcripts. Alternatively, other pumps have been described with ability of extruding quinolones. NorB is a member of the MFS superfamily and pumps out norfloxacin and ciprofloxacin as NorA does, but in addition it can recognize moxifloxacin and sparfloxacin as substrates, causing, when overexpressed, an eightfold increase in the MICs of the first two antibiotics and a fourfold increase in the others.

In addition, NorG has been shown to bind directly to norB promoter and cause a positive effect by increasing norB transcripts. Its overexpression causes a fourfold increase in the MIC of norfloxacin and moxifloxacin and reproducible twofold increase in the MIC of ciprofloxacin and sparfloxacin, resulting in the same substrate profile as NorB.

No basal expression has been detected in standard laboratory conditions suggesting that activation must occur in order to detect it.

The first gene encodes for a transcriptional regulator with strong homology with MarR, which acts as a repressor of its own transcription and MepA expression. The second gene encodes the efflux pump, while the third gene codifies for a protein with no homology to any protein with known function.

There are few reports about the prevalence of the overexpression of these efflux pumps in clinical isolates. Mutations such as point mutations, insertions or deletions, in mepR , norA , norB and norC promoters, were described in overexpressing strains respectively.

Efflux pump as a mechanism of fluoroquinolone resistance in S. The first one is PmrA, a protein belonging to the MFS superfamily showing substantial homology with NorA, and whose overexpression leads to an increase in the MIC of norfloxacin and ciprofloxacin which can decrease up to fourfold upon reserpine addition. In general, it appears in addition to QRDR mutations and it is not well associated with strains showing a high level of norfloxacin resistance.

However, reserpine only seems to inhibit PatA contribution to resistance, whereas PatB would not be affected. As expected from its structure, Qnr determinants did not seem to produce a change in intracellular quinolone accumulation nor did it cause drug inactivation.

Moreover, Topo IV, the secondary target of quinolones in Enterobacteriaceae , also seems to be protected from quinolones by Qnr. Since the first report of this mechanism of resistance to quinolones, a large number of studies addressed to find this gene in different collections of clinical isolates have been reported.

Up to the present, three qnr genes have been identified: the qnrA gene found in K. If Qnr is the only mechanism of resistance to quinolones present, the MIC of ciprofloxacin will increase only to 0.

It is thought that this low level of resistance to the antibacterial agent makes it possible for bacteria populations to raise concentrations that facilitate the occurrence of secondary mutations and thus the high level of resistance. Recently, a new mechanism of transferable quinolone resistance has been reported: enzymatic inactivation of certain quinolones. This new mechanism consists of a gene named qepA that encodes for an efflux pump.

QepA showed high similarity with members of the Major Facilitator Superfamily responsible for resistance to hydrophilic quinolones such as norfloxacin and ciprofloxacin. Recently, after analysis of the prevalence of the qepA and qnr genes in a collection of E. Neither of these latter mechanisms affects the action of nalidixic acid. National Center for Biotechnology Information , U.

Journal List Microb Biotechnol v. Microb Biotechnol. Published online Dec Find articles by Ernest Giralt. Find articles by Jordi Vila.

Author information Article notes Copyright and License information Disclaimer. This article has been cited by other articles in PMC. Structure of the quinolones From the structural perspective, quinolones are heterocycles with a bicyclic core structure Fig. Open in a separate window.

Figure 1. Figure 2. Figure 3. Table 5 Fluoroquinolone efflux transporters characterized to date and their clinical implication. References Abouzeed Y. Antimicrob Agents Chemother. J Antimicrob Chemother. Cloning and characterization of SmeDEF, a novel multidrug efflux pump from Stenotrophomonas maltophilia. Expression of multidrug efflux pump smeDEF by clinical isolates of Stenotrophomonas maltophilia. Molecular characterization of the soxRS genes of Escherichia coli : two genes control a superoxide stress regulon.

Nucleic Acids Res. J Bacteriol. Selection of quinolone resistance in Streptococcus pneumoniae exposed in vitro to subinhibitory drug concentrations.