Pimozide Inhibits the AcrAB-TolC Efflux Pump in Escherichia coli

Comparative reassessment of AcrB efflux inhibitors reveals differential impact of specific pump mutations on the activity of potent compounds

Multidrug resistance poses global challenges, particularly with regard to Gram-negative bacterial infections. In view of the lack of new antibiotics, drug enhancers, such as efflux pump inhibitors (EPIs), have increasingly come into focus. A number of chemically diverse agents have been reported to inhibit AcrB, the main multidrug transporter in Escherichia coli, and homologs in other Gram-negative bacteria. However, due to the often varying methodologies used for their characterization, results remain difficult to compare. In this study, using a defined selection of antibiotics known to be efflux substrates, we reevaluated 38 published compounds for their in vitro EPI activity. When examined in an E. coli strain with stable wild-type AcrB overexpression, we found 17 compounds showing at least fourfold enhancing potency with more than 2 out of 10 test drugs (belonging to eight antibiotic classes). Pyranopyridines (MBX series) were confirmed as the most potent inhibitors among agents reported so far. A new and surprising finding was that their activity, unlike that of the pyridylpiperazine EPI BDM88855, was highly susceptible to the AcrB double-mutation G141D_N282Y, which had previously been shown to diminish drug enhancing of 1-(1-naphthylmethyl)piperazine in a predominantly substrate-specific manner. Conversely, transmembrane region mutation V411A, while eliminating the drug potentiating of the BDM compound, did not decrease the activity of the MBX EPIs. Besides comparative reassessment of the potency of reported EPIs, the study demonstrated the usefulness of mutagenesis approaches providing tools for an initial discrimination of EPIs regarding their mode of function.IMPORTANCEInfections with difficult-to-treat multidrug-resistant bacteria pose an urgent global threat in view of the stagnating development of new antimicrobial substances. Efflux pumps in Gram-negative pathogens are known to substantially contribute to multidrug resistance making them promising targets for chemotherapeutic interventions to restore the efficacy of conventional antibiotics. In the present study, the in vitro activity of previously reported efflux pump inhibitors was reassessed using standardized conditions. Relevant drug sensitizing activity could be proven for almost half of the tested compounds. Further characterization of potent inhibitors was achieved by investigating the impact of specific efflux pump mutations. A double-mutation previously known to decrease the activity of the arylpiperazine 1-(1-naphthylmethyl)piperazine also impaired that of the highly efficient pyranopyridine efflux pump inhibitors. Our findings provide direct comparability of reported efflux pump inhibitors and contribute to the elucidation of their mode of action.

Development of pharmacophore models for AcrB protein and the identification of potential adjuvant candidates for overcoming efflux-mediated colistin resistance

Growing multi-drug resistance (MDR) among ESKAPE pathogens is a huge challenge. Increased resistance to last-resort antibiotics, like colistin, has further aggravated this. Efflux is identified as a major route of colistin resistance. So, finding an FDA-approved efflux inhibitor for potential application as an adjuvant to colistin was the primary objective of this study. E. coli-AcrB pump inhibitors and substrates were used to develop and validate the pharmacophoric model. Drugs confirming this pharmacophore were subjected to molecular docking to identify hits for the AcrB binding pocket. The efflux inhibition potential of the top hit was validated through the in vitro evaluation of the minimum inhibitory concentration (MIC) in combination with colistin. The checkerboard assay was done to demonstrate synergism, which was further corroborated by the Time-kill assay. Ten common pharmacophore hypotheses were successfully generated using substrate/inhibitors. Following enrichment analysis, AHHNR.100 was identified as the top-ranked hypothesis, and 207 unique compounds were found to conform to this hypothesis. The multi-step docking of these compounds against the AcrB protein revealed argatroban as the top non-antibiotic hit. This significantly inhibited the efflux activity of colistin-resistant clinical isolates K. pneumoniae (n = 1) and M. morganii (n = 2). Further, their combination with colistin enhanced the susceptibility of these isolates, and the effect was found to be synergistic. Accordingly, the time-kill assay of this combination showed 8-log and 2-log reductions against K. pneumoniae and M. morganii, respectively. In conclusion, this study found argatroban as a bacterial efflux inhibitor that can be potentially used to overcome efflux-mediated resistance.

Combined Structure- and Ligand-Based Approach for the Identification of Inhibitors of AcrAB-TolC in Escherichia coli

The inhibition of efflux pumps is a promising approach to combating multidrug-resistant bacteria. We have developed a combined structure- and ligand-based model, using OpenEye software, for the identification of inhibitors of AcrB, the inner membrane protein component of the AcrAB-TolC efflux pump in Escherichia coli. From a database of 1391 FDA-approved drugs, 23 compounds were selected to test for efflux inhibition in E. coli. Seven compounds, including ivacaftor (25), butenafine (19), naftifine (27), pimozide (30), thioridazine (35), trifluoperazine (37), and meloxicam (26), enhanced the activity of at least one antimicrobial substrate and inhibited the efflux pump-mediated removal of the substrate Nile Red from cells. Ivacaftor (25) inhibited efflux dose dependently, had no effect on an E. coli strain with genomic deletion of the gene encoding AcrB, and did not damage the bacterial outer membrane. In the presence of a sub-minimum inhibitory concentration (MIC) of the outer membrane permeabilizer colistin, ivacaftor at 1 μg/mL reduced the MICs of erythromycin and minocycline by 4- to 8-fold. The identification of seven potential AcrB inhibitors shows the merits of a combined structure- and ligand-based approach to virtual screening.

Computational assessment of phytochemicals of medicinal plants from Mexico as potential inhibitors of Salmonella enterica efflux pump AcrB protein

The AcrAB-TolC efflux pump (EP) confers multidrug resistance to Salmonella enterica, a major etiological agent of foodborne infections. Phytochemicals that inhibit the functions of AcrAB-TolC EP present ideal candidates for reversal of antibiotic resistance. Progressive technological advancements, have facilitated the development of computational methods that offer a rapid low-cost approach to screen and identify phytochemicals with inhibitory potential against EP. In this study, 71 phytochemicals derived from plants used for medicinal purposes in Mexico were screened for their potential as inhibitors of Salmonella AcrB protein using in silico approaches including molecular docking and molecular dynamics (MD) simulation. Consequently, naringenin, 5-methoxypsoralen, and licarin A were identified as candidate inhibitors of AcrB protein. The three phytochemicals bound distal/deep pocket (DP) and hydrophobic trap (HPT) residues of AcrB protein critical for interactions with inhibitors, with estimated binding free energies of -95.5 kJ/mol, -97.4 kJ/mol, and -143.8 kJ/mol for naringenin, 5-methoxypsoralen, and licarin A, respectively. Data from the 50 ns MD simulation study revealed stability of the protein-ligand complex and alterations in the AcrB protein DP conformation upon binding of phytochemicals to the DP and HPT regions. Based on the estimated binding free energy and interactions with three out of five residues lining the hydrophobic trap, licarin A demonstrated the highest inhibitory potential, supporting its further application as a candidate for overcoming drug resistance in pathogens. Communicated by Ramaswamy H. Sarma.

Discovery of AcrAB-TolC pump inhibitors: Virtual screening and molecular dynamics simulation approach

AcrAB-TolC tripartite efflux pump, which belongs to the RND superfamily, is a main multi-drug efflux system of Escherichia coli (E. coli) because of the broad resistance on various antibiotics. With the discovering of efflux pump inhibitors (EPIs), a combination between these and antibiotics is one of the most promising therapies. Therefore, building a virtual screening model with prediction capacities for the efflux pump inhibitory activities of candidates from DrugBank and ZINC15 dataset, is one of the key goals of this project. Based on the database of 170 diverse chemical structures collected from 28 research journals, two 2D-QSAR models and a 3D-pharmacophore model have been performed. On the AcrB protein (PDB 4DX7), two binding sites have been discovered that match to the hydrophobic trap in the distal pocket and the switch loop in the proximal pocket. After virtual screening processes, twenty candidate AcrAB-TolC inhibitors have been subjected to molecular dynamics simulations, binding free energy calculations and ADMET predictions. The results indicate that three compounds namely DB09233, DB02581, and DB15224 are potential inhibitors with ΔGbind of -42.30 ± 4.58, -40.76 ± 7.30 and -31.06 ± 7.63 kcal.mol-1, respectively.Communicated by Ramaswamy H. Sarma.

Medicinal Chemistry of Inhibitors Targeting Resistant Bacteria

The discovery of antibiotics was a revolutionary feat that provided countless health benefits. The identification of penicillin by Alexander Fleming initiated the era of antibiotics, represented by constant discoveries that enabled effective treatments for the different classes of diseases caused by bacteria. However, the indiscriminate use of these drugs allowed the emergence of resistance mechanisms of these microorganisms against the available drugs. In addition, the constant discoveries in the 20th century generated a shortage of new molecules, worrying health agencies and professionals about the appearance of multidrug-resistant strains against available drugs. In this context, the advances of recent years in molecular biology and microbiology have allowed new perspectives in drug design and development, using the findings related to the mechanisms of bacterial resistance to generate new drugs that are not affected by such mechanisms and supply new molecules to be used to treat resistant bacterial infections. Besides, a promising strategy against bacterial resistance is the combination of drugs through adjuvants, providing new expectations in designing new antibiotics and new antimicrobial therapies. Thus, this manuscript will address the main mechanisms of bacterial resistance under the understanding of medicinal chemistry, showing the main active compounds against efflux mechanisms, and also the application of the use of drug delivery systems, and finally, the main potential natural products as adjuvants or with promising activity against resistant strains.

Efflux pump inhibitors for bacterial pathogens: From bench to bedside

With the advent of antibiotics, bacterial infections were supposed to be a thing of past. However, this instead led to the selection and evolution of bacteria with mechanisms to counter the action of antibiotics. Antibiotic efflux is one of the major mechanisms, whereby bacteria pump out the antibiotics from their cellular interior to the external environment using special transporter proteins called efflux pumps. Inhibiting these pumps seems to be an attractive strategy at a time when novel antibiotic supplies are dwindling. Molecules capable of inhibiting these pumps, known as efflux pump inhibitors (EPIs), have been viewed as potential therapeutic agents that can rejuvenate the activity of antibiotics that are no longer effective against bacterial pathogens. EPIs follow some general mechanisms of efflux inhibition and are derived from various natural as well as synthetic sources. This review focuses on EPIs and identifies the challenges that have kept these futuristic therapeutics away from the commercial realm so far.

Effectiveness of Efflux Pump Inhibitors as Biofilm Disruptors and Resistance Breakers in Gram-Negative (ESKAPEE) Bacteria

Antibiotic resistance represents a significant threat to the modern healthcare provision. The ESKAPEE pathogens (Enterococcus faecium., Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp. and Escherichia coli), in particular, have proven to be especially challenging to treat, due to their intrinsic and acquired ability to rapidly develop resistance mechanisms in response to environmental threats. The development of biofilm has been characterised as an essential contributing factor towards antimicrobial-resistance and tolerance. Several studies have implicated the involvement of efflux pumps in antibiotic resistance, both directly, via drug extrusion and indirectly, through the formation of biofilm. As a result, the underlying mechanism of these pumps has attracted considerable interest due to the potential of targeting these protein structures and developing novel adjunct therapies. Subsequent investigations have revealed the ability of efflux pump-inhibitors (EPIs) to block drug-extrusion and disrupt biofilm formation, thereby, potentiating antibiotics and reversing resistance of pathogen towards them. This review will discuss the potential of EPIs as a possible solution to antimicrobial resistance, examining different challenges to the design of these compounds, with an emphasis on Gram-negative ESKAPEE pathogens.

Reversing Antimicrobial Resistance in Multidrug-Resistant Klebsiella pneumoniae of Clinical Origin Using 1-(1-Naphthylmethyl)-Piperazine

Eleven clinical Klebsiella pneumoniae fluoroquinolone-resistant isolates were tested to access the potential of adjuvant therapies to reduce antimicrobial resistance using fixed concentrations of the chemosensitizers chlorpromazine (CPZ), thioridazine (TZ), phenylalanine-arginine-β-naphthylamide (PAβN), and 1-(1-naphthylmethyl)-piperazine-(NMP) with varying concentrations of antimicrobial agents nalidixic acid (NAL), ciprofloxacin (CIP), moxifloxacin (MXF), tetracycline (TET), and chloramphenicol (CHL). Ethidium bromide dye was used together with the chemosensitizers to investigate permeabilization effects. NMP was assessed for its capacity to reduce the mass of biofilm alone and in combination with CIP and MXF. Of the selected chemosensitizers, NMP exhibited the greatest capacity to reverse resistance and inhibit efflux, based on the concentrations tested. Susceptibility to antimicrobial agents including (fluoro)quinolones, TET, and CHL were found to be increased in the presence of NMP, in a concentration-dependent manner. PAβN also demonstrated similar effects when combined with the chemosensitizers tested. In the case of half of the isolates studied, NMP alone reduced preformed biofilm biomass. Combinations of latter along with CIP or MXF were also found to reduce the mass of preformed biofilm, in the case of only some of the bacterial isolates. The capacity of NMP to reduce antimicrobial resistance could be of relevance as a strategy to limit bacterial colonization on abiotic surfaces.

DNA Damage Repair and Drug Efflux as Potential Targets for Reversing Low or Intermediate Ciprofloxacin Resistance in E. coli K-12

Ciprofloxacin is a potent antibacterial drug that is widely used in human clinical applications. As a consequence of its extensive use, resistance has emerged in almost all clinically relevant bacterial species. A mean to combat the observed ciprofloxacin resistance is by reversing it via co-administration of a potentiating compound, also known as a helper drug. Here, we report on the current advances in identifying ciprofloxacin helper drugs, and put them into perspective of our own findings. We searched for potential helper drug targets in Escherichia coli strains with different levels of ciprofloxacin resistance using transcriptomics i.e., RNAseq and by deletion of genes associated with hyper-susceptibility to ciprofloxacin. Differential gene expression analysis of the highly ciprofloxacin resistant uropathogenic E. coli strain, ST131 UR40, treated with a clinically relevant concentration of ciprofloxacin (2 μg/mL), showed that the transcriptome was unaffected. Conversely, genetic screening of 23 single gene deletions in the high-level ciprofloxacin resistant laboratory derived E. coli strain, LM693, led to a significant decrease in the minimal inhibitory concentration for several genes, including genes encoding the AcrAB-TolC efflux pump, SOS-response proteins and the global regulator Fis. In addition, deletion of acrA, tolC, recA, or recC rendered two E. coli strains with intermediate susceptibility to ciprofloxacin fully susceptible according to the CLSI recommended breakpoint. Our results corroborate the AcrAB-TolC efflux pump and the SOS response proteins, RecA and RecC, as potential targets for ciprofloxacin helper drugs in treatment of human bacterial infections caused by E. coli strains with intermediate sensitivity to ciprofloxacin.

Quantifying the Evolutionary Conservation of Genes Encoding Multidrug Efflux Pumps in the ESKAPE Pathogens To Identify Antimicrobial Drug Targets

Increasing rates of antibiotic-resistant bacterial infection are one of the most pressing contemporary global health concerns. The ESKAPE pathogen group represents the leading cause of these infections, and upregulation of efflux pump expression is a significant mechanism of resistance in these pathogens. This has resulted in substantial interest in the development of efflux pump inhibitors to combat antibiotic-resistant infections; however, no widespread treatments have been developed to date. Our study evaluates an often-underappreciated aspect of resistance—the impact of evolutionary selection. We evaluate selection on all annotated efflux genes in all sequenced ESKAPE pathogens, providing critical context for and insight into current and future development of efflux-targeting treatments for resistant bacterial infections.

Increasing rates of antibiotic-resistant bacterial infection are one of the most pressing contemporary global health concerns. The ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) have been identified as the leading global cause of multidrug-resistant bacterial infections, and overexpression of multidrug efflux (MEX) transport systems has been identified as one of the most critical mechanisms facilitating the evolution of multidrug resistance in ESKAPE pathogens. Despite efforts to develop efflux pump inhibitors to combat antibiotic resistance, the need persists to identify additional targets for future investigations. We evaluated evolutionary pressures on 110 MEX-encoding genes from all annotated ESKAPE organism genomes. We identify several MEX genes under stabilizing selection—representing targets which can facilitate broad-spectrum treatments with evolutionary constraints limiting the potential emergence of escape mutants. We also examine MEX systems being evaluated as drug targets, demonstrating that divergent selection may underlie some of the problems encountered in the development of effective treatments—specifically in relation to the NorA system in S. aureus. This study provides a comprehensive evolutionary context to efflux in the ESKAPE pathogens, which will provide critical context to the evaluation of efflux systems as antibiotic targets.

IMPORTANCE Increasing rates of antibiotic-resistant bacterial infection are one of the most pressing contemporary global health concerns. The ESKAPE pathogen group represents the leading cause of these infections, and upregulation of efflux pump expression is a significant mechanism of resistance in these pathogens. This has resulted in substantial interest in the development of efflux pump inhibitors to combat antibiotic-resistant infections; however, no widespread treatments have been developed to date. Our study evaluates an often-underappreciated aspect of resistance—the impact of evolutionary selection. We evaluate selection on all annotated efflux genes in all sequenced ESKAPE pathogens, providing critical context for and insight into current and future development of efflux-targeting treatments for resistant bacterial infections.

Mode of action of the 2-phenylquinoline efflux inhibitor PQQ4R against Escherichia coli

Efflux pump inhibitors are of great interest since their use as adjuvants of bacterial chemotherapy can increase the intracellular concentrations of the antibiotics and assist in the battle against the rising of antibiotic-resistant bacteria. In this work, we have described the mode of action of the 2-phenylquinoline efflux inhibitor (4-(2-(piperazin-1-yl)ethoxy)-2-(4-propoxyphenyl) quinolone - PQQ4R), against Escherichia coli, by studding its efflux inhibitory ability, its synergistic activity in combination with antibiotics, and compared its effects with the inhibitors phenyl-arginine-β-naphthylamide (PAβN) and chlorpromazine (CPZ). The results showed that PQQ4R acts synergistically, in a concentration dependent manner, with antibiotics known to be subject to efflux in E. coli reducing their MIC in correlation with the inhibition of their efflux. Real-time fluorometry assays demonstrated that PQQ4R at sub-inhibitory concentrations promote the intracellular accumulation of ethidium bromide inhibiting its efflux similarly to PAβN or CPZ, well-known and described efflux pump inhibitors for Gram-negative bacteria and whose clinical usage is limited by their levels of toxicity at clinical and bacteriological effective concentrations. The time-kill studies showed that PQQ4R, at bactericidal concentrations, has a rapid antimicrobial activity associated with a fast decrease of the intracellular ATP levels. The results also indicated that the mode of action of PQQ4R involves the destabilization of the E. coli inner membrane potential and ATP production impairment, ultimately leading to efflux pump inhibition by interference with the energy required by the efflux systems. At bactericidal concentrations, membrane permeabilization increases and finally ATP is totally depleted leading to cell death. Since drug resistance mediated by the activity of efflux pumps depends largely on the proton motive force (PMF), dissipaters of PMF such as PQQ4R, can be regarded as future adjuvants of conventional therapy against E. coli and other Gram-negative bacteria, especially their multidrug resistant forms. Their major limitation is the high toxicity for human cells at the concentrations needed to be effective against bacteria. Their future molecular optimization to improve the efflux inhibitory properties and reduce relative toxicity will optimize their potential for clinical usage against multi-drug resistant bacterial infections due to efflux.

Efflux as a mechanism of antimicrobial drug resistance in clinical relevant microorganisms: the role of efflux inhibitors

INTRODUCTION

Microbial resistance against antibiotics is a serious threat to the effective treatment of infectious diseases. Several mechanisms exist through which microorganisms can develop resistance against antimicrobial drugs, of which the overexpression of genes to produce efflux pumps is a major concern. Several efflux transporters have been identified in microorganisms, which infer resistance against specific antibiotics and even multidrug resistance. Areas covered: This paper focuses on microbial resistance against antibiotics by means of the mechanism of efflux and gives a critical overview of studies conducted to overcome this problem by combining efflux pump inhibitors with antibiotics. Information was obtained from a literature search done with MEDLINE, Pubmed, Scopus, ScienceDirect, OneSearch and EBSCO host. Expert opinion: Efflux as a mechanism of multidrug resistance has presented a platform for improved efficacy against resistant microorganisms by co-administration of efflux pump inhibitors with antimicrobial agents. Although proof of concept has been shown for this approach with in vitro experiments, further research is needed to develop more potent inhibitors with low toxicity which is clinically effective.

Novel Piperazine Arylideneimidazolones Inhibit the AcrAB-TolC Pump in Escherichia coli and Simultaneously Act as Fluorescent Membrane Probes in a Combined Real-Time Influx and Efflux Assay

In this study, we tested five compounds belonging to a novel series of piperazine arylideneimidazolones for the ability to inhibit the AcrAB-TolC efflux pump. The biphenylmethylene derivative (BM-19) and the fluorenylmethylene derivative (BM-38) were found to possess the strongest efflux pump inhibitor (EPI) activities in the AcrAB-TolC-overproducingEscherichia colistrain 3-AG100, whereas BM-9, BM-27, and BM-36 had no activity at concentrations of up to 50 μM in a Nile red efflux assay. MIC microdilution assays demonstrated that BM-19 at 1/4 MIC (intrinsic MIC, 200 μM) was able to reduce the MICs of levofloxacin, oxacillin, linezolid, and clarithromycin 8-fold. BM-38 at 1/4 MIC (intrinsic MIC, 100 μM) was able to reduce only the MICs of oxacillin and linezolid (2-fold). Both compounds markedly reduced the MIC of rifampin (BM-19, 32-fold; and BM-38, 4-fold), which is suggestive of permeabilization of the outer membrane as an additional mechanism of action. Nitrocefin hydrolysis assays demonstrated that in addition to their EPI activity, both compounds were in fact weak permeabilizers of the outer membrane. Moreover, it was found that BM-19, BM-27, BM-36, and BM-38 acted as near-infrared-emitting fluorescent membrane probes, which allowed for their use in a combined influx and efflux assay and thus for tracking of the transport of an EPI across the outer membrane by an efflux pump in real time. The EPIs BM-38 and BM-19 displayed the most rapid influx of all compounds, whereas BM-27, which did not act as an EPI, showed the slowest influx.

The ins and outs of RND efflux pumps in Escherichia coli

Infectious diseases remain one of the principal causes of morbidity and mortality in the world. Relevant authorities including the WHO and CDC have expressed serious concern regarding the continued increase in the development of multidrug resistance among bacteria. They have also reaffirmed the urgent need for investment in the discovery and development of new antibiotics and therapeutic approaches to treat multidrug resistant (MDR) bacteria. The extensive use of antimicrobial compounds in diverse environments, including farming and healthcare, has been identified as one of the main causes for the emergence of MDR bacteria. Induced selective pressure has led bacteria to develop new strategies of defense against these chemicals. Bacteria can accomplish this by several mechanisms, including enzymatic inactivation of the target compound; decreased cell permeability; target protection and/or overproduction; altered target site/enzyme and increased efflux due to over-expression of efflux pumps. Efflux pumps can be specific for a single substrate or can confer resistance to multiple antimicrobials by facilitating the extrusion of a broad range of compounds including antibiotics, heavy metals, biocides and others, from the bacterial cell. To overcome antimicrobial resistance caused by active efflux, efforts are required to better understand the fundamentals of drug efflux mechanisms. There is also a need to elucidate how these mechanisms are regulated and how they respond upon exposure to antimicrobials. Understanding these will allow the development of combined therapies using efflux inhibitors together with antibiotics to act on Gram-negative bacteria, such as the emerging globally disseminated MDR pathogen Escherichia coli ST131 (O25:H4). This review will summarize the current knowledge on resistance-nodulation-cell division efflux mechanisms in E. coli, a bacteria responsible for community and hospital-acquired infections, as well as foodborne outbreaks worldwide.

RND-type drug efflux pumps from Gram-negative bacteria: molecular mechanism and inhibition

Drug efflux protein complexes confer multidrug resistance on bacteria by transporting a wide spectrum of structurally diverse antibiotics. Moreover, organisms can only acquire resistance in the presence of an active efflux pump. The substrate range of drug efflux pumps is not limited to antibiotics, but it also includes toxins, dyes, detergents, lipids, and molecules involved in quorum sensing; hence efflux pumps are also associated with virulence and biofilm formation. Inhibitors of efflux pumps are therefore attractive compounds to reverse multidrug resistance and to prevent the development of resistance in clinically relevant bacterial pathogens. Recent successes on the structure determination and functional analysis of the AcrB and MexB components of the AcrAB-TolC and MexAB-OprM drug efflux systems as well as the structure of the fully assembled, functional triparted AcrAB-TolC complex significantly contributed to our understanding of the mechanism of substrate transport and the options for inhibition of efflux. These data, combined with the well-developed methodologies for measuring efflux pump inhibition, could allow the rational design, and subsequent experimental verification of potential efflux pump inhibitors (EPIs). In this review we will explore how the available biochemical and structural information can be translated into the discovery and development of new compounds that could reverse drug resistance in Gram-negative pathogens. The current literature on EPIs will also be analyzed and the reasons why no compounds have yet progressed into clinical use will be explored.

The challenge of efflux-mediated antibiotic resistance in Gram-negative bacteria

The global emergence of multidrug-resistant Gram-negative bacteria is a growing threat to antibiotic therapy. The chromosomally encoded drug efflux mechanisms that are ubiquitous in these bacteria greatly contribute to antibiotic resistance and present a major challenge for antibiotic development. Multidrug pumps, particularly those represented by the clinically relevant AcrAB-TolC and Mex pumps of the resistance-nodulation-division (RND) superfamily, not only mediate intrinsic and acquired multidrug resistance (MDR) but also are involved in other functions, including the bacterial stress response and pathogenicity. Additionally, efflux pumps interact synergistically with other resistance mechanisms (e.g., with the outer membrane permeability barrier) to increase resistance levels. Since the discovery of RND pumps in the early 1990s, remarkable scientific and technological advances have allowed for an in-depth understanding of the structural and biochemical basis, substrate profiles, molecular regulation, and inhibition of MDR pumps. However, the development of clinically useful efflux pump inhibitors and/or new antibiotics that can bypass pump effects continues to be a challenge. Plasmid-borne efflux pump genes (including those for RND pumps) have increasingly been identified. This article highlights the recent progress obtained for organisms of clinical significance, together with methodological considerations for the characterization of MDR pumps.