New Methods for the Identification of Efflux Mediated MDR Bacteria, Genetic Assessment of Regulators and Efflux Pump Constituents, Characterization of Efflux Systems and Screening for Inhibitors of Efflux Pumps

Enhanced Efficacy of Ciprofloxacin and Tobramycin against Staphylococcus aureus When Combined with Corydalis Tuber and Berberine through Efflux Pump Inhibition

One way that bacteria develop antibiotic resistance is by reducing intracellular antibiotic concentrations through efflux pumps. Therefore, enhancing the efficacy of antibiotics using efflux pump inhibitors provides a way to overcome this type of resistance. Notably, an increasing number of pathogenic Staphylococcus aureus strains have efflux pump genes. In this study, the extract from Corydalis ternata Nakai tuber (Corydalis Tuber) at 512 mg/L was demonstrated to have an antibiotic synergistic effect with ciprofloxacin at 2 mg/L and tobramycin at 1024 mg/L against methicillin-resistant S. aureus (MRSA). Berberine, an isoquinoline alkaloid identified in Corydalis Tuber, was identified as contributing to this effect. Ethidium bromide efflux pump activity assays showed that Corydalis Tuber extract and berberine inhibited efflux, suggesting that they are efflux pump inhibitors. Molecular docking simulations suggested that berberine binds to S. aureus efflux pump proteins MepA, NorA, NorB, and SdrM. Additionally, berberine and Corydalis Tuber extract inhibit biofilm formation, which can confer antibiotic resistance. This study's findings suggest that Corydalis Tuber, a traditional herbal medicine, and berberine, a medicinal supplement, act as S. aureus efflux pump inhibitors, synergistically increasing the efficacy of ciprofloxacin and tobramycin and showing promise as a treatment for antibiotic-resistant S. aureus infections, including MRSA.

Meldrum's acid derivates are MepA efflux pump inhibitors: In vitro and in silico essays

Efflux pumps are proteins capable of expelling antibiotics from bacterial cells, have emerged as a major mechanism of bacterial resistance. In the ongoing pursuit to overcome and reduce bacterial resistance, novel substances are being explored as potential efflux pump inhibitors. Meldrum's acid, a synthetic molecule widely studied for its role in synthesizing bioactive compounds, holds promise in this regard. Therefore, the objective of this study is to evaluate the antibacterial activity of three derivatives of Meldrum's acid and assess their ability to inhibit efflux mechanisms, employing both in silico and in vitro approaches. The antibacterial activity of the derivatives was assessed using a broth microdilution testing method. Surprisingly, the derivatives did not exhibit direct antibacterial activity on their own. However, they displayed a significant effect in enhancing the efficacy of antibiotics, suggesting a potential role in potentiating their effects. Furthermore, fluorescence emission assays using ethidium bromide indicated that the derivatives could potentially block efflux pumps, as they exhibited fluorescence levels comparable to the positive control. To further investigate their inhibitory capacity, molecular docking studies were conducted in silico, revealing binding interactions similar to ciprofloxacin and carbonyl cyanide 3-chlorophenylhydrazone, known efflux pump inhibitors. These findings highlight the potential of Meldrum's acid derivatives as effective inhibitors of efflux pumps. By targeting these mechanisms, the derivatives offer a promising avenue to enhance the effectiveness of antibiotics and combat bacterial resistance. This study underscores the importance of exploring novel strategies in the fight against bacterial resistance and provides valuable insights into the potential of Meldrum's acid derivatives as efflux pump inhibitors. Further research and exploration in this field are warranted to fully exploit their therapeutic potential.

Isoniazid-N-acylhydrazones as promising compounds for the anti-tuberculosis treatment

Tuberculosis (TB), a disease caused by Mycobacterium tuberculosis complex, still presents significant numbers of incidence and mortality, in addition to several cases of drug resistance. Resistance, especially to isoniazid, which is one of the main drugs used in the treatment, has increased. In this context, N-acylhydrazones derived from isoniazid have shown important anti-Mycobacterium tuberculosis activity. Hence, this work aimed to determine the anti-TB potential of 11 isoniazid-N-acylhydrazones (INH-acylhydrazones). For this purpose, the determination of minimum inhibitory concentration (MIC) against M. tuberculosis H37Rv and clinical isolates was carried out. Drug combination, minimum bactericidal concentration, cytotoxicity, and in silico parameters were also performed. INH-acylhydrazones (2), (8), and (9) had MIC for M. tuberculosis H37Rv similar to or lower than isoniazid, and bactericidal activity was observed. In addition, these compounds showed low cytotoxicity, with a selectivity index greater than 3,000. Interesting results were also obtained in the drug combination assay, with synergistic combinations with isoniazid, ethambutol, and rifampicin. In the in silico study, INH-acylhydrazones behaved similarly to INH, but with improvements in some aspects. Based on these findings, it is concluded that compounds (2), (8), and (9) are considered promising scaffolds and warrant further investigation for designing future antimicrobial drugs.

A Highly Substituted Ring-Fused 2-Pyridone Compound Targeting PrfA and the Efflux Regulator BrtA in Listeria monocytogenes

Listeria monocytogenes is a facultative Gram-positive bacterium that causes listeriosis, a severe foodborne disease. We previously discovered that ring-fused 2-pyridone compounds can decrease virulence factor expression in Listeria by binding and inactivating the PrfA virulence activator. In this study, we tested PS900, a highly substituted 2-pyridone that was recently discovered to be bactericidal to other Gram-positive pathogenic bacteria, such as Staphylococcus aureus and Enterococcus faecalis. We show that PS900 can interact with PrfA and reduce the expression of virulence factors. Unlike previous ring-fused 2-pyridones shown to inactivate PrfA, PS900 had an additional antibacterial activity and was found to potentiate sensitivity toward cholic acid. Two PS900-tolerant mutants able to grow in the presence of PS900 carried mutations in the brtA gene, encoding the BrtA repressor. In wild-type (WT) bacteria, cholic acid binds and inactivates BrtA, thereby alleviating the expression of the multidrug transporter MdrT. Interestingly, we found that PS900 also binds to BrtA and that this interaction causes BrtA to dissociate from its binding site in front of the mdrT gene. In addition, we observed that PS900 potentiated the effect of different osmolytes. We suggest that the increased potency of cholic acid and osmolytes to kill bacteria in the presence of PS900 is due to the ability of the latter to inhibit general efflux, through a yet-unknown mechanism. Our data indicate that thiazolino 2-pyridones constitute an attractive scaffold when designing new types of antibacterial agents. IMPORTANCE Bacteria resistant to one or several antibiotics are a very large problem, threatening not only treatment of infections but also surgery and cancer treatments. Thus, new types of antibacterial drugs are desperately needed. In this work, we show that a new generation of substituted ring-fused 2-pyridones not only inhibit Listeria monocytogenes virulence gene expression, presumably by inactivating the PrfA virulence regulator, but also potentiate the bactericidal effects of cholic acid and different osmolytes. We identified a multidrug repressor as a second target of 2-pyridones. The repressor-2-pyridone interaction displaces the repressor from DNA, thus increasing the expression of a multidrug transporter. In addition, our data suggest that the new class of ring-fused 2-pyridones are efficient efflux inhibitors, possibly explaining why the simultaneous addition of 2-pyridones together with cholic acid or osmolytes is detrimental for the bacterium. This work proves conclusively that 2-pyridones constitute a promising scaffold to build on for future antibacterial drug design.

Exploring Efflux as a Mechanism of Reduced Susceptibility towards Biocides and Fluoroquinolones in Staphylococcus pseudintermedius

Staphylococcus pseudintermedius is the main bacterial cause of skin and soft tissue infections (SSTIs) in companion animals, particularly dogs. The emergence of methicillin-resistant S. pseudintermedius (MRSP) strains, frequently with multidrug resistance phenotypes is a public health concern. This study aimed to evaluate efflux, a resistance mechanism still poorly characterized in S. pseudintermedius, as a contributor to biocide and fluoroquinolone resistance. Susceptibility to the efflux pump substrates ethidium bromide (EtBr), tetraphenylphosphonium bromide (TPP) and ciprofloxacin (CIP) was evaluated by minimum inhibitory concentration (MIC) determination for 155 SSTIs-related S. pseudintermedius in companion animals. EtBr and TPP MIC distributions were analyzed to estimate cut-off (CO_WT) values. The effect of the efflux inhibitors (EIs) thioridazine and verapamil was assessed upon MICs and fluorometric EtBr accumulation assays, performed with/without glucose and/or EIs. This approach detected a non-wild type population towards TPP with increased efflux, showed to be strain-specific and glucose-dependent. Resistance to fluoroquinolones was mainly linked to target gene mutations, yet a contribution of efflux on CIP resistance levels could not be ruled out. In sum, this study highlights the relevance of efflux-mediated resistance in clinical S. pseudintermedius, particularly to biocides, and provides a methodological basis for further studies on the efflux activity on this important pathogen of companion animals.

Antibacterial Effect of 16 Essential Oils and Modulation of mex Efflux Pumps Gene Expression on Multidrug-Resistant Pseudomonas aeruginosa Clinical Isolates: Is Cinnamon a Good Fighter?

The purpose of the study was to describe the antimicrobial activity of 16 common essential oils (EOs) on multidrug-resistant (MDR) Pseudomonas aeruginosa clinical isolates, including the determination of the effects on mex efflux pumps gene expression. Seventy-two clinical isolates of P. aeruginosa collected between 2020-2022 were screened for susceptibility to EOs using Kirby-Bauer disk diffusion to identify potential candidates for future alternative therapies. The minimal inhibitory concentration (MIC) was further determined for the EO that proved antibacterial activity following the disk diffusion screening. Positive and negative controls were also used for method validation. Since cinnamon EO exhibited the best antimicrobial activity, it was further used to evaluate its influence on mex A, B, C, E, and X efflux pumps gene expression using real-time RT-PCR. Cinnamon EO inhibited all P. aeruginosa strains, followed by thyme EO (37.5%, n = 27) and lavender EO (12.5%, n = 9). The other EOs were less efficient. The MIC detection showed that cinnamon at a concentration of 0.05% v/v inhibited all MDR P. aeruginosa isolates. Thyme, turmeric, peppermint, basil, clove, and lavender EOs presented various results, most of them having activity at concentrations higher than 12.5% v/v. By studying the activity of cinnamon EO on mex efflux pumps, it was found that mexA and mexB (66.5%) were generally under-expressed. The remarkable results produced using the very low concentrations of cinnamon EO, with 100% antimicrobial activity against multi-, extended-, and pan- drug-resistant (MDR, XDR, PDR) P. aeruginosa clinical isolates, completed with the severe alteration of the RNA messaging system, supports its potential to be used as adjuvant treatment, with impact on therapeutic results.

Evaluation of antibacterial activity and reversal of the NorA and MepA efflux pump of estragole against Staphylococcus aureus bacteria

The antibacterial activity of the monoterpene estragole was evaluated against two strains of bacteria with an efflux pump mechanism, which are Staphylococcus aureus 1199B and S. aureus K2068, which have a NorA and MepA pump, respectively. For that, the methodology proposed by CLSI with modifications was followed, and to evaluate the reversal of the efflux pump, subinhibitory concentrations (MIC/8) of estragole and standard pump inhibitors, CCCP and Chlorpromazine were used and it was verified whether they managed to modulate the action of Norfloxacin, Ciprofloxacin and Ethidium Bromide, an indicator of an efflux pump. It was observed that estragole positively modulated norfloxacin and ethidium bromide against the strain of S. aureus 1199B and that it also managed to reduce the MIC of ethidium bromide against the strain of S. aureus K2068. In the non-clinical acute toxicity tests with estragole, the animals treated with the dose of 625 mg/kg/v.o. showed no clinical signs of toxicity, according to the parameters evaluated. These results are promising, since it places estragole as a possible inhibitor of the efflux pump, thus managing to inhibit this mechanism of action in the strains tested.

Chemical synthesis, molecular docking and MepA efflux pump inhibitory effect by 1,8-naphthyridines sulfonamides

This study aimed to evaluate the antibacterial activity and to verify, in silico and in vitro, the inhibition of efflux mechanisms using a series of synthesized 1,8-naphthyridines sulfonamides against Staphylococcus aureus strains carrying MepA efflux pumps. The chemical synthesis occurred through the thermolysis of the Meldrum's acid adduct. The sulfonamide derivatives were obtained by the sulfonylation of 2-amino-5‑chloro-1,8-naphthyridine with commercial benzenesulfonyl chloride. Antibacterial activity was assessed by the broth microdilution test. Efflux pump inhibitory capacity was evaluated in silico by molecular docking and in vitro by analyzing synergistic effects on ciprofloxacin and ethidium bromide (EtBr) and by EtBr fluorescence emission assays. The following 1,8-naphthyridines were synthesized: 4-methyl-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10a); 2,5-dichloro-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10b); 4-fluoro-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10c); 2,3,4-trifluoro-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10d); 3-trifluoromethyl-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10e); 4‑bromo-2,5-difluoro-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10f). The 1,8-naphthyridines derivatives associated with sulfonamides did not show antibacterial activity. However, they showed a favorable pharmacokinetic profile with possible MepA efflux pump inhibitory action, demonstrated in molecular docking. In addition to the promising results in reducing the concentration of intracellular EtBr. 1,8-naphthyridines act as putative agents in the inhibitory action of the MepA efflux pump.

Measuring Efflux and Permeability in Mycobacteria

Mycobacteria are intrinsically resistant to most antimicrobials, which is generally attributed to the impermeability of their cell wall that considerably limits drug uptake. Moreover, like in other pathogenic bacteria, active efflux systems have been widely characterized from diverse mycobacterial species in laboratory conditions, showing that they can promote resistance by extruding noxious compounds prior to their reaching their intended targets. Therefore, the intracellular concentration of a given compound is determined by the balance between permeability, influx, and efflux.Given the urgent need to discover and develop novel antimycobacterial compounds in order to design effective therapeutic strategies, the contributions to drug resistance made by the controlled permeability of the cell wall and the increased activity of efflux pumps must be determined. In this chapter, we will describe a method that allows (1) the measuring of permeability and the quantification of general efflux activity of mycobacteria, by the study of the transport (influx and efflux) of fluorescent compounds, such as ethidium bromide; and (2) the screening of compounds in search of agents that increase the permeability of the cell wall and efflux inhibitors that could restore the effectiveness of antimicrobials that are subject to efflux.

In vitro and in silico inhibitory effects of synthetic and natural eugenol derivatives against the NorA efflux pump in Staphylococcus aureus

Staphylococcus aureus is a Gram-positive bacterium responsible for a number of diseases and has demonstrated resistance to conventional antibiotics. This study aimed to evaluate the antibacterial activity of eugenol and its derivatives allylbenzene, 4-allylanisole, isoeugenol and 4-allyl-2,6-dimethoxyphenol against the S. aureus NorA efflux pump (EP) in association with norfloxacin and ethidium bromide. The antibacterial activity of the compounds was assessed using the broth microdilution method to determine the minimum inhibitory concentration (MIC). A reduction in the MIC of ethidium bromide (a substrate for several efflux pumps) or norfloxacin was used as a parameter of EP inhibition. Molecular modeling studies were used to predict the 3D structure and analyze the interaction of selected compounds with the binding pocket of the NorA efflux pump. Except for 4-allylanisole and allylbenzene, the compounds presented clinically effective antibacterial activity. When associated with norfloxacin against the SA 1199B strain, 4-allyl-2,6-dimethoxyphenol eugenol and isoeugenol caused significant reduction in the MIC of the antibiotic, demonstrating synergistic effects. Similar effects were observed when 4-allyl-2,6-dimethoxyphenol, allylbenzene and isoeugenol were associated with ethidium bromide. Together, these findings indicate a potential inhibition of the NorA pump by eugenol and its derivatives. This in vitro evidence was corroborated by docking results demonstrating favorable interactions between 4-allyl-2,6-dimetoxypheno and the NorA pump mediated by hydrogen bonds and hydrophobic interactions. In conclusion, eugenol derivatives have the potential to be used in antibacterial drug development in strains carrying the NorA efflux pump.

Effect of α-Bisabolol and Its β-Cyclodextrin Complex as TetK and NorA Efflux Pump Inhibitors in Staphylococcus aureus Strains

Efflux pumps are proteins present in the plasma membrane of bacteria, which transport antibiotics and other compounds into the extracellular medium, conferring resistance. The discovery of natural efflux pump inhibitors is a promising alternative. α-Bisabolol is a sesquiterpene isolated from several plants such as Matricaria chamomilla L. and has important properties such as antibacterial and anti-inflammatory activity. Currently, the formation of inclusion complexes with β-Cyclodextrin has been used for improving the physicochemical characteristics of the host molecule. This study evaluated the effect of α-Bisabolol, in isolation and in complexation with β-Cyclodextrin, as TetK and NorA efflux pump inhibitors in Staphylococcus aureus strains. The minimum inhibitory concentration (MIC) was determined. Subsequently, inhibitory activity over the pumps was observed by an MIC reduction for the antibiotics, by using subinhibitory concentrations (MIC/8) in combination with tetracycline and norfloxacin. The MIC of the compounds was ≥1024 μg/mL. α-Bisabolol potentiated the action of tetracycline and reduced the MIC of norfloxacin to a clinically relevant concentration. The complexed substance showed synergism however, the effect of the isolated α-Bisabolol was superior to the complex. These results indicate α-Bisabolol is a potential substance to be used as an efflux pump inhibitor.

Usnic acid modifies MRSA drug resistance through down-regulation of proteins involved in peptidoglycan and fatty acid biosynthesis

Multidrug‐resistant Staphylococcus aureus infections place a huge burden on the healthcare sector and the wider community. An increasing rate of infections caused by methicillin‐resistant Staphylococcus aureus (MRSA) has necessitated the development of alternative agents. We previously reported that usnic acid (UA) has activity against MRSA; here, we report the effect of UA in combination with norfloxacin on the drug resistance of MRSA clinical isolates. We observed that the combination of UA–norfloxacin significantly reduces the bacterial burden in mouse models infected with S. aureus, without causing any detectable associated toxicity. Proteomic analysis indicated that UA–norfloxacin induces oxidative stress within cells, which leads to membrane damage and inhibits metabolic activity and biosynthesis of peptidoglycan and fatty acids. Collectively, this study provides evidence that UA in combination with norfloxacin may be a potential candidate for development into a resistance‐modifying agent for the treatment of invasive MRSA infections.

This is the first report on the drug resistance‐modifying potential of usnic acid (UA) through inhibition of the multidrug resistance (MDR) efflux pump and down‐regulation of proteins involved in peptidoglycan and fatty acid biosynthesis. This compound may be helpful in the management of infection caused by MRSA through (a) lowering the prescribed amount of antibiotics, (b) decreasing MDR generation, and (c) intensifying the efficacy of antibiotics against MRSA/VRSA under both in vitro and in vivo conditions. These results may be helpful in the development of anti‐MRSA drug combinations from economical and non‐toxic natural products.

Multidrug efflux pumps and their role in antibiotic and antiseptic resistance: a pharmacodynamic perspective

INTRODUCTION

Worrying levels of bacterial resistance have been reported worldwide involving the failure of many available antibiotic treatments. Multidrug resistance (MDR) in Gram-negative bacteria is often ascribed to the presence of multiple and different resistance mechanisms in the same strain. RND efflux pumps play a major role and are an attractive target to discover new antibacterial drugs. Areas covered: This review discusses the prevalence of efflux pumps, their overexpression in clinical scenarios, their polyselectivity, their effect on the intracellular concentrations of various antibiotics associated with the alteration of the membrane permeability and their involvement in pathogenicity are discussed. Expert opinion: Efflux pumps are new targets for the development of adjuvant in antibiotic treatments by of efflux pump inhibition. They may allow us to rejuvenate old antibiotics acting on their concentration inside the bacteria and thus potentiating their activity while blocking the release of virulence factors. It is a pharmacodynamic challenge to finalize new combined therapy.

Drug Resistance Reversal Potential of Ursolic Acid Derivatives against Nalidixic Acid- and Multidrug-resistant Escherichia coli

As a part of our drug discovery program, ursolic acid was chemically transformed into six semi-synthetic derivatives, which were evaluated for their antibacterial and drug resistance reversal potential in combination with conventional antibiotic nalidixic acid against the nalidixic acid-sensitive and nalidixic acid-resistant strains of Escherichia coli. Although ursolic acid and its all semi-synthetic derivatives did not show antibacterial activity of their own, but in combination, they significantly reduced the minimum inhibitory concentration of nalidixic acid up to eightfold. The 3-O-acetyl-urs-12-en-28-isopropyl ester (UA-4) and 3-O-acetyl-urs-12-en-28-n-butyl ester (UA-5) derivatives of ursolic acid reduced the minimum inhibitory concentration of nalidixic acid by eightfold against nalidixic acid-resistant and four and eightfold against nalidixic acid-sensitive, respectively. The UA-4 and UA-5 were further evaluated for their synergy potential with another antibiotic tetracycline against the multidrug-resistant clinical isolate of Escherichia coli-KG4. The results showed that both these derivatives in combination with tetracycline reduced the cell viability in concentration-dependent manner by significantly inhibiting efflux pump. This was further supported by the in silico binding affinity of UA-4 and UA-5 with efflux pump proteins. These ursolic acid derivatives may find their potential use as synergistic agents in the treatment of multidrug-resistant Gram-negative infections.

Measuring efflux and permeability in mycobacteria

The intrinsic resistance of mycobacteria to most antimicrobial agents is mainly attributed to the synergy between their relatively impermeable cell wall and efflux systems. The mycobacterial cell wall is rich in lipids and polysaccharides making a compact envelope that limits drug uptake. Changes in cell wall composition or structure lead to variations in susceptibility to drugs. Bacterial efflux pumps are membrane proteins that are capable of actively transporting a broad range of substrates, including drugs, from the cytoplasm to the extracellular environment. Increased expression of efflux pump genes confers a low level resistance phenotype, and under these conditions, bacteria may have greater chances of acquiring chromosomal mutation(s) conferring higher levels of drug resistance. In order to develop effective antimycobacterial therapeutic strategies, the contributions to drug resistance made by the limited permeability of the cell wall and the increased expression of efflux pumps must be understood. In this chapter, we describe a method that allows: (1) the quantification of general efflux activity of mycobacterial strains (clinical isolates, mutants impaired in efflux or permeability) by the study of the transport (influx and efflux) of fluorescent compounds, such as ethidium bromide; and (2) the screening of compounds in search of inhibitors of efflux pumps, which could restore the effectiveness of antimicrobials that are subject to efflux.

Membrane transport systems and the biodegradation potential and pathogenicity of genus Rhodococcus

The Rhodococcus genus contains species with remarkable ability to tolerate toxic compounds and to degrade a myriad of substrates. These substrates have to cross a distinctive cell envelope dominated by mycolic acids anchored in a scaffold of arabinogalactan covalently attached to the cell wall peptidoglycan, and a cellular membrane with phospholipids, whose composition in fatty acids can be rapidly altered in response to environmental conditions. The hydrophobic nature of the cell envelope facilitates the entrance of hydrophobic molecules but some substrates require active transport systems. Additionally, toxic compounds may also be extruded by energy spending efflux systems. In this review, physiological evidences of the use of transport systems by Rhodococcus strains and genomic studies that corroborate their existence are presented and discussed. The recently released complete genomes of several Rhodococcus strains will be the basis for an in silico correlation analysis between the efflux pumps present in the genome and their role on active transport of substrates. These transport systems will be placed on an integrative perspective of the impact of this important genus on biotechnology and health, ranging from bioremediation to antibiotic and biocide resistance.

Sistemas de efluxo multidrogas em Escherichia coli e uso de inibidores como possíveis adjuvantes na terapia da mastite bovina

Vinte e sete isolados de Escherichia coli provenientes de leite de bovinos com mastite clínica foram submetidos a teste de sensibilidade aos principais antimicrobianos usados no tratamento desta doença. Avaliou-se também a efetividade in vitro de dois inibidores de sistemas de efluxo multidrogas, fenilalanina arginyl ß naftilamida (PAβN) e 1-(1-Naphthylmethyl)-piperazine (NMP), utilizando-se a concentração inibitória mínima (CIM) como referência. A CIM e o sistema de efluxo foram detectados com base nas curvas de crescimento, utilizando-se a densidade óptica (D.O550), em diferentes concentrações da droga e na presença e ausência dos inibidores. Apenas quatro isolados apresentaram resistência à ampicilina e ao sulfametoxazol∕trimetoprim, simultaneamente, enquanto todos os 27 foram sensíveis aos demais antimicrobianos. Observaram-se valores para ampicilina variando de 6µg/mL a 250µg/mL e para sulfametoxazol∕trimetoprim de 12µg/mL a 1500µg/mL. Houve redução das CIMs desses antimicrobianos em todos os isolados na presença dos inibidores, exceto para sulfametoxazol∕trimetoprim na presença de NMP. Conclui-se que esses isolados possuem um estreito perfil de resistência e que PAßN apresentou melhor efeito inibitório em relação à ampicilina e ao sulfametoxazol∕trimetoprim, apresentando-se como um candidato a adjuvante no tratamento da mastite.

Efflux pumps of Gram-negative bacteria: what they do, how they do it, with what and how to deal with them

This review discusses the relationship of the efflux pump (EP) system of Gram-negative bacteria to other antibiotic resistance mechanisms of the bacterium such as quorum sensing, biofilms, two component regulons, etc. The genetic responses of a Gram-negative to an antibiotic that render it immune to an antibiotic are also discussed. Lastly, the methods that have been developed for the identification of bacteria that over-express their EP system are presented in detail. Phenothiazines are well-known antipsychotic drugs with reported activity against bacterial EPs and other ancillary antibiotic mechanisms of the organism. Therefore these compounds will also be discussed.

Contribution of different mechanisms to the resistance to fluoroquinolones in clinical isolates of Salmonella enterica

Objectives

To study the potential factors include gene mutation, efflux pump and alteration of permeability associated with quinolone-resistance of Salmonella enterica strains isolated from patients with acute gastroenteritis and to evaluate the degree of synergistic activity of efflux pump inhibitors when combined with ciprofloxacin against resistant isolates.

Methods

Antimicrobial resistance patterns of fifty-eight Salmonella isolates were tested. Five isolates were selected to study the mechanism of resistance associated with quinolone group, including mutation in topoisomerase-encoding gene, altered cell permeability, and expression of an active efflux system. In addition, the combination between antibiotics and efflux pump inhibitors to overcome the microbial resistance was evaluated.

Results

Five Salmonella isolates totally resistant to all quinolones were studied. All isolates showed alterations in outer membrane proteins including disappearance of some or all of these proteins (Omp-A, Omp-C, Omp-D and Omp-F). Minimum inhibitory concentration values of ciprofloxacin were determined in the presence/absence of the efflux pump inhibitors: carbonyl cyanide m-chlorophenylhydrazone, norepinephrin and trimethoprim. Minimum inhibitory concentration values for two of the isolates were 2–4 fold lower with the addition of efflux pump inhibitors. All five Salmonella isolates were amplified for gyrA and parC genes and only two isolates were sequenced. S. Enteritidis 22 had double mutations at codon 83 and 87 in addition to three mutations at parC at codons 67, 76 and 80 whereas S. Typhimurium 57 had three mutations at codons 83, 87 and 119, but no mutations at parC.

Conclusions

Efflux pump inhibitors may inhibit the major AcrAB-TolC in Salmonella efflux systems which are the major efflux pumps responsible for multidrug resistance in Gram-negative clinical isolates.

A Simple Method for Assessment of MDR Bacteria for Over-Expressed Efflux Pumps

It is known that bacteria showing a multi-drug resistance phenotype use several mechanisms to overcome the action of antibiotics. As a result, this phenotype can be a result of several mechanisms or a combination of thereof. The main mechanisms of antibiotic resistance are: mutations in target genes (such as DNA gyrase and topoisomerase IV); over-expression of efflux pumps; changes in the cell envelope; down regulation of membrane porins, and modified lipopolysaccharide component of the outer cell membrane (in the case of Gram-negative bacteria). In addition, adaptation to the environment, such as quorum sensing and biofilm formation can also contribute to bacterial persistence. Due to the rapid emergence and spread of bacterial isolates showing resistance to several classes of antibiotics, methods that can rapidly and efficiently identify isolates whose resistance is due to active efflux have been developed. However, there is still a need for faster and more accurate methodologies. Conventional methods that evaluate bacterial efflux pump activity in liquid systems are available. However, these methods usually use common efflux pump substrates, such as ethidium bromide or radioactive antibiotics and therefore, require specialized instrumentation, which is not available in all laboratories.

In this review, we will report the results obtained with the Ethidium Bromide-agar Cartwheel method. This is an easy, instrument-free, agar based method that has been modified to afford the simultaneous evaluation of as many as twelve bacterial strains. Due to its simplicity it can be applied to large collections of bacteria to rapidly screen for multi-drug resistant isolates that show an over-expression of their efflux systems. The principle of the method is simple and relies on the ability of the bacteria to expel a fluorescent molecule that is substrate for most efflux pumps, ethidium bromide. In this approach, the higher the concentration of ethidium bromide required to produce fluorescence of the bacterial mass, the greater the efflux capacity of the bacterial cells. We have tested and applied this method to a large number of Gram-positive and Gram-negative bacteria to detect efflux activity among these multi-drug resistant isolates. The presumptive efflux activity detected by the Ethidium Bromide-agar Cartwheel method was subsequently confirmed by the determination of the minimum inhibitory concentration for several antibiotics in the presence and absence of known efflux pump inhibitors.

Resistance to Antimicrobials Mediated by Efflux Pumps in Staphylococcus aureus

Resistance mediated by efflux has been recognized in Staphylococcus aureus in the last few decades, although its clinical relevance has only been recognized recently. The existence of only a few studies on the individual and overall contribution of efflux to resistance phenotypes associated with the need of well-established methods to assess efflux activity in clinical isolates contributes greatly to the lack of solid knowledge of this mechanism in S. aureus. This study aims to provide information on approaches useful to the assessment and characterization of efflux activity, as well as contributing to our understanding of the role of efflux to phenotypes of antibiotic resistance and biocide tolerance in S. aureus clinical isolates. The results described show that efflux is an important contributor to fluoroquinolone resistance in S. aureus and suggest it as a major mechanism in the early stages of resistance development. We also show that efflux plays an important role on the reduced susceptibility to biocides in S. aureus, strengthening the importance of this long neglected resistance mechanism to the persistence and proliferation of antibiotic/biocide-resistant S. aureus in the hospital environment.

Involvement of Fe uptake systems and AmpC β-lactamase in susceptibility to the siderophore monosulfactam BAL30072 in Pseudomonas aeruginosa

BAL30072 is a monosulfactam conjugated with an iron-chelating dihydroxypyridone moiety. It is active against Gram-negative bacteria, including multidrug-resistant Pseudomonas aeruginosa. We selected mutants with decreased susceptibilities to BAL30072 in P. aeruginosa PAO1 under a variety of conditions. Under iron-deficient conditions, mutants with overexpression of AmpC β-lactamase predominated. These mutants were cross-resistant to aztreonam and ceftazidime. Similar mutants were obtained after selection at >16× the MIC in iron-sufficient conditions. At 4× to 8× the MIC, mutants with elevated MIC for BAL30072 but unchanged MICs for aztreonam or ciprofloxacin were selected. The expression of ampC and the major efflux pump genes were also unchanged. These BAL30072-specific mutants were characterized by transcriptome analysis, which revealed upregulation of the Fe-dicitrate operon, FecIRA. Whole-genome sequencing showed that this resulted from a single nucleotide change in the Fur-box of the fecI promoter. Overexpression of either the FecI ECF sigma factor or the FecA receptor increased BAL30072 MICs 8- to 16-fold. A fecI mutant and a fecA mutant of PAO1 were hypersusceptible to BAL30072 (MICs < 0.06 μg/ml). The most downregulated gene belonged to the pyochelin synthesis operon, although mutants in pyochelin receptor or synthesis genes had unchanged MICs. The piuC gene, coding for a Fe(II)-dependent dioxygenase located next to the piuA iron receptor gene, was also downregulated. The MICs of BAL30072 for piuC and piuA transposon mutants were increased 8- and 16-fold, respectively. We conclude that the upregulation of the Fe-dicitrate system impacts the expression of other TonB-dependent iron transporters and that PiuA and PiuC contribute to the susceptibility of P. aeruginosa PAO1 to BAL30072.

Exploring the contribution of efflux on the resistance to fluoroquinolones in clinical isolates of Staphylococcus aureus

BACKGROUND

Antimicrobial resistance mediated by efflux systems is still poorly characterized in Staphylococcus aureus, despite the description of several efflux pumps (EPs) for this bacterium. In this work we used several methodologies to characterize the efflux activity of 52 S. aureus isolates resistant to ciprofloxacin collected in a hospital in Lisbon, Portugal, in order to understand the role played by these systems in the resistance to fluoroquinolones.

RESULTS

Augmented efflux activity was detected in 12 out of 52 isolates and correlated with increased resistance to fluoroquinolones. Addition of efflux inhibitors did not result in the full reversion of the fluoroquinolone resistance phenotype, yet it implied a significant decrease in the resistance levels, regardless of the type(s) of mutation(s) found in the quinolone-resistance determining region of grlA and gyrA genes, which accounted for the remaining resistance that was not efflux-mediated. Expression analysis of the genes coding for the main efflux pumps revealed increased expression only in the presence of inducing agents. Moreover, it showed that not only different substrates can trigger expression of different EP genes, but also that the same substrate can promote a variable response, according to its concentration. We also found isolates belonging to the same clonal type that showed different responses towards drug exposure, thus evidencing that highly related clinical isolates may diverge in the efflux-mediated response to noxious agents. The data gathered by real-time fluorometric and RT-qPCR assays suggest that S. aureus clinical isolates may be primed to efflux antimicrobial compounds.

CONCLUSIONS

The results obtained in this work do not exclude the importance of mutations in resistance to fluoroquinolones in S. aureus, yet they underline the contribution of efflux systems for the emergence of high-level resistance. All together, the results presented in this study show the potential role played by efflux systems in the development of resistance to fluoroquinolones in clinical isolates of S. aureus.

Efflux as a mechanism for drug resistance in Mycobacterium tuberculosis

Tuberculosis remains an important global public health problem, with an estimated prevalence of 14 million individuals with tuberculosis worldwide in 2007. Because antibiotic treatment is one of the main tools for tuberculosis control, knowledge of Mycobacterium tuberculosis drug resistance is an important component for the disease control strategy. Although several gene mutations in specific loci of the M. tuberculosis genome have been reported as the basis for drug resistance, additional resistance mechanisms are now believed to exist. Efflux is a ubiquitous mechanism responsible for intrinsic and acquired drug resistance in prokaryotic and eukaryotic cells. Mycobacterium tuberculosis presents one of the largest numbers of putative drug efflux pumps compared with its genome size. Bioinformatics as well as direct and indirect evidence have established relationships among drug efflux with intrinsic or acquired resistance in M. tuberculosis. This minireview describes the current knowledge on drug efflux in M. tuberculosis.

Role of calcium in the efflux system of Escherichia coli

Efflux of antibiotics by Escherichia coli AG100 is performed by a variety of efflux pumps, ensuring survival of the bacterium in widely diverse media. At pH 5, efflux is independent of metabolic energy during the period of time the assay is conducted; at pH 8 it is totally dependent upon metabolic energy. Because calcium ions (Ca(2+)) are important for membrane transport channels and the activity of ATPases that provide energy functions, the role of Ca(2+) in the extrusion of an efflux pump substrate under conditions that challenge the bacterium was investigated. Real-time accumulation and efflux of ethidium bromide (EtBr) by E. coli K-12 AG100 strain [argE3 thi-1 rpsL xyl mtl Δ(gal-uvrB) supE44] was determined by a semi-automated fluorometric method in the presence and absence of Ca(2+) and agents that are known to inhibit access of calcium to enzymes that provide energy. Chlorpromazine (CPZ), an inhibitor of calcium binding to proteins (calcium-dependent enzymes), and ethylene diamine tetra-acetic acid (EDTA), a chelator of Ca(2+), increased accumulation and efflux of EtBr at pH 8 but not at pH 5. Ca(2+) reverses these effects when the assay is conducted at pH 8. In conclusion, the activity of the efflux pump system of E. coli is dependent upon metabolic energy at pH 8. Because at pH 8 hydrolysis of ATP is favoured and contributes protons for activation of the AcrAB-TolC efflux pump, CPZ is suspected of having its effects on accumulation/efflux of EtBr by indirectly affecting ATPase activity that is dependent upon Ca(2+).

Physiological characterisation of the efflux pump system of antibiotic-susceptible and multidrug-resistant Enterobacter aerogenes

Enterobacter aerogenes predominates amongst Enterobacteriaceae species that are increasingly reported as producers of extended-spectrum beta-lactamases. Although this mechanism of resistance to beta-lactams is important, other mechanisms bestowing a multidrug-resistant (MDR) phenotype in this species are now well documented. Amongst these mechanisms is the overexpression of efflux pumps that extrude structurally unrelated antibiotics prior to their reaching their targets. Interestingly, although knowledge of the genetic background behind efflux pumps is rapidly advancing, few studies assess the physiological nature of the overall efflux pump system of this, or for that matter any other, bacterium. The study reported here evaluates physiologically the efflux pump system of an E. aerogenes ATCC reference as well as two strains whose MDR phenotypes are mediated by overexpressed efflux pumps. The activities of the efflux pumps in these strains are modulated by pH and glucose, although the effects of the latter are essentially restricted to pH 8, suggesting the presence of two general efflux pump systems, i.e. proton-motive force-dependent and ABC transporter types, respectively.

Study of the multidrug membrane transporter of single living Pseudomonas aeruginosa cells using size-dependent plasmonic nanoparticle optical probes

Multidrug membrane transporters (efflux pumps) in both prokaryotes and eukaryotes are responsible for impossible treatments of a wide variety of diseases, including infections and cancer, underscoring the importance of better understanding of their structures and functions for the design of effective therapies. In this study, we designed and synthesized two silver nanoparticles (Ag NPs) with average diameters of 13.1 +/- 2.5 nm (8.1-38.6 nm) and 91.0 +/- 9.3 nm (56-120 nm) and used the size-dependent plasmonic spectra of single NPs to probe the size-dependent transport kinetics of MexAB-OprM (multidrug transporter) in Pseudomonas aeruginosa in real time at nanometer resolution. We found that the level of accumulation of intracellular NPs in wild-type (WT) cells was higher than in nalB1 (overexpression of MexAB-OprM) but lower than in DeltaABM (deletion of MexAB-OprM). In the presence of proton ionophores (CCCP, inhibitor of proton motive force), we found that intracellular NPs in nalB1 were nearly doubled. These results suggest that MexAB-OprM is responsible for the extrusion of NPs out of cells and NPs (orders of magnitude larger than conventional antibiotics) are the substrates of the transporter, which indicates that the substrates may trigger the assembly of the efflux pump optimized for the extrusion of the encountered substrates. We found that the smaller NPs stayed inside the cells longer than larger NPs, suggesting the size-dependent efflux kinetics of the cells. This study shows that multisized NPs can be used to mimic various sizes of antibiotics for probing the size-dependent efflux kinetics of multidrug membrane transporters in single living cells.

Fluorometric determination of ethidium bromide efflux kinetics in Escherichia coli

Background

Efflux pump activity has been associated with multidrug resistance phenotypes in bacteria, compromising the effectiveness of antimicrobial therapy. The development of methods for the early detection and quantification of drug transport across the bacterial cell wall is a tool essential to understand and overcome this type of drug resistance mechanism. This approach was developed to study the transport of the efflux pump substrate ethidium bromide (EtBr) across the cell envelope of Escherichia coli K-12 and derivatives, differing in the expression of their efflux systems.

Results

EtBr transport across the cell envelope of E. coli K-12 and derivatives was analysed by a semi-automated fluorometric method. Accumulation and efflux of EtBr was studied under limiting energy supply (absence of glucose and low temperature) and in the presence and absence of the efflux pump inhibitor, chlorpromazine. The bulk fluorescence variations were also observed by single-cell flow cytometry analysis, revealing that once inside the cells, leakage of EtBr does not occur and that efflux is mediated by active transport. The importance of AcrAB-TolC, the main efflux system of E. coli, in the extrusion of EtBr was evidenced by comparing strains with different levels of AcrAB expression. An experimental model was developed to describe the transport kinetics in the three strains. The model integrates passive entry (influx) and active efflux of EtBr, and discriminates different degrees of efflux between the studied strains that vary in the activity of their efflux systems, as evident from the calculated efflux rates: = 0.0173 ± 0.0057 min^-1; = 0.0106 ± 0.0033 min^-1; and = 0.0230 ± 0.0075 min^-1.

Conclusion

The combined use of a semi-automated fluorometric method and an experimental model allowed quantifying EtBr transport in E. coli strains that differ in their overall efflux activity. This methodology can be used for the early detection of differences in the drug efflux capacity in bacteria accounting for antibiotic resistance, as well as for expedite screening of new drug efflux inhibitors libraries and transport studies across the bacterial cell wall.

Mechanisms of drug efflux and strategies to combat them: challenging the efflux pump of Gram-negative bacteria

Chemoresistance presents a general health problem concerning the therapy of infectious disease and cancer. In this context, the worldwide dissemination of "multidrugresistant" (MDR) pathogens has severely reduced the efficacy of our antimicrobial weapons and dramatically increased the frequency of therapeutic failure. Because MDR bacterial infections involve the over-expression of efflux pumps that expel unrelated antibiotics before they can reach their targets, it is necessary to clearly define the molecular and genetic bases of the MDR mechanisms in order to combat these infectious diseases. This characterization of efflux pumps allows the definition of an original anti-resistance weapon, the efflux pump inhibitor (EPI). Several chemical families of EPIs have been now described and characterized. Among them several inhibitor compounds display an efficient activity and inhibit the major AcrAB-TolC and MexAB-OprM efflux systems which are the major efflux pumps responsible for MDR Gram negative clinical isolates. The use of these EPIs induces a significant reduction of resistance to one or more antibiotics to which these isolates were initially resistant. Hence, the EPI when used as an adjuvant to the given antibiotic, restores the activity of the antibiotic. The description of the responsible efflux mechanism at its structural and physiological level will make it possible to develop along intelligent lines an improved new generation of EPIs that can readily be added to the armamentarium of current and past "fallen by the wayside" antibiotic therapies.