Evaluation of phyto-gallic acid as a potential inhibitor of Staphylococcus aureus efflux pump mediated tetracycline resistance: an in-vitro and in-silico study

Plants contain many bioactive compounds with potent antibacterial and efflux pump inhibitory activity (EPI). In this study, gallic acid extracted from pomegranate molasses by analytical HPLC holds promise as an EPI drug for Staphylococcus aureus mediated tetracycline resistance, it lowered the bacterial resistance and reversed the mechanism via tet family efflux pump, using molecular technique and in-silico molecular docking analysis. Extracted gallic acid combined with tetracycline demonstrated a significant decrease in the minimal inhibitory concentration MIC compared to its single activity. Similarly, little growth and lower fluorescence of S. aureus were observed on ethidium bromide (2.5 mg/mL) agar plates, indicating a reversible efflux pump mechanism and a potent EPI activity. Molecular docking demonstrated a promising affinity binding energy between gallic acid and tet efflux genes, opening a new baseline in bacterial infection treatment. PCR for tetK and Qac A/B genes failed to show any relation between tet genes and gallic acid.

Pentafluorosulfanyl-substituted biaryl derivatives as MATE-type transporter inhibitors targeting drug-resistant bacteria

Multidrug and toxin extrusion (MATE) inhibitors improve the antimicrobial susceptibility of drug-resistant bacteria by preventing the efflux of administered antibiotics. In this study, we optimized the chemical structure of a previously identified bacterial-selective MATE inhibitor 1 (EC50 > 30 µM) to improve its activity further. Compound 1 was divided into three fragments (aromatic part, linker part, and guanidine part), and each part was individually optimized. Compound 31 (EC50 = 1.8 µM), a novel pentafluorosulfanyl-containing molecule synthesized following optimized parts, showed antimicrobial activity against MATE-expressing strains at concentrations lower than conventional inhibitor 1 when co-administrated with norfloxacin. Furthermore, 31 was not cytotoxic at effective concentrations. This suggests that compound 31 can be a promising candidate for combating bacterial infections, particularly those resistant to conventional antibiotics by MATE expression.

Detection of Aminoglycoside Modifying Enzyme (AME) genes in Acinetobacter baumannii isolates and the inhibitory effect of efflux pump activity on drug susceptibility pattern

INTRODUCTION

The development of aminoglycoside modifying enzymes (AMEs) and increased efflux activity are considered important aminoglycosides resistance mechanisms.

AIM

This study is focused on the detection of the AMEs gene and assessing the effect of efflux pump inhibitor on the reversal of A. baumannii drug susceptibility.

METHODOLOGY

Bacterial DNA was amplified using AMEs gene-specific primers. Isolates were also investigated for efflux pump activity using efflux pump inhibitor (EPI) i.e. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and the impact of both mechanisms was analyzed.

RESULTS

Among A. baumannii isolates, 55% isolates (n ​= ​22/40) were identified to have aminoglycoside modifying enzymes genes; ant(3')-I gene (50%, 11/22), aac(6')-Ib gene (45.4%, 10/22), aph(3')-I gene (18.1%, 4/22) and aac(3)-I (9.1%, 2/22). Total 70% isolates have shown MIC alteration in different classes of drugs in response to EPI-CCCP. Such alteration was found in 100% amikacin sensitive and 58.6% amikacin resistant, 93.7% and 57.1% gentamicin sensitive and resistant isolates respectively.

CONCLUSION

The presence of aminoglycosides modifying enzymes was frequent among aminoglycosides resistant A. baumannii isolates and the coexistence of efflux pumps activity also plays an important role to increase drug resistance.

REPOSITORIES

Genbank and their accession numbers are MT903331[aac(3)-I], MT903332 MT903333 [ant(3')-I], MT903334, MT903335 [aph(3')-I)] and MT903336, MT940242 [ aac(6')-Ib].

Optimization of pyridylpiperazine-based inhibitors of the Escherichia coli AcrAB-TolC efflux pump

Multidrug-resistant Escherichia coli is a continuously growing worldwide public health problem, in which the well-known AcrAB-TolC tripartite RND efflux pump is a critical driver. We have previously described pyridylpiperazines as a novel class of allosteric inhibitors of E. coli AcrB which bind to a unique site in the protein transmembrane domain, allowing for the potentiation of antibiotic activity. Here, we show a rational optimization of pyridylpiperazines by modifying three specific derivatization points of the pyridine core to improve the potency and the pharmacokinetic properties of this chemical series. In particular, this work found that the introduction of a primary amine to the pyridine through ester (29, BDM91270) or oxadiazole (44, BDM91514) based linkers allowed for analogues with improved antibiotic boosting potency through AcrB inhibition. In vitro studies, using genetically engineered mutants, showed that this improvement in potency is mediated through novel interactions with distal acidic residues of the AcrB binding pocket. Of the two leads, compound 44 was found to have favorable physico-chemical properties and suitable plasma and microsomal stability. Together, this work expands the current structure-activity relationship data on pyridylpiperazine efflux pump inhibitors, and provides a promising step towards future in vivo proof of concept of pyridylpiperazines as antibiotic potentiators.

Design and synthesis of benzochromene derivatives as AcrB inhibitors for the reversal of bacterial multidrug resistance

A series of novel benzo[h]chromene compounds were designed, synthesized and evaluated for their biological activity as AcrB inhibitors. The compounds were assessed for their ability to potentiate the effect of antibiotics. Compounds with antibiotic-potentiating effects were then evaluated for inhibition of Nile Red efflux, and for off-target effects including activity on the outer and inner bacterial membranes and toxicity. Six compounds were identified to reduce the MIC values of at least one of the tested antibiotics by at least 4-fold, and further reduced the MICs in the presence of a membrane permeabilizer. The identified compounds were also able to inhibit Nile Red efflux at concentrations between 50 μM and 200 μM. The compounds did not disrupt the bacterial outer membrane nor display toxicity in a nematode model (Caenorhabditis elegans). The 4-methoxyphenoxy)propoxy derivative compound G6 possessed the most potent antibacterial potentiation with erythromycin by 8-fold even without the presence of a membrane permeabilizer. Furthermore, H6, G6, G10 and G11 completely abolished the Nile Red efflux at a concentration of 50 μM. The 3,4-dihydro-2H-benzo[h]chromen-5-yl)(morpholino)methanone core appears to be a promising chemical skeleton to be further studied in the discovery of more putative AcrB inhibitors.

Deciphering the Role of β-Lactamase Inhibitors, Membrane Permeabilizers and Efflux Pump Inhibitors as Emerging Targets in Antibiotic Resistance

Antimicrobial drugs have been noticed to have reduce activity effective due to upsurge witnessed in resistance of microbes. To deal with viewpoint of such a circumstance, we must seek ways to prevent it or atleast mitigate its effects in order to provide its activity against the microbes. Hence, novel antimicrobials are the one of the most promising solution for ending antimicrobial resistance. Furthermore, due to the less development of newer antimicrobials in recent years, the only way to combat microbial resistance are various synergistic approaches of exploring antimicrobial drug combinations. This combination's efficacy is due to a synergistic chemical that re-sensitizes the resistant microbial strain. It has been observed that classes of β-lactamases inhibitors, efflux pump inhibitors and membrane permeabilizers are of particular relevance, since they can break resistance to the most commonly used antimicrobials. This review explains the readers that how these synergistic combinations can help to reduce or eliminate the microbial resistance supported with clinical evidence.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-022-01045-6.

Potentiating the intracellular killing of Staphylococcus aureus by dihydroquinazoline analogues as NorA efflux pump inhibitor

Staphylococcus aureus is an emerging human pathogen that has become difficult to treat due to its high resistance against wide range of drugs. Emergence of drug resistant isolates has further convoluted the treatment process. Among different resistance mechanisms, efflux pump proteins play a central role and has made itself a direct approach for therapeutic exploration. To demarcate the role of dihydroquinazoline analogues as NorA efflux pump inhibitor in S. aureus1199B (NorA over producing) strain total seventeen analogues were synthesized and tested for their modulatory effects on norfloxacin and Etbr resistance. Further accumulation assays, bacterial time kill kinetics, cytotoxicity assay were also carried out. The intracellular killing ability of analogues, as EPI was determined using THP-1 monocytes. The binding interaction of analogues with NorA was also predicted. Dihydroquinazoline analogues notably reduced the MIC of norfloxacin and Etbr in S. aureus1199B. In addition to their very low toxicity, they showed high Etbr and norfloxacin accumulation respectively. Further effective over time log reduction in bacterial kill kinetics in presence of these analogues confirmed their role as NorA efflux pump inhibitor. FESEM analysis clearly depicted their effect on the cell surface morphology owing to its lyses. The most significant finding of this study was the ability of analogues to significantly reduce the intracellular S. aureus1199B in human THP-1 monocytes in presence of norfloxacin. Our study has shown for the first time the possibility of developing the dihydroquinazoline analogues as NorA efflux pump inhibitors for S. aureus and control its infection.

Reversal of Tetracycline Resistance by Cepharanthine, Cinchonidine, Ellagic Acid and Propyl Gallate in a Multi-drug Resistant Escherichia coli

Bacterial resistance to antibiotics is an increasing threat to global healthcare systems. We therefore sought compounds with potential to reverse antibiotic resistance in a clinically relevant multi-drug resistant isolate of Escherichia coli (NCTC 13400). 200 natural compounds with a history of either safe oral use in man, or as a component of a traditional herb or medicine, were screened. Four compounds; ellagic acid, propyl gallate, cinchonidine and cepharanthine, lowered the minimum inhibitory concentrations (MICs) of tetracycline, chloramphenicol and tobramycin by up to fourfold, and when combined up to eightfold. These compounds had no impact on the MICs of ampicillin, erythromycin or trimethoprim. Mechanistic studies revealed that while cepharanthine potently suppressed efflux of the marker Nile red from bacterial cells, the other hit compounds slowed cellular accumulation of this marker, and/or slowed bacterial growth in the absence of antibiotic. Although cepharanthine showed some toxicity in a cultured HEK-293 mammalian cell-line model, the other hit compounds exhibited no toxicity at concentrations where they are active against E. coli NCTC 13400. The results suggest that phytochemicals with capacity to reverse antibiotic resistance may be more common in traditional medicines than previously appreciated, and may offer useful scaffolds for the development of antibiotic-sensitising drugs.

Adjunctive therapy for multidrug-resistant bacterial infections: Type III secretion system and efflux inhibitors

The increasing prevalence of multidrug-resistant (MDR) bacterial infections has created a crucial need for new therapeutics that avoid or minimize existing resistance mechanisms. In this review, we describe the development of novel classes of small-molecule adjunctive agents targeting either a bacterial virulence factor, the Pseudomonas aeruginosa type III secretion system (T3SS), or an intrinsic resistance factor, resistance-nodulation-cell division superfamily (RND) efflux pumps of the Enterobacteriaceae. These agents are designed to be administered with antibacterials to improve their efficacy. T3SS inhibition rescues host innate immune system cells from injection with bacterial toxins, whereas RND efflux pump inhibition increases antibiotic susceptibility, in both cases improving the efficacy of the combined antibacterial.

Expression of RND efflux pumps mediated antibiotic resistance in Pseudomonas aeruginosa clinical strains

Resistance-Nodulation-Division (RND) efflux pumps are responsible for multidrug resistance in Pseudomonas aeruginosa. The present study aimed to evaluate the overexpression of RND efflux pumps and its role in the antibiotic resistance of P. aeruginosa clinical isolates. A number of 122 isolates were obtained from three military hospitals in Tehran, Iran. In order to determine the antibiotic resistance, the isolates were identified and assessed by the disk diffusion and agar dilution methods. This study investigated the gene expression of four multi-drug efflux pump systems (MexAB-OprM, MexCD-OprJ, MexEF-OprN and MexXY (-OprA)) and its correlation with antibiotic resistance. The isolates indicated that the highest resistance rate was against ticarcillin (80%), followed by ciprofloxacin (74%) and meropenem (71%). Most of them expressed mexB (69%), mexC (28.7%), mexE (43.4%), and mexY (74.6%), suggesting that mexB and mexY were highly expressed in the studied strains. The overexpression of mexB and mexY was significantly more prevalent in the ICU wards (p = 0.033). Furthermore, there was a significant correlation between the expression of RND-type efflux pumps and the resistance to most anti-pseudomonal antibiotics.

Efflux pumps in multidrug-resistant Acinetobacter baumannii: Current status and challenges in the discovery of efflux pumps inhibitors

Acinetobacter baumannii is an ESKAPE pathogen known to cause fatal nosocomial infections. With the surge of multidrug resistance (MDR) in the bacterial system, effective treatment measures have become very limited. The MDR in A. baumannii is contributed by various factors out of which efflux pumps have gained major attention due to their broad substrate specificity and wide distribution among bacterial species. The efflux pumps are involved in the MDR as well as contribute to other physiological processes in bacteria, therefore, it is critically important to inhibit efflux pumps in order to combat emerging resistance. The present review provides insight about the different efflux pump systems in A. baumannii and their role in multidrug resistance. A major focus has been put on the different strategies and alternate therapeutics to inhibit the efflux system. This includes use of different efflux pump inhibitors-natural, synthetic or combinatorial therapy. The use of phage therapy and nanoparticles for inhibiting efflux pumps have also been discussed here. Moreover, the present review provides the knowledge of barriers in development of efflux pump inhibitors (EPIs) and their approval for commercialization. Here, different prospectives have been discussed to improve the therapeutic development process and make it more compatible for clinical use.

Atomistic insight and modeled elucidation of conessine towards Pseudomonas aeruginosa efflux pump

Drug-resistant Pseudomonas aeruginosa efflux pump extrudes antibiotics from cells for survival. Efflux pump inhibitor (EPI) thus becomes an interesting alternative to handle the drug-resistant bacteria. Conessine, a natural steroidal alkaloid from Holarrhena antidysenterica, previously exhibited efflux pump inhibitory potential. Our molecular docking and molecular dynamics (MD) studies provided atomistic information as well as the interaction of conessine with bacterial MexB efflux pump in phospholipid bilayer membrane to further the previous experimental report. Herein, the binding site and proposed mode of action of conessine were identified compared to known/commercial EPIs such as PAβN or designed-synthetic P9D. Our results explained conessine binding mode of action as an effective agent against the MexB efflux pump. The MD simulation also suggested that conessine was able to affect glycine loop (G-loop) flexibility, and the reduced G-loop flexibility due to conessine could hinder an antibiotics extrusion. In addition, our study suggested the conessine core structure buried in a hydrophobic region in the efflux pump similar to other known EPIs. Our finding could cope as a key for the design and development of the conessine derivative as novel EPI against P. aeruginosa.Communicated by Ramaswamy H. Sarma.

Efflux Pump Inhibitors Against Nontuberculous Mycobacteria

Over the last years, nontuberculous mycobacteria (NTM) have emerged as important human pathogens. Infections caused by NTM are often difficult to treat due to an intrinsic multidrug resistance for the presence of a lipid-rich outer membrane, thus encouraging an urgent need for the development of new drugs for the treatment of mycobacterial infections. Efflux pumps (EPs) are important elements that are involved in drug resistance by preventing intracellular accumulation of antibiotics. A promising strategy to decrease drug resistance is the inhibition of EP activity by EP inhibitors (EPIs), compounds that are able to increase the intracellular concentration of antimicrobials. Recently, attention has been focused on identifying EPIs in mycobacteria that could be used in combination with drugs. The aim of the present review is to provide an overview of the current knowledge on EPs and EPIs in NTM and also, the effect of potential EPIs as well as their combined use with antimycobacterial drugs in various NTM species are described.

Effect of hydroxyamines derived from lapachol and norlachol against Staphylococcus aureus strains carrying the NorA efflux pump

Isolated substances and those organically synthesized have stood out over the years for their therapeutic properties, including their antibacterial activity. These compounds may be an alternative to the production of new antibiotics or may have the ability to potentiate the action of preexisting ones. In this context, the objective of this study was to evaluate the in vitro antibacterial and efflux pump inhibitory activity of hydroxyamines derived from lapachol and norlachol, more specifically the compounds 2-(2-Hydroxyethylamino)-3-(3-methyl-2-butenyl)-1,4 dihydro-1,4-naphthalenedione, 2-(2-Hydroxyethylamino)-3-(2-methyl-propenyl)[1,4]naphthoquinone and 2-(3-Hydroxypropylamino)-3-(3-methyl-2-butenyl)-[1,4]naphthoquinone, against Staphylococcus aureus strains carrying the NorA efflux pump mechanism. The substances were synthesized from 2-hydroxy-quinones, lapachol and nor-lapachol, obtaining the corresponding 2-methoxylated derivatives via dimethyl sulfate alkylation in a basic medium, which then reacted chemoselectively with 2-ethanolamine and 3-propanolamine to form the corresponding amino alcohols. All three molecules underwent a virtual structure-based analysis (docking). The antibacterial activity of the substances was measured by determining their Minimum Inhibitory Concentration (MIC) and a microdilution assay was performed to verify efflux pump inhibition using the substances at a sub-inhibitory concentration. The results were subjected to statistical analysis using an analysis of variance (ANOVA) followed by Bonferroni's post hoc test. The substances obtained MIC values ≥1024 μg/mL, however, a significant reduction of their MICs was observed when the substances were associated with norfloxacin and ethidium bromide, with this effect being attributed to efflux pump inhibition. Following a virtual analysis based on its structure (docking), information regarding the affinity of new ligands for the ABC efflux pump were observed, thus contributing to the understanding of their mechanism of molecular interactions and the discovery of functional ligands associated with a reduction in bacterial resistance.

Targeting antimicrobial drug resistance with marine natural products

The rise and spread of antimicrobial resistance represents one of the most pressing health issues of today. Antimicrobial resistance in micro-organisms can arise due to a multiplicity of factors, including permeability changes in the cell membrane, increase of drug efflux pumps, enzymatic modification or inactivation of the antibiotic, target site modification, alternative metabolic pathways and biofilm formation. The marine environment is a valuable source of diverse natural products with a huge variety of biological activities. Among them, antimicrobial compounds show promising biological activities against numerous drug-resistant bacteria and fungi, making marine natural products a very promising resource in the search for novel antimicrobial agents. This review summarises the state-of-art of marine natural products with antibacterial and antifungal properties against drug-resistant micro-organisms. These natural products were categorised based on their chemical structure, and their respective sources and activities are highlighted. The chemical diversity associated with these marine-derived molecules is enormous, including peptides, polyketides, alkaloids, sterols, terpenoids, lactones, halogenated compounds, nucleosides, etc., some of which have rare substructures. Some of the marine compounds mentioned do not have intrinsic antimicrobial activity but potentiate the antimicrobial effect of other antimicrobials via inhibition of efflux pumps. Although these agents are still in preclinical studies, evidence of their in vivo efficacy suggest research of new drugs from the ocean to overcome antimicrobial resistance in order to fulfil an unmet medical need.

The search of potential inhibitors of the AcrAB-TolC system of multidrug-resistant Escherichia coli: an in silico approach

The number of infections caused by multidrug antibiotic resistance (MDR) species is increasing globally. The efflux pump system, AcrAB-TolC, confers Escherichia coli resistance to many antibiotics and results in poor treatment outcomes. Different rational drug design techniques were employed to search for a safe and effective AcrAB-TolC system inhibitor. Ligand docking was performed to analyze the binding of different ArcB substrates and/or inhibitors in the different AcrAB crystal structure binding sites. The validated docking site using the established docking preferences was used to perform virtual high-throughput screening on a large library of compounds. Domperidone, a known and safe over-the-counter antiemetic drug, was proposed as an effective ArcB inhibitor. Microbiological studies confirmed the computational results and domperidone reversed the resistance to the antibiotics: levofloxacin and ciprofloxacin in the MDR E. coli stains with an effect that surpassed the effect of the known efflux pump inhibitor, reserpine. In addition, it was able to increase both antibiotic effects on susceptible strains. This finding suggests that the antibiotic-domperidone combination can be used clinically to treat infections caused by multidrug-resistant E. coli strains.

Resistance modulatory and efflux-inhibitory activities of capsaicinoids and capsinoids

Capsaicinoids are reported to have a bunch of promising pharmacological activities, among them antibacterial effects against various strains of bacteria. In this study the effect on efflux pumps of mycobacteria was investigated. The importance of efflux pumps, and the inhibition of these, is rising due to their involvement in antibiotic resistance development. In order to draw structure and activity relationships we tested natural and synthetical capsaicinoids as well as synthetical capsinoids. In an accumulation assay these compounds were evaluated for their ability to accumulate ethidium bromide into mycobacterial cells, a well-known substrate for efflux pumps. Capsaicin and dihydrocapsaicin, the two most abundant capsaicinoids in Capsicum species, proved to be superior efflux pump inhibitors compared to the standard verapamil. A dilution series showed dose dependency of both compounds. The compound class of less pungent capsinoids qualified for further investigation as antibacterials against Mycobacterium smegmatis.

Steroidal alkaloids and conessine from the medicinal plant Holarrhena antidysenterica restore antibiotic efficacy in a Galleria mellonella model of multidrug-resistant Pseudomonas aeruginosa infection

Background

This study aimed to evaluate the efficacy of combinations of steroidal alkaloids and conessine from the Thai medicinal plant Holarrhena antidysenterica with antibiotics against Pseudomonas aeruginosa strains possessing different efflux-pump-mediated multidrug-resistant (MDR) phenotypes in a Galleria mellonella infection model.

Methods

P. aeruginosa strains with defined mutations that result in the overexpression of the MexAB-OprM, MexCD-OprJ and MexEF-OprN efflux pumps, and a strain with all three of these pumps deleted, were used. In vitro, the effect of combinations of steroidal alkaloids and conessine with antibiotics was compared with antibiotic treatment alone via MIC determination and time-kill assays. Efficacy of combinations of the steroidal alkaloids and conessine with levofloxacin were compared with monotherapies against infections in G. mellonella larvae by measuring larval mortality and bacterial burden.

Results

Combination therapies of conessine or steroidal alkaloids with levofloxacin enhanced bacterial inhibition in vitro and restored antibiotic efficacy in vivo compared to the constituent monotherapies. Neither conessine nor the steroidal alkaloids induced any detectable toxicity in G. mellonella larvae. The enhanced efficacy of the combination treatments was most pronounced with conessine and correlated with reduced larval burden of infecting P. aeruginosa. Notably, the enhanced efficacy of conessine/levofloxacin combinations was only detected in the parent strain and strains that overexpressed the MexAB-OprM or MexEF-OprN efflux systems.

Conclusions

Steroidal alkaloids from Holarrhena antidysenterica, and particularly the principal active ingredient conessine, restored levofloxacin efficacy against resistant P. aeruginosa strains possessing efflux-mediated MDR phenotypes. The compounds should be investigated further as a potential novel therapy.

Electronic supplementary material

The online version of this article (10.1186/s12906-018-2348-9) contains supplementary material, which is available to authorized users.

A novel bi-functional chalcone inhibits multi-drug resistant Staphylococcus aureus and potentiates the activity of fluoroquinolones

Staphylococcus aureus is the leading cause of bacteraemia and the dwindling supply of effective antibacterials has exacerbated the problem of managing infections caused by this bacterium. Isoliquiritigenin (ISL) is a plant flavonoid that displays therapeutic potential against S. aureus. The present study identified a novel mannich base derivatives of ISL, IMRG4, active against Vancomycin intermediate S. aureus (VISA). IMRG4 damages the bacterial membranes causing membrane depolarization and permeabilization, as determined by loss of salt tolerance, flow cytometric analysis, propidium idodie and fluorescent microscopy. It reduces the intracellular invasion of HEK-293 cells by S. aureus and decreases the staphylococcal load in different organs of infected mice models. In addition to anti-staphylococcal activity, IMRG4 inhibits the multidrug efflux pump, NorA, which was determined by molecular docking and EtBr efflux assays. In combination, IMRG4 significantly reduces the MIC of norfloxacin for clinical strains of S. aureus including VISA. Development of resistance against IMRG4 alone and in combination with norfloxacin was low and IMRG4 prolongs the post-antibiotic effect of norfloxacin. These virtues combined with the low toxicity of IMRG4, assessed by MTT assay and haemolysis, makes it an ideal candidate to enter drug development pipeline against S. aureus.

Essential oils from tropical medicinal herbs and food plants inhibit biofilm formation in vitro and are non-cytotoxic to human cells

The biofilm inhibition and eradication potential of essential oils (EOs) extracted from six tropical medicinal herbs and food plants [Psiadia arguta (PA), Psiadia terebinthina (PT), Citrus grandis (CGp), Citrus hystrix (CH), Citrus reticulata (CR), and Cinnamomum zeylanicum (CZ)] were assessed. The mechanism of inhibition was studied via quenching of efflux pump. Cytotoxicity was evaluated using Artemia salina assay and cell lines [human cervix carcinoma (HeLa), human lung fibroblast (MRC-5), and murine melanoma (B16F10)]. EOs of CH, CR, PA, and PT were found to be prospective antibiofilm agents (IC₅₀ of 0.29, 0.59, 0.22, and 0.11 mg/mL against Staphylococcus epidermidis; 0.39, 0.54, 0.09, and 0.13 mg/mL against Escherichia coli; and 0.54, 0.90, 0.44 and 0.51 mg/mL against Candida albicans for CH, CR, PA, and PT, respectively). The simultaneous actions of the EOs and efflux pump inhibitor impacted on the resistance of the biofilms. LC₅₀ of the EOs ranged from 223 to 583 µg/mL against A. salina. The non-cytotoxic concentration of the EOs varied from 200 to 300 µg/mL (HeLa and MRC-5), and 150-200 µg/mL (B16F10). EOs from these tropical medicinal herbs and food plants are useful sources of new antimicrobials with low cytotoxicity which could open new horizons in the drug development process.

Attaching NorA efflux pump inhibitors to methylene blue enhances antimicrobial photodynamic inactivation of Escherichia coli and Acinetobacter baumannii in vitro and in vivo

Resistance of bacteria to antibiotics is a public health concern worldwide due to the increasing failure of standard antibiotic therapies. Antimicrobial photodynamic inactivation (aPDI) is a promising non-antibiotic alternative for treating localized bacterial infections that uses non-toxic photosensitizers and harmless visible light to produce reactive oxygen species and kill microbes. Phenothiazinium photosensitizers like methylene blue (MB) and toluidine blue O are hydrophobic cations that are naturally expelled from bacterial cells by multidrug efflux pumps, which reduces their effectiveness. We recently reported the discovery of a NorA efflux pump inhibitor-methylene blue (EPI-MB) hybrid compound INF55-(Ac)en-MB that shows enhanced photodynamic inactivation of the Gram-positive bacterium methicillin-resistant Staphylococcus aureus (MRSA) relative to MB, both in vitro and in vivo. Here, we report the surprising observation that INF55-(Ac)en-MB and two related hybrids bearing the NorA efflux pump inhibitors INF55 and INF271 also show enhanced aPDI activity in vitro (relative to MB) against the Gram-negative bacteria Escherichia coli and Acinetobacter baumannii, despite neither species expressing the NorA pump. Two of the hybrids showed superior effects to MB in murine aPDI infection models. The findings motivate wider exploration of aPDI with EPI-MB hybrids against Gram-negative pathogens and more detailed studies into the molecular mechanisms underpinning their activity.

Detection of floR gene and active efflux mechanism of Escherichia coli in Ningxia, China

Florfenicol is an antibiotic, a fluorinated structural analogue of thiamphenicol and chloramphenicol, approved exclusively for use in Asia for aquaculture since the 1980's. Our study examined the prevalence of florfenicol resistance in bovine mastitis Escherichia coli isolates. A total of 245 E. coli isolates were collected from bovine mastitis in Ningxia Province, China between May 2016 to July 2017 and screened for florfenicol resistance gene, floR gene by PCR analysis. About 7.35% (15/245) of the isolates were positive for floR gene. Minimal inhibitory concentration (MIC) results showed that 9 isolates were susceptible strains and 6 isolates were highly resistant to florfenicol. HPLC (high performance liquid chromatography) results showed that the amounts of florfenicol was significantly improved in the presence of carbonyl cyanide 3-chlorophenylhydrazone (CCCP) as an efflux pump inhibitor. This, therefore, indicates that the employment of florfenicol in conjunction with CCCP in future drug formulations should be considered.

Design, synthesis and biological activity evaluation of novel 4-subtituted 2-naphthamide derivatives as AcrB inhibitors

A novel series of 4-substituted 2-naphthamide derivatives were designed, synthesized and evaluated for their biological activity. In particular, the ability of the compounds to potentiate the action of antibiotics, to inhibit Nile Red efflux and to target AcrB specifically was investigated. The results indicated that most of the 4-substituted 2-naphthamide derivatives were able to synergize with the antibiotics tested, and inhibit Nile Red efflux by AcrB in the resistant phenotype. Subsequent exclusion of compounds with off target effects such as outer- or inner membrane permeabilization identified compounds 7c, 7g, 12c, 12i and 13g as efflux pump inhibitors (EPIs). Particularly, compounds 7c, 7g and 12i were found to be the most potent EPIs, which synergized with the two substrates tested at lower concentrations than that of parent A3, demonstrating an improvement in potency as compared to A3. Additionally, when the outer membrane of E. coli was permeabilized, compound 12c displayed a huge increase in efficacy and was able to synergize with erythromycin at a concentration that was 16 times lower than that of the parent A3. Hence we were able to design and synthesize compounds that displayed significant increase in efficacy as EPIs against AcrB.

The hydrophobic trap-the Achilles heel of RND efflux pumps

Resistance-nodulation-division (RND) superfamily efflux pumps play a major role in multidrug resistance (MDR) of Gram-negative pathogens by extruding diverse classes of antibiotics from the cell. There has been considerable interest in developing efflux pump inhibitors (EPIs) of RND pumps as adjunctive therapies. The primary challenge in EPI discovery has been the highly hydrophobic, poly-specific substrate binding site of the target. Recent findings have identified the hydrophobic trap, a narrow phenylalanine-lined groove in the substrate-binding site, as the "Achilles heel" of the RND efflux pumps. In this review, we will examine the hydrophobic trap as an EPI target and two chemically distinct series of EPIs that bind there.

Kinetic analysis of the inhibition of the drug efflux protein AcrB using surface plasmon resonance

Multidrug efflux protein complexes such as AcrAB-TolC from Escherichia coli are paramount in multidrug resistance in Gram-negative bacteria and are also implicated in other processes such as virulence and biofilm formation. Hence efflux pump inhibition, as a means to reverse antimicrobial resistance in clinically relevant pathogens, has gained increased momentum over the past two decades. Significant advances in the structural and functional analysis of AcrB have informed the selection of efflux pump inhibitors (EPIs). However, an accurate method to determine the kinetics of efflux pump inhibition was lacking. In this study we standardised and optimised surface plasmon resonance (SPR) to probe the binding kinetics of substrates and inhibitors to AcrB. The SPR method was also combined with a fluorescence drug binding method by which affinity of two fluorescent AcrB substrates were determined using the same conditions and controls as for SPR. Comparison of the results from the fluorescent assay to those of the SPR assay showed excellent correlation and provided validation for the methods and conditions used for SPR. The kinetic parameters of substrate (doxorubicin, novobiocin and minocycline) binding to AcrB were subsequently determined. Lastly, the kinetics of inhibition of AcrB were probed for two established inhibitors (phenylalanine arginyl β-naphthylamide and 1-1-naphthylmethyl-piperazine) and three novel EPIs: 4-isobutoxy-2-naphthamide (A2), 4-isopentyloxy-2-naphthamide (A3) and 4-benzyloxy-2-naphthamide (A9) have also been probed. The kinetic data obtained could be correlated with inhibitor efficacy and mechanism of action. This study is the first step in the quantitative analysis of the kinetics of inhibition of the clinically important RND-class of multidrug efflux pumps and will allow the design of improved and more potent inhibitors of drug efflux pumps. This article is part of a Special Issue entitled: Beyond the Structure-Function Horizon of Membrane Proteins edited by Ute Hellmich, Rupak Doshi and Benjamin McIlwain.

Conessine as a novel inhibitor of multidrug efflux pump systems in Pseudomonas aeruginosa

Background

Holarrhena antidysenterica has been employed as an ethnobotanical plant for the treatment of dysentery, diarrhoea, fever, and bacterial infections. Biological activities of the principle compound, conessine including anti-diarrhoea and anti-plasmodial effects were documented. Our previous study reported potency of Holarrhena antidysenterica extract and conessine as resistance modifying agents against extensively drug-resistant Acinetobacter baumannii. This study aimed to investigate (i) whether conessine, a steroidal alkaloid compound, could act as a resistance modifying agent against multidrug-resistant Pseudomonas aeruginosa, and (ii) whether MexAB-OprM efflux pump involved in the mechanism.

Methods

Conessine combined with various antibiotics were determined for synergistic activity against P. aeruginosa PAO1 strain K767 (wild-type), K1455 (MexAB-OprM overexpressed), and K1523 (MexB deletion). H33342 accumulation assay was used to evaluate efflux pump inhibition while NPN uptake assay was assessed membrane permeabilization.

Results

Conessine significantly reduced MICs of all antibiotics by at least 8-fold in MexAB-OprM overexpressed strain. The levels were comparable to those obtained in wild-type strain for cefotaxime, levofloxacin, and tetracycline. With erythromycin, novobiocin, and rifampicin, MICs were 4- to 8-fold less than MICs of the wild-type strain. Loss of MexAB-OprM due to deletion of mexB affected susceptibility to almost all antibiotics, except novobiocin. Synergistic activities between other antibiotics (except novobiocin) and conessine observed in MexB deletion strain suggested that conessine might inhibit other efflux systems present in P. aeruginosa. Inhibition of H33342 efflux in the tested strains clearly demonstrated that conessine inhibited MexAB-OprM pump. In contrast, the mode of action as a membrane permeabilizer was not observed after treatment with conessine as evidenced by no accumulation of 1-N-phenylnaphthylamine.

Conclusions

The results suggested that conessine could be applied as a novel efflux pump inhibitor to restore antibiotic activity by inhibiting efflux pump systems in P. aeruginosa. The findings speculated that conessine may also have a potential to be active against homologous resistance–nodulation–division (RND) family in other Gram-negative pathogens.

Development of an accumulation assay and evaluation of the effects of efflux pump inhibitors on the retention of chlorhexidine digluconate in Pseudomonas aeruginosa and Staphylococcus aureus

BACKGROUND

Chlorhexidine digluconate (CHG) is used as a disinfectant. The emergence of pathogens resistant to the biocide raises health concern. Information on specific efflux mechanisms utilised by bacteria to confer reduced susceptibility to the biocide, may be used to develop ways of preventing the efflux of the biocide from nosocomial pathogens resulting in higher disinfection activity. The aim of the study was to evaluate the role of ATP-binding cassette transporters on the transport of CHG in bacteria.

METHODS

Clinical strains of Pseudomonas aeruginosa, Staphylococcus aureus and their respective laboratory strains ATCC 27853 and ATCC 9144 were used for susceptibility tests. The minimum inhibitory concentration (MIC) of CHG with or without an efflux pump inhibitor [reserpine or carbonyl cyanide m-chlorophenylhydrazone (CCCP)] was determined using the broth microdilution method. A spectrophotometric method to quantify the amount of chlorhexidine in a sample was developed, validated and used to quantify CHG within P. aeruginosa and S. aureus cells.

RESULTS

In the presence of reserpine, the MIC of CHG against the clinical strains of P. aeruginosa and S. aureus decreased from 6.3 to 3.2 µg/ml but showed no change against both ATCC isolates. The MIC of CHG in the presence of CCCP for both strains of P. aeruginosa remained unchanged but showed a reduction for both isolates of S. aureus. The suitability of the spectrophotometric method developed for quantifying the amount of CHG accumulated in microbial cells was validated and used successfully to quantify CHG accumulated within bacterial cells.

CONCLUSION

The spectrophotometric determination of CHG within microbial cells may be used to quantify CHG in microbial cells. Only the clinical strain of P. aeruginosa showed significant efflux of CHG suggesting the participation of efflux transporters in the pumping out of CHG from this isolate. The use of efflux pump inhibitors together with the biocide may be explored to preventing the efflux of the biocide from P. aeruginosa resulting in order to increase disinfection activity.

Inhibition of the multidrug efflux pump LmrS from Staphylococcus aureus by cumin spice Cuminum cyminum

Staphylococcus aureus is a serious causative agent of infectious disease. Multidrug-resistant strains like methicillin-resistant S. aureus compromise treatment efficacy, causing significant morbidity and mortality. Active efflux represents a major antimicrobial resistance mechanism. The proton-driven multidrug efflux pump, LmrS, actively exports structurally distinct antimicrobials. To circumvent resistance and restore clinical efficacy of antibiotics, efflux pump inhibitors are necessary, and natural edible spices like cumin are potential candidates. The mode of cumin antibacterial action and underlying mechanisms behind drug resistance inhibition, however, are unclear. We tested the hypothesis that cumin inhibits LmrS drug transport. We found that cumin inhibited bacterial growth and LmrS ethidium transport in a dosage-dependent manner. We demonstrate that cumin is antibacterial toward a multidrug-resistant host and that resistance modulation involves multidrug efflux inhibition.

Inhibition of the NorA multi-drug transporter by oxygenated monoterpenes

The aim of this study was to investigate intrinsic antimicrobial activity of three monoterpenes nerol, dimethyl octanol and estragole, against bacteria and yeast strains, as well as, investigate if these compounds are able to inhibit the NorA efflux pump related to fluoroquinolone resistance in Staphylococcus aureus. Minimal inhibitory concentrations (MICs) of the monoterpenes against Staphylococcus aureus, Escherichia coli and Candida albicans strains were determined by micro-dilution assay. MICs of the norfloxacin against a S. aureus strain overexpressing the NorA protein were determined in the absence or in the presence of the monoterpenes at subinhibitory concentrations, aiming to verify the ability of this compounds act as efflux pump inhibitors. The monoterpenes were inactive against S. aureus however the nerol was active against E. coli and C. albicans. The addition of the compounds to growth media at sub-inhibitory concentrations enhanced the activity of norfloxacin against S. aureus SA1199-B. This result shows that bioactives tested, especially the nerol, are able to inhibit NorA efflux pump indicating a potential use as adjuvants of norfloxacin for therapy of infections caused by multi-drug resistant S. aureus strains.

Contribution of mexAB-oprM and mexXY (-oprA) efflux operons in antibiotic resistance of clinical Pseudomonas aeruginosa isolates in Tabriz, Iran

Overexpression of efflux pumps is one of the most important mechanisms that contributes to intrinsic and acquired resistance to antibiotics in Pseudomonas aeruginosa. The present study evaluated the role of MexAB-OprM and MexXY (-OprA) efflux pump overexpression in antibiotics resistance of P. aeruginosa clinical isolates. One-hundred clinical isolates of P. aeruginosa were obtained from four hospitals of Tabriz city in Northwest Iran. Isolates were identified and evaluated by the disk diffusion method and agar dilution in order to determine antibiotic resistance. Effect of Phenylalanine Arginine beta-Naphthylamide (PAβN) on susceptibility to various anti-Pseudomonas antimicrobials and expression levels of mexB and mexY using quantitative real-time PCR were determined in the clinical isolates. Random Amplified Polymorphic DNA Typing (RAPD-PCR) was used for genotyping of the isolates. The most and least effective antibiotics tested were colistin and ofloxacin, respectively. Seventy-one percent of the isolates were found as multidrug resistant (resistant to at least three different classes of antibiotics). Among ciprofloxacin and levofloxacin resistant isolates, 39.6% and 28.5% of them showed four-fold reduction in MIC with PAβN, respectively. Sixty-two percent and 65% of isolates overexpressed mexB and mexY, respectively. Sixty six isolates showed overexpression of both mexB and mexY efflux genes. Moreover, 76% and 88.7% of MDR isolates were mexB and mexY overexpressed, respectively. There were 30 different RAPD types in this study which were clustered into 6 clones. The study indicated that there is a significant correlation between the expression of efflux pumps and the resistance to most anti-pseudomonal antibiotics.

Dithiazole thione derivative as competitive NorA efflux pump inhibitor to curtail multi drug resistant clinical isolate of MRSA in a zebrafish infection model

Multi drug resistant (MDR) pathogens pose a serious threat to public health since they can easily render most potent drugs ineffective. Efflux pump inhibitors (EPI) can be used to counter the MDR phenotypes arising due to increased efflux. In the present study, a series of dithiazole thione derivatives were synthesized and checked for its antibacterial and efflux pump inhibitory (EPI) activity. Among 10 dithiazole thione derivatives, real-time efflux studies revealed that seven compounds were potent EPIs relative to CCCP. Zebrafish toxicity studies identified four non-toxic putative EPIs. Both DTT3 and DTT9 perturbed membrane potential and DTT6 was haemolytic. Among DTT6 and DTT10, the latter was less toxic as evidenced by histopathology studies. Since DTT10 was non-haemolytic, did not affect the membrane potential, and was least toxic, it was chosen further for in vivo study, wherein DTT10 potentiated effect of ciprofloxacin against clinical strain of MRSA and reduced bacterial burden in muscle and skin tissue of infected zebrafish by ~ 1.7 and 2.5 log fold respectively. Gene expression profiling of major efflux transport proteins by qPCR revealed that clinical isolate of MRSA, in the absence of antibiotic, upregulated NorA, NorB and MepA pump, whereas it downregulates NorC and MgrA relative to wild-type strain of Staphylococcus aureus. In vitro studies with NorA mutant strains and substrate profiling revealed that at higher concentrations DTT10 is likely to function as a competitive inhibitor of NorA efflux protein in S. aureus, whereas at lower concentrations it might inhibit ciprofloxacin efflux through NorB and MepA as implied by docking studies. A novel non-toxic, non-haemolytic dithiazole thione derivative (DTT10) was identified as a potent competitive inhibitor of NorA efflux pump in S. aureus using in silico, in vitro and in vivo studies. This study also underscores the importance of using zebrafish infection model to screen and evaluate putative EPI for mitigating MDR strains of S. aureus.

Antibacterial activities and structure-activity relationships of a panel of 48 compounds from Kenyan plants against multidrug resistant phenotypes

In the current study forty eight compounds belonging to anthraquinones, naphthoquinones, benzoquinones, flavonoids (chalcones and polymethoxylated flavones) and diterpenoids (clerodanes and kauranes) were explored for their antimicrobial potential against a panel of sensitive and multi-drug resistant Gram-negative and Gram-positive bacteria. The minimal inhibitory concentration (MIC) determinations on the tested bacteria were conducted using modified rapid INT colorimetric assay. To evaluate the role of efflux pumps in the susceptibility of Gram-negative bacteria to the most active compounds, they were tested in the presence of phenylalanine arginine β-naphthylamide (PAβN) (at 30 µg/mL) against selected multidrug resistance (MDR) bacteria. The anthraquinone, emodin, naphthaquinone, plumbagin and the benzoquinone, rapanone were active against methicillin resistant Staphylococcus aureus (MRSA) strains of bacteria with MIC values ranging from 2 to 128 μg/mL. The structure activity relationships of benzoquinones against the MDR Gram-negative phenotype showed antibacterial activities increasing with increase in side chain length. In the chalcone series the presence of a hydroxyl group at C3′ together with a methoxy group and a second hydroxyl group in meta orientation in ring B of the chalcone skeleton appeared to be necessary for minimal activities against MRSA. In most cases, the optimal potential of the active compounds were not attained as they were extruded by bacterial efflux pumps. However, the presence of the PAβN significantly increased the antibacterial activities of emodin against Gram-negative MDR E. coli AG102, 100ATet; K. pneumoniae KP55 and KP63 by >4–64 g/mL. The antibacterial activities were substantially enhanced and were higher than those of the standard drug, chloramphenicol. These data clearly demonstrate that the active compounds, having the necessary pharmacophores for antibacterial activities, including some quinones and chalcones are substrates of bacterial efflux pumps and therefore should be combined to efflux pump inhibitors in the fight against MDR bacterial infections.

A clerodane diterpene from Polyalthia longifolia as a modifying agent of the resistance of methicillin resistant Staphylococcus aureus

BACKGROUND

Staphylococcus aureus infections are raising serious concern across the world. The effectiveness of conventional drugs is continuously decreasing due to global emergence of multidrug resistance (MDR) and therefore, new resistance-modifying agents (RMAs) are highly needed.

HYPOTHESIS

Clerodane diterpene 16α-hydroxycleroda-3,13(14)-Z-dien-15,16-olide (CD) from leaves of Polyalthia longifolia (Sonn.) Thwaites (Annonaceae) as RAM will be useful in improving the current treatment strategies for staphylococcal infections.

STUDY DESIGN

In the present study, we determine the resistance-modifying activity of CD using clinical isolates of MRSA. Further, the influence of CD on innate immune response was also evaluated in vitro and in vivo. The nature of potential interactions was determined by fractional inhibitory concentration indices (FICIs) calculated from microdilution assays and time-kill curves.

RESULTS

The result of in vitro combination study showed that CD significantly reduced MIC of fluoroquinolones up to 16-folds (FICI 0.315-0.500), while in S. aureus infected Swiss albino mice model, combination of CD with norfloxacin, significantly (p<0.01, p<0.001) lowered the systemic microbial burden in blood, liver, kidney, lung and spleen tissues in comparison to CD, norfloxacin alone as well as untreated control. Flow cytometry analysis clearly showed that CD significantly inhibited EtBr efflux and extended post-antibiotic effect. In qRT-PCR analysis, CD alone as well as in combination, significantly modulated the expression of various efflux pump genes including norA up to 2-fold in clinical isolate MRSA-ST2071. Further, the in vitro combination study of the CD (10, 5, 2.5µg/ml) along with the norfloxacin (10µg/ml) depicted a significant decline in the pro-inflammatory cytokines, IL6 and TNF-α. In septic shock mice model, CD did not exhibit any significant changes in the level of pro-inflammatory cytokines.

CONCLUSION

This is the first report on drug resistance-modifying potential of CD through inhibition of MDR efflux pump.

SILA-421 activity in vitro against rifampicin-susceptible and rifampicin-resistant Mycobacterium tuberculosis, and in vivo in a murine tuberculosis model

Due to the emergence of multidrug-resistant and extensively drug-resistant tuberculosis (TB), there is an urgent need for new TB drugs or for compounds that improve the efficacy of currently used drugs. In this study, time-kill kinetics of SILA-421 as a single drug and in combination with isoniazid (INH), rifampicin (RIF), moxifloxacin (MXF) or amikacin (AMK) against Mycobacterium tuberculosis were assessed. Therapeutic efficacy in vivo in a mouse TB model was also studied. Further in vitro analysis was performed with a RIF-susceptible and RIF-resistant strains of M. tuberculosis. When used as a single drug, SILA-421 in vitro showed concentration-dependent and time-dependent bactericidal activity. SILA-421 also enhanced the activity of INH and RIF, resulting in synergy in the case of INH. Emergence of INH resistance following exposure to INH can be prevented by the addition SILA-421. SILA-421 had no additional value in combination with MXF or AMK. Furthermore, SILA-421 enhanced the activity of RIF towards a RIF-resistant strain and resulted in complete elimination of RIF-resistant mycobacteria. Unfortunately, in mice with TB induced by a Beijing genotype strain, addition of SILA-421 to an isoniazid-rifampicin-pyrazinamide regimen for 13 weeks did not result in enhanced therapeutic efficacy.

Structure-activity relationships of a novel pyranopyridine series of Gram-negative bacterial efflux pump inhibitors

Recently we described a novel pyranopyridine inhibitor (MBX2319) of RND-type efflux pumps of the Enterobacteriaceae. MBX2319 (3,3-dimethyl-5-cyano-8-morpholino-6-(phenethylthio)-3,4-dihydro-1H-pyrano[3,4-c]pyridine) is structurally distinct from other known Gram-negative efflux pump inhibitors (EPIs), such as 1-(1-naphthylmethyl)-piperazine (NMP), phenylalanylarginine-β-naphthylamide (PAβN), D13-9001, and the pyridopyrimidine derivatives. Here, we report the synthesis and biological evaluation of 60 new analogs of MBX2319 that were designed to probe the structure activity relationships (SARs) of the pyranopyridine scaffold. The results of these studies produced a molecular activity map of the scaffold, which identifies regions that are critical to efflux inhibitory activities and those that can be modified to improve potency, metabolic stability and solubility. Several compounds, such as 22d-f, 22i and 22k, are significantly more effective than MBX2319 at potentiating the antibacterial activity of levofloxacin and piperacillin against Escherichia coli.

Thioridazine for treatment of tuberculosis: promises and pitfalls

The articles by De Knegt et al. and Singh et al. in a recent issue of this Journal address one of the current debates regarding the potential role of thioridazine in the treatment of tuberculosis. This commentary presents a summary of the available evidence, and, emphasizing the need for further research, asks the question: "How far can we go in repurposing thioridazine?"

Effect of galbanic Acid, a sesquiterpene coumarin from ferula szowitsiana, as an inhibitor of efflux mechanism in resistant clinical isolates of Staphylococcus aureus

Galbanic acid, a sesquiterpene coumarin from Ferula szowitsiana roots, was investigated for its potentiating effect on the antimicrobial activity of antibiotics as well as ethidium bromide, in 6 multidrug resistance (MDR) clinical isolates of Staphylococcus aureus. Galbanic acid had inhibitory effect on none of the isolated bacteria tested (up to 800 μg /ml). The MIC range of ciprofloxacin, tetracycline and ethidium bromide, against all tested S. aureus were 10-80, 10-80 and 4-16 μg/ml, respectively. These were reduced to ≤2.5-5, 2.5-5 and 0.5-2 μg/ml in the presence of galbanic acid (300 μg /ml) or verapamil (100 μg /ml). The rate of ethidium bromide (2 μg /ml) accumulation in clinical isolates was enhanced with galbanic acid (300 μg /ml). There is also a decrease in loss of ethidium bromide from bacteria in the presence of galbanic acid. Similar results were obtained when verapamil (100 μg /ml) was used as an efflux pump inhibitor. Galbanic acid, like verapamil, a typical inhibitor of efflux pump, reduced the MIC of ethidium bromide and tested antibiotics. Since efflux is the only known reported mechanism for ethidium bromide resistance, the reduction in ethidium bromide MIC and enhanced accumulation as well as decreased efflux of ethidium bromide in the presence of galbanic acid, can be attributed to this efflux inhibitory properties.

Effects of efflux pump inhibitors on the multidrug resistance of Helicobacter pylori

AIM: To evaluate the effects of efflux pump inhibitors (EPIs) on the multidrug resistance (MDR) of Helicobacter pylori

(H.pylori).

METHODS: H.pylori strains were isolated and cultured on Brucella agar plates containing 10% sheep blood. The MDR of H.pylori strains was induced with chloramphenicol. The susceptibility of multidrug-resistant H.pylori strains and their parental strains to nine antibiotics was assessed by agar dilution test in the presence and absence of EPIs such as CCCP, reserpine and pantoprazole. The minimal inhibitory concentrations (MICs) of different antibiotics against multidrug-resistant H.pylori strains were determined. Similarly, the impact of five proton pump inhibitors (PPIs) on the MICs of these antibiotics was also tested and compared.

RESULTS: Four multidrug-resistant H.pylori strains were induced successfully. The susceptibility of these multidrug-resistant strains to some antibiotics such as cefotaxime were partly restored by CCCP and pantoprazole, but not by reserpine. Of the five PPIs tested, rabeprazole reduced the MICs of metronidazole and amoxicillin against multidrug-resistant H.pylori strains to one forth and one third, respectively, pantoprazole reduced both of them to one half, and the remaining two PPIs showed no obvious effects.

CONCLUSION: Some EPIs can potentiate the activity of antibiotics that are putative substrates of the efflux pump system of H.pylori. Of all PPIs tested, rabeprazole is the most effective one to reduce the MDR of H.pylori.