INTRODUCTION:

Staphylococcus aureus is the most pathogenic specie of Staphylococcus and one of the most dangerous human pathogens because of its virulence, the severity of its infections and its ability to develop antibiotic resistance‎^1,2,3,4. It exists commensally on the skin and mucous membranes and can colonize many sites in human body especially the anterior nares. All that provides a reservoir from which S. aureus is introduced in the host when its defense is compromised and thus causing high risk infections‎^1,2. Infections of S. aureus include: (i) superficial lesions such as wound and burn infections⁵ and cutaneous infections⁶ such as impetigo, folliculitis, cellulitis and focal and nodular abscesses (ii) toxins such as staphylococcal food poisoning, scalded skin syndrome and toxic shock syndrome, and (iii) systemic and life-threatening conditions such as endocarditis, osteomyelitis, septic arthritis, prosthetic joint infections, pneumonia, brain abscesses, meningitis, and bacteremia^7,8,9.

Antibiotic resistance in S. aureus:

Staphylococcus aureus is one of the ESCAPE bacteria that develop resistance to a wide range of antimicrobials. The mechanisms of antimicrobial resistance in S. aureus include: (1) Producing enzymes that inactivate or destroy the antibiotic, (2) Active efflux, (3) Reducing the bacterial cell wall permeability and (4) Developing alternative metabolic pathways to those inhibited by the antimicrobial¹⁰.

Efflux is responsible for S. aureus resistance to a wide range of antibiotics as fluoroquinolones, macrolides, chloramphenicol and tetracyclines. One of fluoroquinolones resistance mechanism is the induction of NorA multidrug resistance efflux pump encoded by norA gene‎‎¹¹. Also, one mechanism of macrolides resistance mechanisms in S. aureus is the efflux mediated by msrA gene^12,13. Chloramphenicol resistance is mediated by efflux pumps encoded by either fexA¹⁴ or cfr genes¹⁵. Resistance to tetracycline is mediated by efflux pumps encoded by tetK and tetL genes which confer resistance to tetracycline and doxycycline‎¹⁶.

Efflux in S. aureus:

Efflux pumps are membrane hydrophobic proteins capable of expelling noxious molecules to detoxify the cells and exporting specific drug classes or structurally diverse compounds and toxins using ATP or ion gradients as a source of energy‎¹⁷. They extrude antimicrobials as antibiotics and biocides as an accidental function^18,19, however, efflux-mediated resistance of antimicrobials is increasingly identified in bacteria and became an important resistance mechanism²⁰.

Active drug efflux pumps are classified into two transporter types: primary active transporters and secondary active transporters. Both types consist of integral membrane transporter proteins that attach to antibacterials and actively catalyze their translocation in an outward direction²¹. The primary active transporters use the energy in the form of ATP to transport drugs through the hydrolysis of ATP^22,23. On the other hand, the secondary active transporters indirectly utilize the energy derived from ATP hydrolysis and transport molecules across an electrochemical concentration gradient by coupling with another compound, where H+ or Na+ is the driving force^24,25.

Secondary active membrane transporters are highly substrate specific, and are involved in the transport of sugars, vitamins, peptides, fatty acids and amino acids²⁶. The substrate recognition sites on these transporters are used as antimicrobial drug targets²⁵.

Bacterial secondary active transporters, involved in antimicrobial drug mechanisms, are classified based on their sequence and functional similarities into: (1) the major facilitator superfamily (MFS); (2) the small multidrug resistance (SMR) family; (3) the resistance-nodulation-cell division (RND) superfamily; (4) the multidrug and toxic compound extrusion (MATE) family; and (5) the adenosine-triphosphate (ATP)-binding cassette (ABC) superfamily²⁷. The transporters of MFS, SMR, MATE and RND efflux systems utilize the proton motive force to control their substrates efflux by an antiport H+: drug mechanism, except the MATE family, that also are able to use the sodium membrane gradient as a source of energy. However, the ABC superfamily transporters are primary ones that utilize ATP to control the extrusion of their substrates^25,27.

Also, efflux pumps systems can be classified depending on their specificities for substrates into: specific, extruding only one antibiotic or class of antibiotics, or, multidrug efflux pumps that are able to extrude more than one class of antibiotics and/or other antimicrobial compounds²⁸. Multidrug efflux pumps (MDR) have great importance as they cause resistance to multiple drugs, promote cross-resistance between antibiotics and other antimicrobials and improve the bacterial fitness and survival making the treatment difficult^20,29. They include: MFS, RND, MATE, SMR and the recently described proteobacterial antimicrobial compound efflux (PACE) family³⁰^,31,³². Moreover, MDR efflux pumps can be classified into chromosomally-encoded that are responsible for the intrinsic resistance (NorA, NorB, MepA and MdeA) or plasmid-encoded efflux pumps (QacA/B) that provide the transferable mode of resistance^33,34.

In Gram-positive bacteria (GPB), drug resistance is mainly mediated by cytoplasmic membrane located efflux transporters. On the other hand, efflux pumps in Gram-negative bacteria (GNB) are more complex because of their multi-layered cell envelop: the outer membrane and the inner cytoplasmic membrane, which are separated by a periplasmic space that form a tripartite drug efflux protein channel. In GNB, RND efflux pumps mainly involved in their intrinsic antibiotic resistance and expel a broad spectrum of antibiotics and biocides as fluoroquinolones, β-lactams, tetracycline and linezolid^35,36 On the other hand, in GPB, efflux pumps belong to four families: MFS, SMR, ABC and MATE^37,38,39,40. MFS efflux proteins are composed of 380–520 amino acids that are putatively arranged into 12 membrane-spanning helices, with a large cytoplasmic loop between helices six and seven^37,41, while, SMR transporters consist of approximately 110 amino acids and contain four transmembrane helices³⁷. In case of MATE transporters, they consist of 400–700 amino acids that form 12 transmembrane helices. The ABC transporters consists of four conserved domains: two transmembrane domains and two nucleotide-binding domains⁴².

In S. aureus, MFS efflux pumps include NorA, NorB, NorC, MdeA, SdrM, QacA/B, Tet^38, TetK, LmrB, Bmr and Bmr3^{‎37,‎41,‎43,} while, SMR efflux pumps include Smr, QacG, QacH and QacJ^‎37,‎44. Only one MATE efflux pump present in S. aureus, MepA‎‎⁴⁵. Examples of ABC efflux pumps in S. aureus are Sav1866, AbcA^‎46 and MsrA‎‎⁴⁷.

Assessment of Efflux Activity:

Several techniques have been used to identify the bacterial active efflux as using the radiolabelled substrates, fluorometric assays or the decrease in the minimum inhibitory concentration (MIC) of the antibiotic in the presence of efflux inhibitors⁴⁸. Most studies that assess the efflux-mediated resistance in S. aureus uses the MIC method^49,50, but this method is laborious and depends on the susceptibility of the efflux system(s) to the efflux inhibitor, which can greatly differ^51,52.

Some techniques use ethidium bromide (EtBr, substrate for the majority of S. aureus multidrug efflux pumps) to assess S. aureus efflux activity such as EtBr-Agar Cartwheel method⁵³. This method depends on using EtBr as a marker for indirect efflux assessment through the determination of MICs of EtBr^54,55 in the absence/presence of efflux inhibitors^48,56 and evaluating the capacity of cells to retain/extrude EtBr⁵³. In addition, it permits the screening of large number of clinical isolates to detect the isolates with active efflux activity. Other techniques use the assays that evaluate the efflux activity directly depending on the real-time fluorometry which extensively characterize the efflux activity and also confirm the results of EtBr-agar Cartwheel method on a real-time basis⁵⁷.

Acridine orange agar method is another technique that used acridine orange (AO; N,N,N’,N’-tetramethylacridine-3,6- diamine) to assess the efflux activity of S. aureus. It is a non-toxic fluorescent chromophore, cell permeable, interacts with DNA and RNA and a substrate for bacterial efflux pumps^58,59. Therefore, it is used to develop a non-toxic, fluorescent based agar system to screen the bacterial efflux pump systems. The method based on the same principle of EtBr method. The AO agar method can readily identify Gram-positive and Gram-negative bacterial strains that overexpress the efflux pump systems⁶⁰. Despite, both methods are used, many countries discourage the use and the disposal of EtBr because it is a toxic and carcinogenic agent⁶¹.

Inhibition of efflux activity in S. aureus:

Efflux pump inhibitors (EPIs) are promising therapeutic compounds that cause efflux pumps inhibition by one or more mechanisms, leading to inactive drug efflux. When used in combination with antibiotics, they can restore and improve antibiotics activity against efflux mediated bacterial resistance as they increase the intracellular antibiotics concentrations that were extruded by efflux pumps. Therefore, they decrease the intrinsic antibiotic resistance and reverse the acquired resistance associated with overexpression of efflux pumps^62,63,64.

There are many strategies or different approaches to overcome the efflux pump activity such as: (1) modifying antibiotics chemical design to decrease their affinity for efflux pump binding sites; (2) increasing antibiotics influx by using membrane permeabilizers; (3) inhibiting the functional assembly of efflux pump components; (4) eliminating the energy that support efflux; (5) inserting a molecular plug inside the membrane channels that expel the antibiotic; (6) down-regulating the expression of efflux pump genes and/or reducing the active efflux level in the bacterial envelope; and (7) generating a dynamic competition, between the antibiotic and substrate during efflux⁶⁵. But, the most effective EPI strategy is the competitive binding with higher binding affinities and subsequently blocking and/or reducing flexibility of the binding sites to interfere with the binding of other drug substrates⁶⁶.

1. Synthetic EPIs:

1.1. Proton pump inhibitors:

Proton pump inhibitors as omeprazole and lansoprazole are synthetic inhibitors of H+ and/or K+ ATPase pump leading to the elimination of the energy that support the efflux. They are less potent than reserpine in increasing the activity of fluoroquinolones⁶⁷. The synthetic omeprazole derivatives showed improved EPI activities when used in combination with fluoroquinolones⁶⁸. However, these compounds showed low activity and can't achieve the required effectiveness to be used as EPIs⁶⁹.

1.2. P-glycoprotein inhibitors:

P-glycoprotein (P-gp) is an energy-dependent membrane-bound efflux protein that is located in the mammalian plasma membrane and belongs to the mammalian ABC efflux systems⁷⁰. Bacterial MFS and MATE transporters and P-glycoprotein aren't structurally homologs, but they have similar substrate profiles. Therefore, some mammalian MDR inhibitors also affect the bacterial efflux^67,71. Elacridar, a synthetic P-gp inhibitor, was found to inhibit EtBr efflux and potentiate the antibiotics activity in S. aureus. It inhibited the activity of NorA-and MepA efflux pumps and potentiated norfloxacin and ciprofloxacin activities⁶⁹. Other P-gp inhibitors such as biricodar and timcodar could inhibit the MDR and had low in-vivo toxicity^72,73. They enhanced the action of many antibiotics against S. aureus. They represent a class of bacterial efflux inhibitors that can be used in combination with antibiotics⁷³.

Phenothiazines (as thioridazine and prochlorperazine), their derivatives (as methylene blue, promethazine, chlorpromazine) and thioxanthenes (as trans-(E)-flupentixol) act as dopamine receptor antagonists and are used as antiemetic and neuroleptic agents. They are p-glycoprotein inhibitors that can inhibit NorA and MepA function in S. aureus and potentiated norfloxacin activity^74,75.

1.3. COX-2 inhibitors:

Celecoxib, a nonsteroidal anti-inflammatory drug and a specific COX-2 inhibitor, was found to have an inhibitory effect on EtBr efflux in S. aureus. Moreover, synthetic celecoxib analogue, 3-(4-chlorophenyl)-1-(4-nitrophenyl)-1,4- dihydropyrazolo[4,3-c]-[1,2] benzothiazine 5,5-dioxide (dPBD), showed a superior EPI activity and had a synergistic activity with ciprofloxacin⁷⁶.

1.4. Piperine analogues:

Piperine, the major constituent in black pepper (Piper nigrum), and its derivative, piperidine had an inhibitory action against NorA and MdeA efflux pumps of S. aureus^77,78 The synthetic piperine analogues were found to be more potent than piperine, improved ciprofloxacin activity and effectively decreased the in vitro emergence of ciprofloxacin-resistant S. aureus⁷⁹.

1.5. Phenylpiperidine selective serotonin reuptake inhibitors (PSSRIs)-based EPIs:

Phenylpiperidine selective serotonin reuptake inhibitors (PSSRIs) had been shown to inhibit the function of some MDR efflux pumps. Paroxetine is the first identified PSSRI that inhibited both NorA and MepA efflux pumps in S. aureus. Some studies changed in paroxetine chemical structure by either replacing the benzo [d] ^1,3 dioxole moiety of paroxetine with 2-chloro-5-bromo-phenyl fragment or exchanging the phenoxyl fragment into arylidene one. These structure changes yielded two compounds that had an improved NorA efflux inhibition activity compared to paroxetine^80,81. Also, the deletion of fluorophenyl substituent of paroxetine yielded a compound with selective MepA efflux inhibition activity^64,80.

1.6. Indole-based inhibitors:

Synthetic indole derivatives had been found to have a potent effect against bacteria, viruses, fungi and Leishmania parasites^82,83. Also, both natural and synthetic structurally simple indole derivatives display potent antibacterial activities and had interesting pharmacological properties. For example, bromo-indoles derivatives had antifouling properties, while, 2,3-substituted indole ring derivatives had antitubercular activity^84,85. Also, indole derivatives, in which chromene ring was appended to 2-indole and were resulted from the reaction between salicylaldehydes, substituted acetoacetanilides and indoles, had been found to have a potent antibacterial effect against Staphylococcus aureus^84,86.

Synthetic substituted 5-hydroxyindole was found to be the key precursor of the target indoles. The reaction on the hydroxyl-indole moiety with N,N-dimethyl-chloroethane had been found to yield a potent efflux pump inhibitor^87,88. Also, 5-nitro-2-phenyl-1H-indole (INF 55) is a potent inhibitor of NorA efflux pump that had been found to increase ciprofloxacin activity against S. aureus⁸⁹.

1.7. Mesoionic compounds:

Mesoionic compounds, such as 1,4-diphenyl-5-(5-nitro- 2-furanyl)-1,3,4-triazolium-2-thiol chloride and 4-phenyl-5-(5-nitro-2-furanyl)-1,3,4-thiadiazolium-2-phenylamine chloride, have been found to enhance the activity of tetracycline and erythromycin against S. aureus because they had a unique heterocyclic structure and electrical character that make them able to cross the bacterial cellular membranes and give their action^90,91.

1.8. Berberine–INF 55 hybrids:

In hybrid molecule, the antibiotic and the EPI are linked covalently into a single molecule to match the pharmacokinetic and pharmacodynamic properties of the two agents when administered together and to guarantee the delivery of equimolar concentrations^92,93. Hybrid SS14, is a hybrid of berberine (had antibacterial activity) and INF 55 (EPI, 5-nitro-2-phenyl-1H-indole) which had been found to have significantly stronger antibacterial activity against S. aureus compared with each agent alone^92,94. Another hybrid of berberine and INF 55, showed greater activity than SS14 and berberine alone against S. aureus⁹³.

1.9. Substituted dihydronaphthalene:

Both 3-(3,4-dihydronapth-2-yl)-propenoic acid isobutyl amide (3-PIA) and 4-methyl-N-[2-(1-methyl-1H-pyrrol-2-yl)-1H-benzimidazol-5-yl] benzene sulfonamide are substituted dihydronaphthalene compounds that had an inhibitory effect against NorA efflux pump and enhanced ciprofloxacin activity^95,96.

2. Naturally occurring EPIs:

2.1. Reserpine:

Reserpine is a plant alkaloid that is extracted from Rauwolfia serpentina roots and used as antipsychotic and antihypertensive drug^97,98. It was the first clinical drug that reverse S. aureus MDR phenotype⁹⁹. It targeted the MFS and RND superfamily efflux pumps⁹⁷. It potentiated tetracycline, norfloxacin, ciprofloxacin and levofloxacin activity and reversed NorA-mediated resistance in S. aureus by eliminating the energy that support the efflux^67,71. However, it has a nephrotoxic effect, therefore, its clinical use with antibiotics had not been applied, but it is often used as a reference control for the evaluation of other EPIs^68,100.

2.2. Resin glycosides:

Resin glycosides are glycosyl derivatives of monohydroxy and dihydroxy C₁₄ and C₁₆ fatty acids and their sugar units are D-glucose and epimers of pentoses. The O-glycosidic linkage links the monosaccharide residues with each other and with the aglycone. Orizabins IX and orizabins XV are two resin glycosides that are isolated from morning glory. They had been found to have potent EPI activities¹⁰¹. Orizabin IX and other resin glycosides as murucoidin I, VI and VII and stoloniferin I had been found to potentiate norfloxacin activity against S. aureus^69,102.

2.3. Diterpenes:

Ferruginol (isolated from Chamaecyparis lawsoniana), carnosic acid and carnosol (isolated from rosemary (Rosmarinus officinalis)) and totarol (isolated from Chamaecyparis nootkatensis) are diterpenes that had been showed to have an EPI activity and improved the potency of ciprofloxacin, norfloxacin, erythromycin and tetracycline against S. aureus that contained NorA, MsrA and TetK efflux pumps^103,104.

2.4. Flavonoids:

The 5’-methoxyhydnocarpin-D (5’-MHC), flavonolignan isolated from Mimosa diplotricha, showed a NorA efflux pump inhibitory activity and potentiated the effect of fluoroquinolones as norfloxacin by reducing the active efflux level in the bacterial envelope. When used in combination with berberine, it significantly increases the antibacterial activity of berberine against S. aureus^105,106,107.

Isoflavones including genistein, orobol and biochanin A, isolated from Lupinus argenteus plant, have been found to block the MDR efflux pumps and thus improving the activity of norfloxacin and berberine against clinical S. aureus¹⁰⁸.

Tiliroside, kaempferol-3-O-b-d-(6-E-p-coumaroyl) glucopyranoside, enhanced the activities of ciprofloxacin, norfloxacin, ofloxacin and lomefloxacin¹⁰⁹. Also, it reduced the inhibitory concentrations of EtBr and acriflavine dyes and benzalkonium chloride and cetrimide biocides⁶⁹.

Baicalein, isolated from thyme, showed weak antimicrobial effect and improved the activity of ciprofloxacin, β-lactams (oxacillin, ampicillin and cefmetazole) and tetracycline by reducing the active efflux level in the bacterial envelope^110,111.

Kaempferol-3-O-a-l-(2,4-bis-E-p-coumaroyl) rhamnoside (KCR), isolated from Persea linguem and 4-phenoxy-4’-dimethylaminoethoxychalcone (4-DAEC) enhanced the activity of ciprofloxacin and inhibited EtBr efflux in S. aureus^112,113. Also, 4’,6’-dihydroxy-3’,5’-dimethyl-2’- methoxychalcone, isolated from Dalea versicolor, have been found to have an inhibitory effect against NorA efflux pump¹¹⁴.

Catechin gallates as epicatechin gallate and epigallocatechin gallate (isolated from green tea) have been found to reverse the resistance of methicillin resistant Staphylococcus aureus (MRSA), but weakly inhibited NorA efflux pump (only at a high concentration)^115,116. Epigallocatechin gallate have been found to have an inhibitory activity on TetK efflux pump and thus improved tetracycline activity against tetracycline-resistant S. aureus^115,117 and also enhanced the activity of erythromycin and ciprofloxacin¹¹⁷.

2.5. 4’,5’-O-dicaffeoylquinic acid:

It is isolated from wormwood (Artemisia absinthium) and can be used along with antibiotics and dyes to inhibit the efflux pumps in community-acquired MRSA¹¹⁸. It is a NorA inhibitor that potentiated the activity of berberine and norfloxacin against S. aureus^69,119.

2.6. N-trans-feruloyl 4’-O-methyldopamine:

It is a polyphenolic amide isolated from Mirabilis jalapa Linn. (Nyctaginaceae). Both N-trans-feruloyl 4’-O-methyldopamine and its synthetic derivative, N-trans-3,4-O-dimethylcaffeoyl tryptamine (more potent than its parent molecule), inhibited norfloxacin efflux and enhancing its activity against S. aureus¹²⁰.

2.7. Other natural EPI:

A penta-substituted pyridine derivative (2,6-dimethyl-4-phenylpyridine-3,5-dicarboxylic acid diethyl ester isolated from Jatropha elliptica), coumarins (isolated from Mesua ferrea), olympicin A (isolated from Hypericum olympicum), sarothrin (5,7,40-trihydroxy-3,6,8-trimethoxyflavone isolated from Alkanna orientalis), pheophorbide (a phorphyrin isolated from Berberis plants) and capsaicin (8-methyl-N-vanillyl-6-nonenamide, the major constituent of Capsicum annuum) have been found to have NorA efflux pump inhibitory activities and improved ciprofloxacin and norfloxacin activities against S. aureus ^{107,121,122,123,124,125}.

3. In-vivo clinical trials on efflux:

Only few EPIs had reached the clinical trials, where they failed to be clinically used because of the EPI toxic effects at concentrations at which they are active, low in vivo efficacy, poor solubility and poor pharmacokinetic properties^126,127,128.

According to Lomovskaya et al. (2007)¹²⁹ , the EPI, bis (pyrimidine) sulfonamide, had been developed clinically. It was tested in aerosol formulation form in combination with ciprofloxacin to treat the respiratory infections (caused by MDR Gram‐negative bacteria, as P. aeruginosa) in patients with ventilator‐associated pneumonia and cystic fibrosis^130,131. It was tested in phase I clinical trials, but unfortunately its development was stopped due to tolerability issues¹²⁹.

Xu et al. (2018)¹³² found that verapamil improved bedaquiline efficacy in a murine TB model, but it had no effect on clofazimine oral bioavailability or efficacy in mice. Also, Xu et al. (2018)¹³² found that verapamil has adjunctive activity in vivo and that adjunctive effect may be due to enhanced systemic exposure to companion drugs on mammalian transporters, rather than the inhibition of bacterial efflux pumps. Thus, Xu et al. (2018) concluded that there may be no advantage in administering verapamil versus increasing the doses of companion drugs¹³².

According to Rineh et al. (2018)¹³³, 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, as methylene blue (MB) and toluidine blue O, are hydrophobic cations that are expelled by multidrug bacterial efflux pumps. Rineh et al. (2018)¹³³ reported that the NorA efflux pump inhibitor-methylene blue (EPI-MB) hybrid compound INF55-(Ac)en−MB showed enhanced photodynamic inactivation of MRSA both in vitro and in vivo. Also, Rineh et al. (2018) reported that INF55-(Ac)en−MB and two related hybrids having INF55 and INF271 showed an enhanced aPDI activity in vitro against Escherichia coli and Acinetobacter baumannii and that the two hybrids showed superior effects in murine aPDI infection models¹³³.

According to Sundaramoorthy et al. (2019)¹³⁴, inhibition of MarR by salicylate was used to overcome colistin resistance in E. coli (U3790) strain, but, MarR inhibition by salicylate triggered the expression of AcrAB efflux pumps. Therefore, to fully restore the sensitivity of colistin against colistin resistant in E. coli (U3790) strain, efflux pump inhibitor (synthesized benzochromene derivative; BC1) was used. Therefore, a combination of salicylate (MarR inhibitor), efflux pump inhibitor (BC1) and colistin was used for zebrafish infection. The study of Sundaramoorthy et al. (2019)¹³⁴ found that this combination caused a remarkable reduction in cell counts of U3790 in time-kill assay, was highly effective in reducing the bioburden in infected muscle tissue and effectively restored colistin sensitivity in colistin-resistant clinical isolate of E. coli in vitro and in vivo. However, the reduction in cell counts observed in vitro was not noted in vivo, which could be due to poor solubility of BC1^134.

Hazlett et al. (2019) reported that glycyrrhizin, when combined with ciprofloxacin, decreased the MDR in vivo as it enhanced ciprofloxacin activity, reduced the ocular disease and myeloperoxidase activity and that glycyrrhizin has antimicrobial and anti-inflammatory effects on MDR isolates that caused severe keratitis in vivo¹³⁵.

CONCLUSION AND FUTURE PERSPECTIVE:

The development of new drugs against S. aureus is in decline, but EPIs can potentiate the effect of the existing drugs by overcoming the activity of MDR efflux pumps. Also, most EPIs haven't been approved for clinical use until today because of low stability, low selectivity, high cytotoxicity and the strong pharmacological effects of EPI in eukaryotic systems, especially in human^136,137. Moreover, much is still unknown about the pharmacokinetics, pharmacodynamics and toxicity of the current EPIs^48,138. However, there are some promising trials have been conducted on glycyrrhizin, bis (pyrimidine) sulfonamide, INF55-(Ac)en−Methylene blue and salicylate and BC1 combination.

Therefore, searching for compounds that have efflux-inhibitory activities among the existing pharmaceuticals, the compounds that are isolated from natural sources or the synthesis of novel derivatives is very important to provide antibiotic–EPI combination therapies, as hybrid molecules, for treating the infections of S. aureus.

CONFLICT OF INTEREST:

The authors have no conflicts of interest regarding this investigation.

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Received on 14.11.2021 Modified on 24.12.2021

Research J. Pharm. and Tech 2022; 15(9):4300-4308.

DOI: 10.52711/0974-360X.2022.00722