Discovery of 4-acetyl-3-(4-fluorophenyl)-1-(p-tolyl)-5-methylpyrrole as a dual inhibitor of human P-glycoprotein and Staphylococcus aureus Nor A efflux pump

Photosensitizer associated with efflux pump inhibitors as a strategy for photodynamic therapy against bacterial resistance

Antimicrobial resistance is currently one of the biggest challenges in controlling infectious diseases and was listed among the top 10 threats to global health by the World Health Organization (WHO) in 2023. The antibiotics misuse has led to the widespread emergence of antimicrobial resistance, marking the beginning of the alarming increase in antibiotic resistance. In this context, Antimicrobial Photodynamic Therapy (aPDT) has garnered significant attention from the scientific community due to its potential to effectively eliminate multidrug-resistant pathogenic bacteria and its low propensity to induce drug resistance, which bacteria can quickly develop against traditional antibiotic treatments. However, some efflux pumps can expel diverse substrates from inside the cell, including photosensitizers used in aPDT, contributing to multidrug-resistance mechanisms. Efflux Pump Inhibitors are potential solutions to combat resistance mediated by these pumps and can play a crucial role in enhancing aPDT's effectiveness against multidrug-resistant bacteria. Therefore, combining efflux pumps inhibitors with photosensitizers can possible to eliminate the pathogen more efficiently. This review summarizes the mechanisms in which bacteria resist conventional antibiotic treatment, with a particular emphasis on efflux pump-mediated resistance, and present aPDT as a promising strategy to combat antibiotic resistance. Additionally, we highlighted several molecules of photosensitizer associated with efflux pump inhibitors as potential strategies to optimize aPDT, aiming to offer a perspective on future research directions on aPDT for overcoming the limitations of antibiotic resistance.

Progress on synthesis and structure-activity relationships of lamellarins over the past decade

Lamellarins are polyaromatic alkaloids isolated from marine organisms, including mollusks, tunicates, and sponges. Currently, over 60 structurally distinct natural lamellarins have been reported, and most of them exhibit promising biological activities, such as topoisomerase inhibition, mitochondrial function inhibition, multidrug resistance reversing, and anti-HIV activity. There has also been a significant progress on the synthetic study of lamellarins which has been regularly updated by numerous medicinal chemists as well. This review provides a detailed summary of the synthesis, pharmacology, and structural modification of lamellarins over the past decades.

Piperine analog PGP-41 treatment overcomes paclitaxel resistance in NCI/ADR-RES ovarian cells by inhibition of MDR1

Chemoresistance is one of the leading causes of the failure of chemotherapy. Overexpression of P-glycoprotein (P-gp) in cancer cells is one of the most important contributing factors toward the development of chemoresistance. This study was designed to synthesize the derivatives of dihydronaphthyl and to evaluate the P-gp inhibition activity of these compounds. Among all the compounds, PGP-41 showed the most potent P-gp inhibition activity in colorectal adenocarcinoma LS-180 cells. This compound showed potent P-gp inhibition activity in chemoresistant ovarian cell line NCI/ADR-RES. Paclitaxel is one of the first lines of drugs for treating ovarian cancer and is a substrate of P-gp; therefore, NCI/ADR-RES cells are highly resistant to treatment with paclitaxel. Based on this information, we evaluated PGP-41 to overcome the paclitaxel resistance of NCI/ADR-RES cells. PGP-41 was able to sensitize the NCI/ADR-RES cells to the treatment of paclitaxel, which was evident by the reduced IC50 value of paclitaxel from 6.64 μM to 0.12 μM. The sensitization of NCI/ADR-RES cells by PGP-41 was comparable to that of elacridar and Zosuquidar. Further studies revealed that the PGP-41 exerts its effect by downregulating the expression of P-gp. Reduction of P-gp activity leads to the accumulation of higher intracellular concentration of paclitaxel, and thus allowing it to interact with its targets, which further helps in its increased efficacy. Paclitaxel was able to arrest the sensitized NCI/ADR-RES cells into G2M phase, which ultimately led to the induction of apoptotic proteins and the death of cancer cells. Being a different scaffold from zosuquidar and elacridar, further studies are required to develop PGP-41 into a potential drug to overcome chemoresistance in cancer cells.

Inhibition of Erythromycin and Erythromycin-Induced Resistance among Staphylococcus aureus Clinical Isolates

The increasing incidence of erythromycin and erythromycin-induced resistance to clindamycin among Staphylococcus aureus (S. aureus) is a serious problem. Patients infected with inducible resistance phenotypes may fail to respond to clindamycin. This study aimed to identify the prevalence of erythromycin and erythromycin-induced resistance and assess for potential inhibitors. A total of 99 isolates were purified from various clinical sources. Phenotypic detection of macrolide-lincosamide-streptogramin B (MLSB)-resistance phenotypes was performed by D-test. MLSB-resistance genes were identified using PCR. Different compounds were tested for their effects on erythromycin and inducible clindamycin resistance by broth microdilution and checkerboard microdilution methods. The obtained data were evaluated using docking analysis. Ninety-one isolates were S. aureus. The prevalence of constitutive MLSB, inducible MLSB, and macrolide-streptogramin (MS) phenotypes was 39.6%, 14.3%, and 2.2%, respectively. Genes including ermC, ermA, ermB, msrA, msrB, lnuA, and mphC were found in 82.6%, 5.8%, 7.7%, 3.8%, 3.8%, 13.5%, and 3.8% of isolates, respectively. Erythromycin resistance was significantly reduced by doxorubicin, neomycin, and omeprazole. Quinine, ketoprofen, and fosfomycin combated and reversed erythromycin/clindamycin-induced resistance. This study highlighted the significance of managing antibiotic resistance and overcoming clindamycin treatment failure. Doxorubicin, neomycin, omeprazole, quinine, ketoprofen, and fosfomycin could be potential inhibitors of erythromycin and inducible clindamycin resistance.

Tackling multidrug-resistant Staphylococcus aureus by natural products and their analogues acting as NorA efflux pump inhibitors

Staphylococcus aureus (S. aureus) is a pathogen responsible for various community and hospital-acquired infections with life-threatening complications like bacteraemia, endocarditis, meningitis, liver abscess, and spinal cord epidural abscess. Antibiotics have been used to treat microbial infections since the introduction of penicillin in 1940. In recent decades, the abuse and misuse of antibiotics in humans, animals, plants, and fungi, including the treatment of non-microbial diseases, have led to the rapid emergence of multidrug-resistant pathogens with increased virulence. Bacteria have developed several complementary mechanisms to avoid the effects of antibiotics. These mechanisms include chemical transformations and enzymatic inactivation of antibiotics, modification of antibiotics' target site, and reduction of intracellular antibiotics concentration by changes in membrane permeability or by the overexpression of efflux pumps (EPs). The strategy to check antibiotic resistance includes synthesis of the antibiotic analogues, or antibiotics are given in combination with the adjuvant. The inhibitors of multidrug EPs are considered promising alternative therapeutic options with the potential to revive the effects of antibiotics and reduce bacterial virulence. Natural products played a vital role in drug discovery and significantly contributed to the area of infectious diseases. Also, natural products provide lead compounds that sometimes need modification based on structural and biological properties to meet the drug criteria. This review discusses natural products and their derived compounds as NorA efflux pump inhibitors (EPIs).

Medicinal Chemistry of Inhibitors Targeting Resistant Bacteria

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

Antifungal Azoles as Tetracycline Resistance Modifiers in Staphylococcus aureus

Staphylococcus aureus has developed resistance to antimicrobials since their first use. The S. aureus major facilitator superfamily (MFS) efflux pump Tet(K) contributes to resistance to tetracyclines. The efflux pump diminishes antibiotic accumulation, and biofilm hampers the diffusion of antibiotics. None of the currently known compounds have been approved as efflux pump inhibitors (EPIs) for clinical use. In the current study, we screened clinically approved drugs for possible Tet(K) efflux pump inhibition. By performing in silico docking followed by in vitro checkerboard assays, we identified five azoles (the fungal ergosterol synthesis inhibitors) showing putative EPI-like potential with a fractional inhibitory concentration index of ≤0.5, indicating synergism. The functionality of the azoles was confirmed using ethidium bromide (EtBr) accumulation and efflux inhibition assays. In time-kill kinetics, the combination treatment with butoconazole engendered a marked increase in the bactericidal capacity of tetracycline. When assessing the off-target effects of the azoles, we observed no disruption of bacterial membrane permeability and polarization. Finally, the combination of azoles with tetracycline led to a significant eradication of preformed mature biofilms. This study demonstrates that azoles can be repurposed as putative Tet(K) EPIs and to reduce biofilm formation at clinically relevant concentrations. IMPORTANCE Staphylococcus aureus uses efflux pumps to transport antibiotics out of the cell and thus increases the dosage at which it endures antibiotics. Also, efflux pumps play a role in biofilm formation by the excretion of extracellular matrix molecules. One way to combat these pathogens may be to reduce the activity of efflux pumps and thereby increase pathogen sensitivity to existing antibiotics. We describe the in silico-based screen of clinically approved drugs that identified antifungal azoles inhibiting Tet(K), a pump that belongs to the major facilitator superfamily, and showed that these compounds bind to and block the activity of the Tet(K) pump. Azoles enhanced the susceptibility of tetracycline against S. aureus and its methicillin-resistant strains. The combination of azoles with tetracycline led to a significant reduction in preformed biofilms. Repurposing approved drugs may help solve the classical toxicity issues related to efflux pump inhibitors.

An Update on Staphylococcus aureus NorA Efflux Pump Inhibitors

BACKGROUND

Methicillin-resistant and vancomycin-resistant Staphylococcus aureus are pathogens causing severe infectious diseases that pose real public health threats problems worldwide. In S. aureus, the most efficient multidrug-resistant system is the NorA efflux pump. For this reason, it is critical to identify efflux pump inhibitors.

OBJECTIVE

In this paper, we present an update of the new natural and synthetic compounds that act as modulators of antibiotic resistance through the inhibition of the S. aureus NorA efflux pump.

RESULTS

Several classes of compounds capable of restoring the antibiotic activity have been identified against resistant-S. aureus strains, acting as NorA efflux pump inhibitors. The most promising classes of compounds were quinolines, indoles, pyridines, phenols, and sulfur-containing heterocycles. However, the substantial degree structural diversity of these compounds makes it difficult to establish good structure- activity correlations that allow the design of compounds with more promising activities and properties.

CONCLUSION

Despite substantial efforts put forth in the search for new antibiotic adjuvants that act as efflux pump inhibitors, and despite several promising results, there are currently no efflux pump inhibitors authorized for human or veterinary use, or in clinical trials. Unfortunately, it appears that infection control strategies have remained the same since the discovery of penicillin, and that most efforts remain focused on discovering new classes of antibiotics, rather than trying to prolong the life of available antibiotics, and simultaneously fighting mechanisms of bacterial resistance.

Clinically-Relevant ABC Transporter for Anti-Cancer Drug Resistance

Multiple drug resistance (MDR), referring to the resistance of cancer cells to a broad spectrum of structurally and mechanistically unrelated drugs across membranes, severely impairs the response to chemotherapy and leads to chemotherapy failure. Overexpression of ATP binding cassette (ABC) transporters is a major contributing factor resulting in MDR, which can recognize and mediate the efflux of diverse drugs from cancer cells, thereby decreasing intracellular drug concentration. Therefore, modulators of ABC transporter could be used in combination with standard chemotherapeutic anticancer drugs to augment the therapeutic efficacy. This review summarizes the recent advances of important cancer-related ABC transporters, focusing on their physiological functions, structures, and the development of new compounds as ABC transporter inhibitors.

Chemical molecular-based approach to overcome multidrug resistance in cancer by targeting P-glycoprotein (P-gp)

Multidrug resistance (MDR) remains one of the major impediments for efficacious cancer chemotherapy. Increased efflux of multiple chemotherapeutic drugs by transmembrane ATP-binding cassette (ABC) transporter superfamily is considered one of the primary causes for cancer MDR, in which the role of P-glycoprotein (P-gp/ABCB1) has been most well-established. The clinical co-administration of P-gp drug efflux inhibitors, in combination with anticancer drugs which are P-gp transport substrates, was considered to be a treatment modality to surmount MDR in anticancer therapy by blocking P-gp-mediated multidrug efflux. Extensive attempts have been carried out to screen for sets of nontoxic, selective, and efficacious P-gp efflux inhibitors. In this review, we highlight the recent achievements in drug design, characterization, structure-activity relationship (SAR) studies, and mechanisms of action of the newly synthetic, potent small molecules P-gp inhibitors in the past 5 years. The development of P-gp inhibitors will increase our knowledge of the mechanisms and functions of P-gp-mediated drug efflux which will benefit drug discovery and clinical cancer therapeutics where P-gp transporter overexpression has been implicated in MDR.

Medicinal chemistry strategies to discover P-glycoprotein inhibitors: An update

The presence of multidrug resistance (MDR) in malignant tumors is one of the primary causes of treatment failure in cancer chemotherapy. The overexpression of the ATP binding cassette (ABC) transporter, P-glycoprotein (P-gp), which significantly increases the efflux of certain anticancer drugs from tumor cells, produces MDR. Therefore, inhibition of P-gp may represent a viable therapeutic strategy to overcome cancer MDR. Over the past 4 decades, many compounds with P-gp inhibitory efficacy (referred to as first- and second-generation P-gp inhibitors) have been identified or synthesized. However, these compounds were not successful in clinical trials due to a lack of efficacy and/or untoward toxicity. Subsequently, third- and fourth-generation P-gp inhibitors were developed but dedicated clinical trials did not indicate a significant therapeutic effect. In recent years, an extraordinary array of highly potent, selective, and low-toxicity P-gp inhibitors have been reported. Herein, we provide a comprehensive review of the synthetic and natural products that have specific inhibitory activity on P-gp drug efflux as well as promising chemosensitizing efficacy in MDR cancer cells. The present review focuses primarily on the structural features, design strategies, and structure-activity relationships (SAR) of these compounds.

Efflux pump inhibitors for bacterial pathogens: From bench to bedside

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

Antimicrobial activity of Mimosa caesalpiniifolia Benth and its interaction with antibiotics against Staphylococcus aureus strains overexpressing efflux pump genes

This study aimed to evaluate the antimicrobial activity of the dichloromethane fraction (DCMF) from the stem bark of Mimosa caesalpiniifolia and its effect on the activity of conventional antibiotics against Staphylococcus aureus strains overexpressing specific efflux pump genes. DCMF showed activity against S. aureus, Staphylococcus epidermidis and Candida albicans. Addition of DCMF at subinhibitory concentrations to the growth media enhanced the activity of norfloxacin, ciprofloxacin and ethidium bromide against S. aureus strains overexpressing norA suggesting the presence of efflux pump inhibitors in its composition. Similar results were verified for tetracycline against S. aureus overexpressing tetK, as well as, for ethidium bromide against S. aureus overexpressing qacC. These results indicate that M. caesalpiniifolia is a source of molecules able to modulate the fluoroquinolone- and tetracycline-resistance in S. aureus probably by inhibition of NorA, TetK and QacC respectively. SIGNIFICANCE AND IMPACT OF THE STUDY: Drug resistance is a common problem in patients with infectious diseases. Dichloromethane fraction from the stem bark of Mimosa caesalpiniifolia showed antimicrobial activity against Gram-positive bacterium Staphylococcus aureus and against Candida albicans, but did not show activity against Gram-negative specie Escherichia coli. Moreover, this fraction was able to potentiate the action of norfloxacin, ciprofloxacin and tetracycline against S. aureus strains overexpressing different efflux pump genes. Thus, Mimosa caesalpiniifolia is a source of efflux pump inhibitors which could be used in combination with fluoroquinolones or tetracycline in the treatment of infectious diseases caused by S. aureus strains overexpressing efflux pump genes.

Ciprofloxacin Enhances the Chemosensitivity of Cancer Cells to ABCB1 Substrates

ABCB1 is one of the major drug efflux transporters that is known to cause multidrug resistance (MDR) in cancer patients receiving chemotherapy for the treatment of solid tumors and hematological malignancies. Inhibition of ABCB1 efflux function is important for maintaining the intracellular concentration of chemotherapeutic drugs. Here, we evaluated ciprofloxacin for its ability to reverse MDR caused by the overexpression of ABCB1. Cytotoxicity of ciprofloxacin was determined by the MTT assay. The chemosensitizing effects of ciprofloxacin were determined in combination with ABCB1 substrates. The intracellular accumulation and efflux of ABCB1 substrates was measured by a scintillation counter, and protein expression was determined by the Western blotting. Vanadate-sensitive ATPase assay was performed to determine the effect of ciprofloxacin on the ATPase activity of ABCB1, and docking analysis was done to determine the interaction of ciprofloxacin with ABCB1. Ciprofloxacin significantly potentiated the cytotoxic effects of ABCB1 substrates in ABCB1-overexpressing cells. Furthermore, ciprofloxacin increased the intracellular accumulation and decreased the efflux of [³H]-paclitaxel without altering the expression of ABCB1. Ciprofloxacin stimulated the ATPase activity of ABCB1 in a concentration-dependent manner. Our findings showed that ciprofloxacin potently inhibits the ABCB1 efflux function and it has potential to be developed as a combination anticancer therapy.

Modulation of antimicrobial efflux pumps of the major facilitator superfamily in Staphylococcus aureus

Variants of the microorganism Staphylococcus aureus which are resistant to antimicrobial agents exist as causative agents of serious infectious disease and constitute a considerable public health concern. One of the main antimicrobial resistance mechanisms harbored by S. aureus pathogens is exemplified by integral membrane transport systems that actively remove antimicrobial agents from bacteria where the cytoplasmic drug targets reside, thus allowing the bacteria to survive and grow. An important class of solute transporter proteins, called the major facilitator superfamily, includes related and homologous passive and secondary active transport systems, many of which are antimicrobial efflux pumps. Transporters of the major facilitator superfamily, which confer antimicrobial efflux and bacterial resistance in S. aureus, are good targets for development of resistance-modifying agents, such as efflux pump inhibition. Such modulatory action upon these antimicrobial efflux systems of the major facilitator superfamily in S. aureus may circumvent resistance and restore the clinical efficacy of therapy towards S. aureus infection.

Boeravinone B, A Novel Dual Inhibitor of NorA Bacterial Efflux Pump of Staphylococcus aureus and Human P-Glycoprotein, Reduces the Biofilm Formation and Intracellular Invasion of Bacteria

This study elucidated the role of boeravinone B, a NorA multidrug efflux pump inhibitor, in biofilm inhibition. The effects of boeravinone B plus ciprofloxacin, a NorA substrate, were evaluated in NorA-overexpressing, wild-type, and knocked-out Staphylococcus aureus (SA-1199B, SA-1199, and SA-K1758, respectively). The mechanism of action was confirmed using the ethidium bromide accumulation and efflux assay. The role of boeravinone B as a human P-glycoprotein (P-gp) inhibitor was examined in the LS-180 (colon cancer) cell line. Moreover, its role in the inhibition of biofilm formation and intracellular invasion of S. aureus in macrophages was studied. Boeravinone B reduced the minimum inhibitory concentration (MIC) of ciprofloxacin against S. aureus and its methicillin-resistant strains; the effect was stronger in SA-1199B. Furthermore, time–kill kinetics revealed that boeravinone B plus ciprofloxacin, at subinhibitory concentration (0.25 × MIC), is as equipotent as that at the MIC level. This combination also had a reduced mutation prevention concentration. Boeravinone B reduced the efflux of ethidium bromide and increased the accumulation, thus strengthening the role as a NorA inhibitor. Biofilm formation was reduced by four–eightfold of the minimal biofilm inhibitory concentration of ciprofloxacin, effectively preventing bacterial entry into macrophages. Boeravinone B effectively inhibited P-gp with half maximal inhibitory concentration (IC₅₀) of 64.85 μM. The study concluded that boeravinone B not only inhibits the NorA-mediated efflux of fluoroquinolones but also considerably inhibits the biofilm formation of S. aureus. Its P-gp inhibition activity demonstrates its potential as a bioavailability and bioefficacy enhancer.

Pharmaceutical Approaches to Target Antibiotic Resistance Mechanisms

There is urgent need for new therapeutic strategies to fight the global threat of antibiotic resistance. The focus of this Perspective is on chemical agents that target the most common mechanisms of antibiotic resistance such as enzymatic inactivation of antibiotics, changes in cell permeability, and induction/activation of efflux pumps. Here we assess the current landscape and challenges in the treatment of antibiotic resistance mechanisms at both bacterial cell and community levels. We also discuss the potential clinical application of chemical inhibitors of antibiotic resistance mechanisms as add-on treatments for serious drug-resistant infections. Enzymatic inhibitors, such as the derivatives of the β-lactamase inhibitor avibactam, are closer to the clinic than other molecules. For example, MK-7655, in combination with imipenem, is in clinical development for the treatment of infections caused by carbapenem-resistant Enterobacteriaceae and Pseudomonas aeruginosa, which are difficult to treat. In addition, other molecules targeting multidrug-resistance mechanisms, such as efflux pumps, are under development and hold promise for the treatment of multidrug resistant infections.

Inhibition of the NorA efflux pump of Staphylococcus aureus by synthetic riparins

AIM

The goal of this study was to increase knowledge about the antimicrobial activity of some synthetic Riparin-derived compounds, alone or in combination with fluoroquinolone antibiotics, against a strain of Staphylococcus aureus resistant to fluoroquinolone by way of overexpression of the NorA efflux pump.

METHODS AND RESULTS

Microdilution tests showed that Riparins A and B did not show any significant antibacterial activity against Staph. aureus strains. On the other hand, the intrinsic antibacterial activity increased with increasing lipophilicity of the compounds, in the following order: Riparin-D (MIC 256 μg ml^-1 ; Log P 2·95) < Riparin-C (MIC 102 μg ml^-1 ; Log P 3·22) < Riparin-E (MIC 16 μg ml^-1 ; Log P 3·57). The addition of all riparins to growth media at subinhibitory concentrations caused an increase in the antibacterial activity of antibiotics against the NorA-overexpressing test strain. Riparin-B, which has two methoxyl groups at the phenethyl moiety, showed the best modulatory effect.

CONCLUSIONS

Riparin-E is a good anti-staphylococci agent, while Riparin-B functions as a NorA efflux pump inhibitor.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our data suggest the possibility of using Riparin-B in combination with norfloxacin or ciprofloxacin for therapy of infections caused by multi-drug resistant Staph. aureus.

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.

Tetrahydropyrrolization of Resveratrol and Other Stilbenes Improves Inhibitory Effects on DNA Oxidation

The inhibitory effect of resveratrol on DNA oxidation caused by 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH) was found to be enhanced if the C=C bond in resveratrol was converted into tetrahydropyrrole by reaction with azomethine ylide (CH2 =N(+) (CH3 )CH2 (-) ). This encouraged us to explore whether the inhibitory activities of other stilbenes could also be increased by the same method. We found that the inhibitory effects of the tetrahydropyrrole derivatives on AAPH-induced oxidation of DNA were higher than those of the corresponding stilbenes, because the tetrahydropyrrole motif can provide hydrogen atoms to be abstracted by radicals. Therefore, the tetrahydropyrrolization offered an advantage for enhancing the antioxidant effects of stilbenes. Notably, (CH3 )3 SiCH2 N(CH3 )CH2 OCH3 (in the presence of CF3 COOH) and (CH3 )3 NO (in the presence of LiN(iPr)2 ) can be used to generate azomethine ylide for the tetrahydropyrrolization of stilbenes containing electron-withdrawing and -donating groups, respectively.

New Is Old, and Old Is New: Recent Advances in Antibiotic-Based, Antibiotic-Free and Ethnomedical Treatments against Methicillin-Resistant Staphylococcus aureus Wound Infections

Staphylococcus aureus is the most common pathogen of wound infections. Thus far, methicillin-resistant S. aureus (MRSA) has become the major causative agent in wound infections, especially for nosocomial infections. MRSA infections are seldom eradicated by routine antimicrobial therapies. More concerning, some strains have become resistant to the newest antibiotics of last resort. Furthermore, horizontal transfer of a polymyxin resistance gene, mcr-1, has been identified in Enterobacteriaceae, by which resistance to the last group of antibiotics will likely spread rapidly. The worst-case scenario, "a return to the pre-antibiotic era", is likely in sight. A perpetual goal for antibiotic research is the discovery of an antibiotic that lacks resistance potential, such as the recent discovery of teixobactin. However, when considering the issue from an ecological and evolutionary standpoint, it is evident that it is insufficient to solve the antibiotic dilemma through the use of antibiotics themselves. In this review, we summarized recent advances in antibiotic-based, antibiotic-free and ethnomedical treatments against MRSA wound infections to identify new clues to solve the antibiotic dilemma. One potential solution is to use ethnomedical drugs topically. Some ethnomedical drugs have been demonstrated to be effective antimicrobials against MRSA. A decline in antibiotic resistance can therefore be expected, as has been demonstrated when antibiotic-free treatments were used to limit the use of antibiotics. It is also anticipated that these drugs will have low resistance potential, although there is only minimal evidence to support this claim to date. More clinical trials and animal tests should be conducted on this topic.