Antibiotic resistance: multidrug efflux proteins, a common transport mechanism?

Natural products acting against S. aureus through membrane and cell wall disruption

Covering: 2015 to 2022Staphylococcus aureus (S. aureus) is responsible for several community and hospital-acquired infections with life-threatening complications such as bacteraemia, endocarditis, meningitis, liver abscess, and spinal cord epidural abscess. In recent decades, the abuse and misuse of antibiotics in humans, animals, plants, and fungi and the treatment of nonmicrobial diseases have led to the rapid emergence of multidrug-resistant pathogens. The bacterial wall is a complex structure consisting of the cell membrane, peptidoglycan cell wall, and various associated polymers. The enzymes involved in bacterial cell wall synthesis are established antibiotic targets and continue to be a central focus for antibiotic development. Natural products play a vital role in drug discovery and development. Importantly, natural products provide a starting point for active/lead compounds that sometimes need modification based on structural and biological properties to meet the drug criteria. Notably, microorganisms and plant metabolites have contributed as antibiotics for noninfectious diseases. In this study, we have summarized the recent advances in understanding the activity of the drugs or agents of natural origin that directly inhibit the bacterial membrane, membrane components, and membrane biosynthetic enzymes by targeting membrane-embedded proteins. We also discussed the unique aspects of the active mechanisms of established antibiotics or new agents.

The probiotic and immunomodulation effects of Limosilactobacillus reuteri RGW1 isolated from calf feces

Introduction

Limosilactobacillus reuteri is a gut symbiont with multiple remarkable beneficial effects on host health, and members of L. reuteri are valuable probiotic agents. However, L. reuteri showed obvious host specificity.

Methods

In our study, a novel L. reuteri RGW1 was isolated from feces of healthy calves, and its potential as a probiotic candidate were assessed, by combining in vitro, in vivo experiments and genomic analysis.

Results and discussion

RGW1 was sensitive to all the antibiotics tested, and it did not contain any virulence factor-coding genes. This isolate showed good tolerance to acid (pH 3.0), 0.3% bile salt, and simulated gastric fluid. Moreover, this isolate showed a high hydrophobicity index (73.7 ± 4.6%) and was able to adhere to Caco-2 cells, and antagonize Escherichia coli F5. Treatment of LPS-induced mice with RGW1 elevated TGF-β and IL-10 levels, while RGW1 cell-free supernatant (RCS) decreased TNF-α levels in the sera. Both RGW1 and RCS increased the villus height and villus height/crypt depth ratio of colon. Genomic analysis revealed the mechanism of the probiotic properties described above, and identified the capacity of RGW1 to biosynthesize L-lysine, folate, cobalamin and reuterin de novo. Our study demonstrated the novel bovine origin L. reuteri RGW1 had multiple probiotic characteristics and immunomodulation effects, and provided a deeper understanding of the relationship between these probiotic properties and genetic features.

Efflux-Mediated bile Resistance in Gram-Positive Pathogens

Gram-positive pathogens are causing many serious infections that affect humans and result in mild to severe diseases worldwide. In order to survive and initiate infection, enteric pathogens must resist the physiochemical defence factors in the human intestinal tract. One of these defence factors is bile, a potent antibacterial like compound in the intestine. Efflux pumps are the important mechanism by which bacteria resist antibacterial agents such as bile. Efflux of antimicrobial substances outside the bacterial cell is considered as a key factor for intestinal colonization and virulence of enteric pathogens. This paper will review the research conducted on efflux–mediated bile resistance in Staphylococcus aureus, Listeria monocytogenes, Enterococcus faecalis and Clostridium perfringens. These bacteria colonize in the human & animal gastrointestinal tract and they have a multiple mechanism to resist the innate defences in the gut and antibacterial activity of bile. However, bile resistance in these bacteria is not fully understood. The evidence from this review suggests that Gram-positive pathogens have the ability to active transport of bile. Further research is needed to know how these pathogens sense bile and how bile regulates its virulence factor. In general, therefore, it seems that understanding the specific mechanism of bile resistance in enteric bacteria including gram-positive pathogens may involve in the development of novel strategies to control and treatment of gastrointestinal infections.

Antibiotic resistance modifying ability of phytoextracts in anthrax biological agent Bacillus anthracis and emerging superbugs: a review of synergistic mechanisms

Background and objectives

The chemotherapeutic management of infections has become challenging due to the global emergence of antibiotic resistant pathogenic bacteria. The recent expansion of studies on plant-derived natural products has lead to the discovery of a plethora of phytochemicals with the potential to combat bacterial drug resistance via various mechanisms of action. This review paper summarizes the primary antibiotic resistance mechanisms of bacteria and also discusses the antibiotic-potentiating ability of phytoextracts and various classes of isolated phytochemicals in reversing antibiotic resistance in anthrax agent Bacillus anthracis and emerging superbug bacteria.

Methods

Growth inhibitory indices and fractional inhibitory concentration index were applied to evaluate the in vitro synergistic activity of phytoextract-antibiotic combinations in general.

Findings

A number of studies have indicated that plant-derived natural compounds are capable of significantly reducing the minimum inhibitory concentration of standard antibiotics by altering drug-resistance mechanisms of B. anthracis and other superbug infection causing bacteria. Phytochemical compounds allicin, oleanolic acid, epigallocatechin gallate and curcumin and Jatropha curcas extracts were exceptional synergistic potentiators of various standard antibiotics.

Conclusion

Considering these facts, phytochemicals represents a valuable and novel source of bioactive compounds with potent antibiotic synergism to modulate bacterial drug-resistance.

The Two-Component System RstA/RstB Regulates Expression of Multiple Efflux Pumps and Influences Anaerobic Nitrate Respiration in Pseudomonas fluorescens

Multidrug resistance (MDR) efflux pumps are involved in bacterial intrinsic resistance to multiple antimicrobials. Expression of MDR efflux pumps can be either constitutive or transiently induced by various environmental signals, which are typically perceived by bacterial two-component systems (TCSs) and relayed to the bacterial nucleoid, where gene expression is modulated for niche adaptation. Here, we demonstrate that RstA/RstB, a TCS previously shown to control acid-induced and biofilm-related genes in Escherichiacoli, confers resistance to multiple antibiotics in Pseudomonas fluorescens by directly regulating the MDR efflux pumps EmhABC and MexCD-OprJ. Moreover, we show that phosphorylation of the conserved Asp52 residue in RstA greatly enhances RstA-DNA interaction, and regulation of the multidrug resistance by RstA/RstB is dependent on the phosphorylation of the RstA Asp52 residue by RstB. Proteome analysis reveals RstA/RstB also positively regulates the efflux pump MexEF-OprN and enzymes involved in anaerobic nitrate respiration and pyoverdine biosynthesis. Our results suggest that, by coupling the expression of multiple efflux pumps and anaerobic nitrate respiration, RstA/RstB could play a role in defense against nitrosative stress caused by anaerobic nitrate respiration.

IMPORTANCE Microenvironmental hypoxia typically increases bacterial multidrug resistance by elevating expression of multidrug efflux pumps, but the precise mechanism is currently not well understood. Here, we showed that the two-component system RstA/RstB not only positively regulated expression of several efflux pumps involved in multidrug resistance, but also promoted expression of enzymes involved in anaerobic nitrate respiration and pyoverdine biosynthesis. These results suggested that, by upregulating expression of efflux pumps and pyoverdine biosynthesis-related enzymes, RstA/RstB could play a role in promoting bacterial tolerance to hypoxia by providing protection against nitrosative stress.

The 1,8-naphthyridines sulfonamides are NorA efflux pump inhibitors

OBJECTIVE

Efflux pumps are transmembrane proteins associated with bacterial resistance mechanisms. Bacteria use these proteins to actively transport antibiotics to the extracellular medium, preventing the pharmacological action of these drugs. This study aimed to evaluate in vitro the antibacterial activity of 1,8-naphthyridines sulfonamides, as well as their ability to inhibit efflux systems of Staphylococcus aureus strains expressing different levels of the NorA efflux pump.

METHODS

The broth microdilution test was performed to assess antibacterial activity. Efflux pump inhibition was evaluated in silico by molecular docking and in vitro by fluorometric tests, and the minimum inhibitory concentration (MIC) was determined. The MIC was determined in the association between 1,8-naphthyridine and norfloxacin or ethidium bromide.

RESULTS

The 1,8-naphthyridines did not show direct antibacterial activity. However, they effectively reduced the MIC of multidrug-resistant bacteria by associating with norfloxacin and ethidium bromide, in addition to increasing the fluorescence emission. In silico analysis addressing the binding between NorA and 1,8-naphthyridines suggests that hydrogen bonds and hydrophilic interactions represent the interactions with the most favourable binding energy, corroborating the experimental data.

CONCLUSION

Our data suggest that 1,8-naphthyridines sulfonamides inhibit bacterial resistance through molecular mechanisms associated with inhibition of the NorA efflux pump in S. aureus strains.

Antimicrobial resistance three ways: healthcare crisis, major concepts and the relevance of biofilms

Worldwide, infections are resuming their role as highly effective killing diseases, as current treatments are failing to respond to the growing problem of antimicrobial resistance (AMR). The social and economic burden of AMR seems ever rising, with health- and research-related organizations rushing to collaborate on a worldwide scale to find effective solutions. Resistant bacteria are spreading even in first-world nations, being found not only in healthcare-related settings, but also in food and in the environment. In this minireview, the impact of AMR in healthcare systems and the major bacteria behind it are highlighted. Ecological aspects of AMR evolution and the complexity of its molecular mechanisms are explained. Major concepts, such as intrinsic, acquired and adaptive resistance, as well as tolerance and heteroresistance, are also clarified. More importantly, the problematic of biofilms and their role in AMR, namely their main resistance and tolerance mechanisms, are elucidated. Finally, some of the most promising anti-biofilm strategies being investigated are reviewed. Much is still to be done regarding the study of AMR and the discovery of new anti-biofilm strategies. Gladly, considerable research on this topic is generated every day and increasingly concerted actions are being engaged globally to try and tackle this problem.

Laboratory Evolution Assays and Whole-Genome Sequencing for the Development and Safety Evaluation of Lactobacillus plantarum With Stable Resistance to Gentamicin

The goal of this work was to use laboratory evolution assays and whole-genome sequencing to develop and test the safety of a probiotic, Lactobacillus plantarum, with high-level of resistance to gentamicin. The evolution of L. plantarum was evaluated under the selective pressure from gentamicin and subsequently when the selective pressure was removed. After 30 days of selective pressure from gentamicin, the minimum inhibitory concentration (MIC) of L. plantarum to gentamicin increased from 4 to 512 μg/mL and remained stable at this level. After removing the selective pressure, the resistance of L. plantarum to gentamicin decreased to 64 μg/mL after 20 days, and remained stable thereafter. Although the MIC declined it was still higher than the cut-off value recommended by EFSA, indicating that the acquisition of gentamicin-resistance was an irreversible process. Using whole-genome sequencing, gene mutations were identified in the strains that had undergone selection pressure from gentamicin as well as in the strains where the selection pressure was subsequently removed. Specifically, four non-synonymous mutations were detected including one single nucleotide polymorphism (SNP), one insertion, and two structural variants (SVs), of which the mutations in genes encoding the drug resistance MFS transporter and transcriptional regulator of AraC family were only detected in the strains under selective pressure from gentamicin. The results indicate that these mutations play an important role in increasing the resistant levels of L. plantarum to gentamicin. The mobility analysis of mutant genes confirmed that they were not located on mobile elements of the genome of highly resistant L. plantarum, indicating that horizontal gene transfer was not possible.

Influence of the T to S mutation at the STMK motif on antibiotic resistance of penicillin binding protein 1A: A comprehensive computational study

The emergence of antibiotic resistance has attracted the attention of scientists and scientific circles over the decades. β-Lactam antibiotics resistance is a worldwide therapeutic challenge in bacterial infections, mediated through several mechanisms of which mutations in Penicillin Binding Proteins (PBPs) are an important issue, making critical therapeutic problems in the human population. Accordingly, investigating the dynamic structures of mutant variants could result in a profound understanding of such a specific resistance. Therefore, this work investigated structural properties sampled by all-atom molecular dynamics (MD) simulations, umbrella sampling, and binding free energy calculations for both a wild-type and a cefotaxime-resistant T to S mutant of PBP1A. The T to S mutation significantly reduces the binding affinity of cefotaxime (a frequently clinically-administrated β-lactam antibiotic) as the PBP1A inhibitor. In the conventional MD simulations presented here, more fluctuations of the mutant's active site cleft margins were detected. The cleft of the mutant protein also opened remarkably more than the wild-type's cleft and displayed more flexibility. Thus, our findings have shown that flexibility of cleft margins of the active site in the mutant PBP1A immediately results in the catalytic cleft opening. In addition, binding free energy calculation suggests that reducing hydrophobic contacts and increasing the polar contribution in the binding energy may play an important role in cefotaxime resistance.

Natural Products as Platforms To Overcome Antibiotic Resistance

Natural products have served as powerful therapeutics against pathogenic bacteria since the golden age of antibiotics of the mid-20th century. However, the increasing frequency of antibiotic-resistant infections clearly demonstrates that new antibiotics are critical for modern medicine. Because combinatorial approaches have not yielded effective drugs, we propose that the development of new antibiotics around proven natural scaffolds is the best short-term solution to the rising crisis of antibiotic resistance. We analyze herein synthetic approaches aiming to reengineer natural products into potent antibiotics. Furthermore, we discuss approaches in modulating quorum sensing and biofilm formation as a nonlethal method, as well as narrow-spectrum pathogen-specific antibiotics, which are of interest given new insights into the implications of disrupting the microbiome.

New Roads Leading to Old Destinations: Efflux Pumps as Targets to Reverse Multidrug Resistance in Bacteria

Multidrug resistance (MDR) has appeared in response to selective pressures resulting from the incorrect use of antibiotics and other antimicrobials. This inappropriate application and mismanagement of antibiotics have led to serious problems in the therapy of infectious diseases. Bacteria can develop resistance by various mechanisms and one of the most important factors resulting in MDR is efflux pump-mediated resistance. Because of the importance of the efflux-related multidrug resistance the development of new therapeutic approaches aiming to inhibit bacterial efflux pumps is a promising way to combat bacteria having over-expressed MDR efflux systems. The definition of an efflux pump inhibitor (EPI) includes the ability to render the bacterium increasingly more sensitive to a given antibiotic or even reverse the multidrug resistant phenotype. In the recent years numerous EPIs have been developed, although so far their clinical application has not yet been achieved due to their in vivo toxicity and side effects. In this review, we aim to give a short overview of efflux mediated resistance in bacteria, EPI compounds of plant and synthetic origin, and the possible methods to investigate and screen EPI compounds in bacterial systems.

ClbM is a versatile, cation-promiscuous MATE transporter found in the colibactin biosynthetic gene cluster

Multidrug transporters play key roles in cellular drug resistance to toxic molecules, yet these transporters are also involved in natural product transport as part of biosynthetic clusters in bacteria and fungi. The genotoxic molecule colibactin is produced by strains of virulent and pathobiont Escherichia coli and Klebsiella pneumoniae. In the biosynthetic cluster is a multidrug and toxic compound extrusion protein (MATE) proposed to transport the prodrug molecule precolibactin across the cytoplasmic membrane, for subsequent cleavage by the peptidase ClbP and cellular export. We recently determined the X-ray structure of ClbM, and showed preliminary data suggesting its specific role in precolibactin transport. Here, we define a functional role of ClbM by examining transport capabilities under various biochemical conditions. Our data indicate ClbM responds to sodium, potassium, and rubidium ion gradients, while also having substantial transport activity in the absence of alkali cations.

Structural and mechanistic diversity of multidrug transporters

The review article surveys recent structural and mechanistic advances in the field of multi-drug and natural product transporters.

Rational design of engineered cationic antimicrobial peptides consisting exclusively of arginine and tryptophan, and their activity against multidrug-resistant pathogens

The emergence of multidrug-resistant (MDR) pathogens underscores the need for new antimicrobial agents to overcome the resistance mechanisms of these organisms. Cationic antimicrobial peptides (CAPs) provide a potential source of new antimicrobial therapeutics. We previously characterized a lytic base unit (LBU) series of engineered CAPs (eCAPs) of 12 to 48 residues demonstrating maximum antibacterial selectivity at 24 residues. Further, Trp substitution in LBU sequences increased activity against both P. aeruginosa and S. aureus under challenging conditions (e.g., saline, divalent cations, and serum). Based on these findings, we hypothesized that the optimal length and, therefore, the cost for maximum eCAP activity under physiologically relevant conditions could be significantly reduced using only Arg and Trp arranged to form idealized amphipathic helices. Hence, we developed a novel peptide series, composed only of Arg and Trp, in a sequence predicted and verified by circular dichroism to fold into optimized amphipathic helices. The most effective antimicrobial activity was achieved at 12 residues in length (WR12) against a panel of both Gram-negative and Gram-positive clinical isolates, including extensively drug-resistant strains, in saline and broth culture and at various pH values. The results demonstrate that the rational design of CAPs can lead to a significant reduction in the length and the number of amino acids used in peptide design to achieve optimal potency and selectivity against specific pathogens.

Mechanisms of antibiotic resistance in Burkholderia pseudomallei: implications for treatment of melioidosis

Burkholderia pseudomallei is the etiologic agent of melioidosis. This multifaceted disease is difficult to treat, resulting in high morbidity and mortality. Treatment of B. pseudomallei infections is lengthy and necessitates an intensive phase (parenteral ceftazidime, amoxicillin-clavulanic acid or meropenem) and an eradication phase (oral trimethoprim-sulfamethoxazole). The main resistance mechanisms affecting these antibiotics include enzymatic inactivation, target deletion and efflux from the cell, and are mediated by chromosomally encoded genes. Overproduction and mutations in the class A PenA β-lactamase cause ceftazidime and amoxicillin-clavulanic acid resistance. Deletion of the penicillin binding protein 3 results in ceftazidime resistance. BpeEF-OprC efflux pump expression causes trimethoprim and trimethoprim-sulfamethoxazole resistance. Although resistance is still relatively rare, therapeutic efficacies may be compromised by resistance emergence due to increased use of antibiotics in endemic regions. Novel agents and therapeutic strategies are being tested and, in some instances, show promise as anti-B. pseudomallei infectives.

The role of antimicrobial peptides in preventing multidrug-resistant bacterial infections and biofilm formation

Over the last decade, decreasing effectiveness of conventional antimicrobial-drugs has caused serious problems due to the rapid emergence of multidrug-resistant pathogens. Furthermore, biofilms, which are microbial communities that cause serious chronic infections and dental plaque, form environments that enhance antimicrobial resistance. As a result, there is a continuous search to overcome or control such problems, which has resulted in antimicrobial peptides being considered as an alternative to conventional drugs. Antimicrobial peptides are ancient host defense effector molecules in living organisms. These peptides have been identified in diverse organisms and synthetically developed by using peptidomimic techniques. This review was conducted to demonstrate the mode of action by which antimicrobial peptides combat multidrug-resistant bacteria and prevent biofilm formation and to introduce clinical uses of these compounds for chronic disease, medical devices, and oral health. In addition, combinations of antimicrobial peptides and conventional drugs were considered due to their synergetic effects and low cost for therapeutic treatment.

Pharmacophore-based strategy for the development of general and specific scFv biosensors for abused antibiotics

We developed fluorescent biosensor systems that are either general or selective to fluoroquinolone antibiotics by using a single-chain variable-fragment (scFv) as a recognition element. The selectivity of these biosensors to fluoroquinolone antibiotics was rationally tuned through the structural modification on the pharmacophore of fluoroquinolone antibiotics and the subsequent selection of scFv receptor modules against these antibiotics-based antigens using phage display. The resulting A2 and F9 scFv's bound to their representative antigen with a moderate affinity (K(D) in micromolar range as determined by surface plasmon resonance). A2 is a specific binder for enrofloxacin and did not cross-react with other fluoroquinolone antibiotics including structurally similar ciprofloxacin, while F9 is a general fluoroquinolone binder that likely bound to the antigen at the common pyridone-carboxylic acid pharmacophore. These scFv-based receptors were successfully applied to the development of one-step fluorescent biosensor which can detect fluoroquinolone antibiotics at concentrations below the level suggested in animal drug application guidelines. The strategy described in this report can be applied to developing convenient field biosensors that can qualitatively detect overused/misused antibiotics in the livestock drinking water.

Biochemical characterization of the multidrug regulator QacR distinguishes residues that are crucial to multidrug binding and induction of qacA transcription

Staphylococcus aureus transcription factor QacR regulates expression of the qacA multidrug efflux determinant. In response to binding cationic lipophilic compounds, including ethidium and rhodamine 6G, QacR dissociates from the qacA operator alleviating repression. Such ligand binding uniformly induces a coil-to-helix transition of residues Thr(89)-Tyr(93) revealing an asymmetric binding pocket in QacR containing two distinct subpockets. Here, the functional significance of hydrophobic, aromatic, and polar residues characteristic of the rhodamine 6G pocket and the proximal Tyr(92), proposed to facilitate the transcriptionally active conformation, was examined. Notably, the presence of Tyr(92) was not essential for QacR structural changes between DNA-bound and induced conformations. Furthermore, although mutation of the majority of residues contacting rhodamine 6G exerted moderate effects on QacR-rhodamine 6G binding, mutation of Leu(54) and Gln(96), and cumulative mutations involving these with Tyr(93) and Tyr(123), imparted a dramatic decrease in QacR-rhodamine 6G binding affinity. This equated with impaired dissociation of QacR from its operator DNA in the presence of this ligand in S. aureus, delineating the important role of these residues in the QacR-rhodamine 6G interaction. Additionally, despite maintaining a high affinity for ethidium, QacR mutants involving Leu(54), Tyr(93), Gln(96), and Tyr(123), which denote the interface between the rhodamine 6G and ethidium subpockets, were unable to be induced from operator DNA in the presence of ethidium in S. aureus. This highlights the significant contribution of these residues to QacR-mediated derepression of qacA transcription following ligand binding in the distal subpocket and may be important for the general mechanism irrespective of the ligand bound.

Antimicrobial resistance in bacteria

The development of antimicrobial resistance by bacteria is inevitable and is considered as a major problem in the treatment of bacterial infections in the hospital and in the community. Despite efforts to develop new therapeutics that interact with new targets, resistance has been reported even to these agents. In this review, an overview is given of the many therapeutic possibilities that exist for treatment of bacterial infections and how bacteria become resistant to these therapeutics.

Synthesis and evaluation of fluoroquinolone derivatives as substrate-based inhibitors of bacterial efflux pumps

Bacterial efflux pump systems contribute to antimicrobial resistance in pathogenic bacteria. The co-administration of bacterial efflux pump inhibitors with antibiotics is being pursued to overcome efflux-mediated resistance to antibiotics. In this study we investigated a strategy for converting broad-spectrum efflux pump substrates, fluoroquinolone antibacterial agents which are inherently recognized by most efflux transporters, into expanded spectrum efflux pump inhibitors. Employing this strategy against organisms expressing efflux pumps from the MFS, MATE and RND classes of pump systems, we report here the identification of an ofloxacin-based EPI that is a potent inhibitor of MFS (NorA) and MATE (MepA) efflux pumps in Staphylococcus aureus. The methods described here outline a process that we feel will be broadly applicable to the systematic identification of bacterial efflux pump inhibitors.

A genomic strategy for cloning, expressing and purifying efflux proteins of the major facilitator superfamily

A genomic strategy for the overexpression of bacterial multidrug and antibiotic resistance membrane efflux proteins in Escherichia coli is described. Expression is amplified so that the encoded proteins from a range of Gram-positive and Gram-negative bacteria comprise 5% to 35% of E. coli inner membrane protein. Depending upon their topology, proteins are produced with RGS(His)(6)-tag or a Strep-tag at the C terminus. These tags facilitate the purification of the overexpressed proteins in milligram quantities for structural studies. The strategy is illustrated for the bicyclomycin resistance efflux protein, Bcr, of E. coli.

The dihydro-β-agarofuran sesquiterpenoids

Dihydro-Beta-agarofuran sesquiterpenoids are a structurally diverse class of natural products based on tricyclic 5,11-epoxy-5Beta,10alpha-eudesman-4-(14)-ene skeleton. Between January 1990 and June 2006, 462 new dihydro-Beta-agarofuran sesquiterpenoids of 74 structural types have been isolated from about 64 species of Celastraceae, 3 species of Hippocrateaceae and one species of Lamiaceae. The present review covers the chemical and biological activity research of dihydro-Beta-agarofuran sesquiterpenoids in the past 16 years. The chemical research includes structural classification into sesquiterpene polyesters and macrolide sesquiterpene pyridine alkaloids, synthesis of dihydro-Beta-agarofuran as well as extraction, isolation and purification methods. The biological activity research includes activities such as multidrug resistance (MDR) reversal activity, HIV inhibition, cytotoxicity, antitumor activity, antifeedant activity and insecticidal activity with some insights to their modes of actions.

Antibacterial drug discovery and structure-based design

Bacterial resistance continues to develop and pose a significant threat, both in hospitals and, more recently, in the community. A focus on other therapeutic areas by the larger pharmaceutical companies has left a shortfall in the pipeline of novel antibacterials. Recently, many new structures have been studied by structure-genomics initiatives, delivering a wealth of targets to consider. Using the tools of structure-based design, antibacterial discovery must exploit these targets to accelerate the process of drug discovery.

The translocation mechanism of P-glycoprotein

Multidrug transporters are involved in mediating the failure of chemotherapy in treating several serious diseases. The archetypal multidrug transporter P-glycoprotein (P-gp) confers resistance to a large number of chemically and functionally unrelated anti-cancer drugs by mediating efflux from cancer cells. The ability to efflux such a large number of drugs remains a biological enigma and the lack of mechanistic understanding of the translocation pathway used by P-gp prevents rational design of compounds to inhibit its function. The translocation pathway is critically dependent on ATP hydrolysis and drug interaction with P-gp is possible at one of a multitude of allosterically linked binding sites. However, aspects such as coupling stoichiometry, molecular properties of binding sites and the nature of conformational changes remain unresolved or the centre of considerable controversy. The present review attempts to utilise the available data to generate a detailed sequence of events in the translocation pathway for this dexterous protein.