A new antibiotic traps lipopolysaccharide in its intermembrane transporter.

Gram-negative bacteria are extraordinarily difficult to kill because their cytoplasmic membrane is surrounded by an outer membrane that blocks the entry of most antibiotics. The impenetrable nature of the outer membrane is due to the presence of a large, amphipathic glycolipid called lipopolysaccharide (LPS) in its outer leaflet1. Assembly of the outer membrane requires transport of LPS across a protein bridge that spans from the cytoplasmic membrane to the cell surface. Maintaining outer membrane integrity is essential for bacterial cell viability, and its disruption can increase susceptibility to other antibiotics2-6. Thus, inhibitors of the seven lipopolysaccharide transport (Lpt) proteins that form this transenvelope transporter have long been sought. A new class of antibiotics that targets the LPS transport machine in Acinetobacter was recently identified. Here, using structural, biochemical and genetic approaches, we show that these antibiotics trap a substrate-bound conformation of the LPS transporter that stalls this machine. The inhibitors accomplish this by recognizing a composite binding site made up of both the Lpt transporter and its LPS substrate. Collectively, our findings identify an unusual mechanism of lipid transport inhibition, reveal a druggable conformation of the Lpt transporter and provide the foundation for extending this class of antibiotics to other Gram-negative pathogens.

Pyridylpiperazine-based allosteric inhibitors of RND-type multidrug efflux pumps.

Efflux transporters of the RND family confer resistance to multiple antibiotics in Gram-negative bacteria. Here, we identify and chemically optimize pyridylpiperazine-based compounds that potentiate antibiotic activity in E. coli through inhibition of its primary RND transporter, AcrAB-TolC. Characterisation of resistant E. coli mutants and structural biology analyses indicate that the compounds bind to a unique site on the transmembrane domain of the AcrB L protomer, lined by key catalytic residues involved in proton relay. Molecular dynamics simulations suggest that the inhibitors access this binding pocket from the cytoplasm via a channel exclusively present in the AcrB L protomer. Thus, our work unveils a class of allosteric efflux-pump inhibitors that likely act by preventing the functional catalytic cycle of the RND pump.

Short-chain aurachin D derivatives are selective inhibitors of E. coli cytochrome bd-I and bd-II oxidases.

Cytochrome bd-type oxidases play a crucial role for survival of pathogenic bacteria during infection and proliferation. This role and the fact that there are no homologues in the mitochondrial respiratory chain qualify cytochrome bd as a potential antimicrobial target. However, few bd oxidase selective inhibitors have been described so far. In this report, inhibitory effects of Aurachin C (AurC-type) and new Aurachin D (AurD-type) derivatives on oxygen reductase activity of isolated terminal bd-I, bd-II and bo3 oxidases from Escherichia coli were potentiometrically measured using a Clark-type electrode. We synthesized long- (C10, decyl or longer) and short-chain (C4, butyl to C8, octyl) AurD-type compounds and tested this set of molecules towards their selectivity and potency. We confirmed strong inhibition of all three terminal oxidases for AurC-type compounds, whereas the 4(1H)-quinolone scaffold of AurD-type compounds mainly inhibits bd-type oxidases. We assessed a direct effect of chain length on inhibition activity with highest potency and selectivity observed for heptyl AurD-type derivatives. While Aurachin C and Aurachin D are widely considered as selective inhibitors for terminal oxidases, their structure-activity relationship is incompletely understood. This work fills this gap and illustrates how structural differences of Aurachin derivatives determine inhibitory potency and selectivity for bd-type oxidases of E. coli.

Identification of Potent Natural Resource Small Molecule Inhibitor to Control Vibrio cholera by Targeting Its Outer Membrane Protein U: An In Silico Approach.

Vibrio cholerae causes the diarrheal disease cholera which affects millions of people globally. The outer membrane protein U (OmpU) is the outer membrane protein that is most prevalent in V. cholerae and has already been recognized as a critical component of pathogenicity involved in host cell contact and as being necessary for the survival of pathogenic V. cholerae in the host body. Computational approaches were used in this study to screen a total of 37,709 natural compounds from the traditional Chinese medicine (TCM) database against the active site of OmpU. Following a sequential screening of the TCM database, we report three lead compounds-ZINC06494587, ZINC85510056, and ZINC95910434-that bind strongly to OmpU, with binding affinity values of -8.92, -8.12, and -8.78 kcal/mol, which were higher than the control ligand (-7.0 kcal/mol). To optimize the interaction, several 100 ns molecular dynamics simulations were performed, and the resulting complexes were shown to be stable in their vicinity. Additionally, these compounds were predicted to have good drug-like properties based on physicochemical properties and ADMET assessments. This study suggests that further research be conducted on these compounds to determine their potential use as cholera disease treatment.

A Convenient Chemoenzymatic Preparation of Chimeric Macrocyclic Peptide Antibiotics with Potent Activity against Gram-Negative Pathogens.

The continuing rise of antibiotic resistance, particularly among Gram-negative pathogens, threatens to undermine many aspects of modern medical practice. To address this threat, novel antibiotics that utilize unexploited bacterial targets are urgently needed. Over the past decade, a number of studies have highlighted the antibacterial potential of macrocyclic peptides that target Gram-negative outer membrane proteins (OMPs). Recently, it was reported that the antibacterial activities of OMP-targeting macrocyclic peptidomimetics of the antimicrobial peptide protegrin-1 are dramatically enhanced upon linking to polymyxin E nonapeptide (PMEN). In this study, we describe a convergent, chemoenzymatic route for the convenient preparation of such conjugates. Specifically, we investigated the use of both amide bond formation and azide-alkyne ligation for connecting an OMP-targeting macrocyclic peptide to a PMEN building block (obtained by enzymatic degradation of polymyxin E). The conjugates obtained via both approaches display potent antibacterial activity against a range of Gram-negative pathogens including multi-drug-resistant isolates.

A ban on BAM: an update on inhibitors of the ß-barrel assembly machinery.

Gram-negative pathogens are a rapidly increasing threat to human health worldwide due to high rates of antibiotic resistance and the lack of development of novel antibiotics. The protective cell envelope of gram-negative bacteria is a major permeability barrier that contributes to the problem by restricting the uptake of antibiotics. On the other hand, its unique architecture also makes it a suitable target for antibiotic interference. In particular, essential multiprotein machines that are required for biogenesis of the outer membrane have attracted attention in antibacterial design strategies. Recently, significant progress has been made in the development of inhibitors of the ß-barrel assembly machine (BAM) complex. Here, we summarize the current state of drug development efforts targeting the BAM complex in pursuit of new antibiotics.

The antibiotic darobactin mimics a ß-strand to inhibit outer membrane insertase.

Antibiotics that target Gram-negative bacteria in new ways are needed to resolve the antimicrobial resistance crisis1-3. Gram-negative bacteria are protected by an additional outer membrane, rendering proteins on the cell surface attractive drug targets4,5. The natural compound darobactin targets the bacterial insertase BamA6-the central unit of the essential BAM complex, which facilitates the folding and insertion of outer membrane proteins7-13. BamA lacks a typical catalytic centre, and it is not obvious how a small molecule such as darobactin might inhibit its function. Here we resolve the mode of action of darobactin at the atomic level using a combination of cryo-electron microscopy, X-ray crystallography, native mass spectrometry, in vivo experiments and molecular dynamics simulations. Two cyclizations pre-organize the darobactin peptide in a rigid ß-strand conformation. This creates a mimic of the recognition signal of native substrates with a superior ability to bind to the lateral gate of BamA. Upon binding, darobactin replaces a lipid molecule from the lateral gate to use the membrane environment as an extended binding pocket. Because the interaction between darobactin and BamA is largely mediated by backbone contacts, it is particularly robust against potential resistance mutations. Our results identify the lateral gate as a functional hotspot in BamA and will allow the rational design of antibiotics that target this bacterial Achilles heel.

Predictive Rules of Efflux Inhibition and Avoidance in Pseudomonas aeruginosa.

Antibiotic-resistant bacteria rapidly spread in clinical and natural environments and challenge our modern lifestyle. A major component of defense against antibiotics in Gram-negative bacteria is a drug permeation barrier created by active efflux across the outer membrane. We identified molecular determinants defining the propensity of small peptidomimetic molecules to avoid and inhibit efflux pumps in Pseudomonas aeruginosa, a human pathogen notorious for its antibiotic resistance. Combining experimental and computational protocols, we mapped the fate of the compounds from structure-activity relationships through their dynamic behavior in solution, permeation across both the inner and outer membranes, and interaction with MexB, the major efflux transporter of P. aeruginosa We identified predictors of efflux avoidance and inhibition and demonstrated their power by using a library of traditional antibiotics and compound series and by generating new inhibitors of MexB. The identified predictors will enable the discovery and optimization of antibacterial agents suitable for treatment of P. aeruginosa infections.IMPORTANCE Efflux pump avoidance and inhibition are desired properties for the optimization of antibacterial activities against Gram-negative bacteria. However, molecular and physicochemical interactions defining the interface between compounds and efflux pumps remain poorly understood. We identified properties that correlate with efflux avoidance and inhibition, are predictive of similar features in structurally diverse compounds, and allow researchers to distinguish between efflux substrates, inhibitors, and avoiders in P. aeruginosa The developed predictive models are based on the descriptors representative of different clusters comprising a physically intuitive combination of properties. Molecular shape (represented by acylindricity), amphiphilicity (anisotropic polarizability), aromaticity (number of aromatic rings), and the partition coefficient (LogD) are physicochemical predictors of efflux inhibitors, whereas interactions with Pro668 and Leu674 residues of MexB distinguish between inhibitors/substrates and efflux avoiders. The predictive models and efflux rules are applicable to compounds with unrelated chemical scaffolds and pave the way for development of compounds with the desired efflux interface properties.

Specific egg yolk immunoglobulin as a promising non-antibiotic biotherapeutic product against Acinetobacter baumannii pneumonia infection.

Acinetobacter baumannii is a serious health threat with a high mortality rate. We have already reported prophylactic effects of IgYs raised against OmpA and Omp34 as well as against inactivated whole-cell (IWC) of A. baumannii in a murine pneumonia model. However, the infection was exacerbated in the mice group that received IgYs raised against the combination of OmpA and Omp34. The current study was conducted to propose reasons for the observed antibody-dependent enhancement (ADE) in addition to the therapeutic effect of specific IgYs in the murine pneumonia model. This phenomenon was hypothetically attributed to topologically inaccessible similar epitopes of OmpA and Omp34 sharing similarity with peptides of mice proteins. In silico analyses revealed that some inaccessible peptides of OmpA shared similarity with peptides of Omp34 and Mus musculus. Specific anti-OmpA and anti-Omp34 IgYs cross-reacted with Omp34 and OmpA respectively. Specific IgYs showed different protectivity against A. baumannii AbI101 in the murine pneumonia model. IgYs triggered against OmpA or IWC of A. baumannii were the most protective antibodies. IgY triggered against Omp34 is ranked next after those against OmpA. The lowest protection was observed in mice received IgYs raised against the combination of rOmpA and rOmp34. In conclusion, specific IgYs against OmpA, Omp34, and IWC of A. baumannii could serve as novel biotherapeutics against A. baumannii pneumonia.

Revelation of Function and Inhibition of Wza Through Single-Channel Studies.

Antibacterial resistance (AR) is causing more and more bacterial infections that cannot be cured by using the antibacterial drugs that are currently available. It is predicted that 10 million people will die every year by 2050 from infections caused by antibacterial resistant strains, surpassing the predicted numbers of deaths caused by cancer. AR is therefore a global challenge and novel antibacterial strategies are in high demand. To this end, the work on exploring the pore properties of a bacterial sugar transporter, WzaK30, has led to the discovery of the first inhibitor against bacterial capsular polysaccharides export.Recently, single-molecule recapitulation of capsular polysaccharide (CPS) export and pore formation properties of Wza barrel peptides have also revealed the possibility of a next-generation of Wza strategies. These strategies are based upon the first examination and understanding of the pore properties of wild-type (WT) and mutant WzaK30 in single-molecule electrical channel recording. The initially reported experimental procedures have been further developed to enable efficient studies of other Wza homologs that are more common in bacterial pathogens causing significant bacterial infections. Therefore, this chapter presents the most recent protocols and logistics behind the research on Wza channel activity, antibacterials, and strategies. The disciplines covered here include computation, molecular biology, biochemistry, electrophysiology, microbiology, and biophysics.

Cellulose membrane modified with LED209 as an antibacterial and anti-adhesion material.

Bacterial adhesion infection caused by medical materials in clinical application has become a serious threat, and it urgently needs new strategies to deal with these clinical challenges. In this work, LED209, a highly selective histidine sensor kinase inhibitor of Gram-negative bacteria, was covalently attached on cellulose membrane (CM) via click reaction. The data of contact angle measurements, ATR-FTIR and X-ray photoelectron spectroscopy confirmed the successful synthesis of LED-CM. In addition, the results of antibacterial activity of the membranes shown that LED-CM exhibited excellent anti-adhesion ability to Enterohemorrhagic Escherichia coli (EHEC), and significantly reduced the formation of bacterial biofilm. Importantly, LED-CM was able to repress the expression of virulence genes in EHEC. Furthermore, LED209-functionalized cellulose membrane indicated no cytotoxicity to mammalian cells. Hence, our present work demonstrated that CM modified with LED209 possessed markedly anti-adhesion activity against EHEC, which offered a potent antimicrobial material for combating bacterial infections.

Configuration Flipping in Distal Pocket of Multidrug Transporter MexB Impacts the Efflux Inhibitory Mechanism.

MexAB-OprM efflux pumps, found in Pseudomonas aeruginosa, play a major role in drug resistance by extruding out drugs and antibiotic molecules from cells. Inhibitors are used to cease the potency of the efflux pumps. In this study, in-silico models are used to investigate the nature of the binding pocket of the MexAB-OprM efflux pump. First, we have performed classical molecular dynamics (MD) simulations to shed light on different aspects of protein-inhibitor interaction in the binding pocket of the pump. Using classical MD simulations, quantum mechanics/molecular mechanics (QM/MM), and various types of analyses, it is found that D13-9001 has a higher binding affinity towards the binding pocket compared to D1 and D2; the results are in sync with the experimental dat. Two stable configurations of D13-9001 are discovered inside the distal pocket which could be one of the primary reasons for the greater efficacy of D13-9001. The free energy barrier upon changing one state to another is calculated by employing umbrella sampling method. Finally, F178 is mutated to have the complete picture as it contributes significantly to the binding energy irrespective of the three inhibitors. Our results may help to design a new generation of inhibitors for such an efflux pump.

Vibrio campbellii chitoporin: Thermostability study and implications for the development of therapeutic agents against Vibrio infections.

Vibrio campbellii (formerly Vibrio harveyi) is a bacterial pathogen that causes vibriosis, which devastates fisheries and aquaculture worldwide. V. campbellii expresses chitinolytic enzymes and chitin binding/transport proteins, which serve as excellent targets for antimicrobial agent development. We previously characterized VhChiP, a chitooligosaccharide-specific porin from the outer membrane of V. campbellii BAA-1116. This study employed far-UV circular dichroism and tryptophan fluorescence spectroscopy, together with single channel electrophysiology to demonstrate that the strong binding of chitoligosaccharides enhanced thermal stability of VhChiP. The alanine substitution of Trp136 at the center of the affinity sites caused a marked decrease in the binding affinity and decreased the thermal stability of VhChiP. Tryptophan fluorescence titrations over a range of temperatures showed greater free-energy changes on ligand binding (ΔG°binding) with increasing chain length of the chitooligosaccharides. Our findings suggest the possibility of designing stable channel-blockers, using sugar-based analogs that serve as antimicrobial agents, active against Vibrio infection.

A chalcone derivative binds a putative allosteric site of YopH: Inhibition of a virulence factor of Yersinia.

Identification of allosteric inhibitors of PTPs has attracted great interest as a new strategy to overcome the challenge of discover potent and selective molecules for therapeutic intervention. YopH is a virulence factor of the genus Yersinia, validated as an antimicrobial target. The finding of a second substrate binding site in YopH has revealed a putative allosteric site that could be further exploited. Novel chalcone compounds that inhibit PTPs activity were designed and synthesized. Compound 3j was the most potent inhibitor, interestingly, with different mechanisms of inhibition for the panel of enzymes evaluated. Further, our results showed that compound 3j is an irreversible non-competitive inhibitor of YopH that binds to a site different than the catalytic site, but close to the well-known second binding site of YopH.

Immunotherapy targeting the Streptococcus pyogenes M protein or streptolysin O to treat or prevent influenza A superinfection.

Viral infections complicated by a bacterial infection are typically referred to as coinfections or superinfections. Streptococcus pyogenes, the group A streptococcus (GAS), is not the most common bacteria associated with influenza A virus (IAV) superinfections but did cause significant mortality during the 2009 influenza pandemic even though all isolates are susceptible to penicillin. One approach to improve the outcome of these infections is to use passive immunization targeting GAS. To test this idea, we assessed the efficacy of passive immunotherapy using antisera against either the streptococcal M protein or streptolysin O (SLO) in a murine model of IAV-GAS superinfection. Prophylactic treatment of mice with antiserum to either SLO or the M protein decreased morbidity compared to mice treated with non-immune sera; however, neither significantly decreased mortality. Therapeutic use of antisera to SLO decreased morbidity compared to mice treated with non-immune sera but neither antisera significantly reduced mortality. Overall, the results suggest that further development of antibodies targeting the M protein or SLO may be a useful adjunct in the treatment of invasive GAS diseases, including IAV-GAS superinfections, which may be particularly important during influenza pandemics.

A new antibiotic selectively kills Gram-negative pathogens.

The current need for novel antibiotics is especially acute for drug-resistant Gram-negative pathogens1,2. These microorganisms have a highly restrictive permeability barrier, which limits the penetration of most compounds3,4. As a result, the last class of antibiotics that acted against Gram-negative bacteria was developed in the 1960s2. We reason that useful compounds can be found in bacteria that share similar requirements for antibiotics with humans, and focus on Photorhabdus symbionts of entomopathogenic nematode microbiomes. Here we report a new antibiotic that we name darobactin, which was obtained using a screen of Photorhabdus isolates. Darobactin is coded by a silent operon with little production under laboratory conditions, and is ribosomally synthesized. Darobactin has an unusual structure with two fused rings that form post-translationally. The compound is active against important Gram-negative pathogens both in vitro and in animal models of infection. Mutants that are resistant to darobactin map to BamA, an essential chaperone and translocator that folds outer membrane proteins. Our study suggests that bacterial symbionts of animals contain antibiotics that are particularly suitable for development into therapeutics.

A small-molecule inhibitor of BamA impervious to efflux and the outer membrane permeability barrier.

The development of new antimicrobial drugs is a priority to combat the increasing spread of multidrug-resistant bacteria. This development is especially problematic in gram-negative bacteria due to the outer membrane (OM) permeability barrier and multidrug efflux pumps. Therefore, we screened for compounds that target essential, nonredundant, surface-exposed processes in gram-negative bacteria. We identified a compound, MRL-494, that inhibits assembly of OM proteins (OMPs) by the ß-barrel assembly machine (BAM complex). The BAM complex contains one essential surface-exposed protein, BamA. We constructed a bamA mutagenesis library, screened for resistance to MRL-494, and identified the mutation bamAE470K BamAE470K restores OMP biogenesis in the presence of MRL-494. The mutant protein has both altered conformation and activity, suggesting it could either inhibit MRL-494 binding or allow BamA to function in the presence of MRL-494. By cellular thermal shift assay (CETSA), we determined that MRL-494 stabilizes BamA and BamAE470K from thermally induced aggregation, indicating direct or proximal binding to both BamA and BamAE470K Thus, it is the altered activity of BamAE470K responsible for resistance to MRL-494. Strikingly, MRL-494 possesses a second mechanism of action that kills gram-positive organisms. In microbes lacking an OM, MRL-494 lethally disrupts the cytoplasmic membrane. We suggest that the compound cannot disrupt the cytoplasmic membrane of gram-negative bacteria because it cannot penetrate the OM. Instead, MRL-494 inhibits OMP biogenesis from outside the OM by targeting BamA. The identification of a small molecule that inhibits OMP biogenesis at the cell surface represents a distinct class of antibacterial agents.

Identification of conformation-selective nanobodies against the membrane protein insertase BamA by an integrated structural biology approach.

The insertase BamA is an essential protein of the bacterial outer membrane. Its 16-stranded transmembrane ß-barrel contains a lateral gate as a key functional element. This gate is formed by the C-terminal half of the last ß-strand. The BamA barrel was previously found to sample different conformations in aqueous solution, as well as different gate-open, gate-closed, and collapsed conformations in X-ray crystallography and cryo-electron microscopy structures. Here, we report the successful identification of conformation-selective nanobodies that stabilize BamA in specific conformations. While the initial candidate generation and selection protocol was based on established alpaca immunization and phage display selection procedures, the final selection of nanobodies was enhanced by a solution NMR-based screening step to shortlist the targets for crystallization. In this way, three crystal structures of BamA-nanobody complexes were efficiently obtained, showing two types of nanobodies that indeed stabilized BamA in two different conformations, i.e., with open and closed lateral gate, respectively. Then, by correlating the structural data with high resolution NMR spectra, we could for the first time assign the BamA conformational solution ensemble to defined structural states. The new nanobodies will be valuable tools towards understanding the client insertion mechanism of BamA and towards developing improved antibiotics.

Crystal structures of multidrug efflux pump MexB bound with high-molecular-mass compounds.

RND-type multidrug efflux pumps have two voluminous multisite drug-binding pockets named the proximal and distal binding pocket. High- and low-molecular-mass drugs bind to these proximal and distal pocket, respectively. Here, we report the crystal structures of MexB of Pseudomonas aeruginosa bound with high-molecular-mass compounds. Contrary to the expectations, lauryl maltose neopentyl glycol (LMNG, MW 1,005), which is a surfactant larger than the proximal pocket-binding drugs, was found to bind to the distal pocket: one of the two hydrophobic alkyl chains was inserted into the hydrophobic pit, which is the binding site of the efflux pump inhibitor ABI-PP. LMNG is a substrate of the MexAB-OprM system and competitively inhibits the export of other substrates by this system. However, LMNG does not inhibit the export of other substrates by the inhibitor-binding-pit mutant F178W, which retains the export activity of LMNG. The crystal structure of this mutant suggested that the alkyl chain of LMNG could no longer be inserted into the pit because of steric hindrance. We also determined the crystal structure of MexB containing the high-molecular-mass compound neopentyl glycol derivative C7NG (MW 1,028), the binding site of which overlapped with LMNG in the distal pocket, indicating that whether a substrate binds to the distal or proximal pockets is controlled not only by its molecular weight but also by its individual molecular characteristic.

Pharmacokinetics and Pharmacodynamics of Murepavadin in Neutropenic Mouse Models.

Murepavadin (POL7080) represents the first member of a novel class of outer membrane protein-targeting antibiotics. It specifically interacts with LptD and inhibits lipopolysaccharide (LPS) transport. Murepavadin is being developed for the treatment of serious infections by Pseudomonas aeruginosa We determined the plasma protein binding and the pharmacokinetics of murepavadin in plasma and epithelial lining fluid (ELF; pulmonary) in infected animals, and we determined the exposure-response relationship. Treatment of CD-1 neutropenic mice was started 2 h after infection using murepavadin at different dosing frequencies for 24 h, and the number of CFU per lung was determined. The sigmoid maximum-effect model was used to fit the dose-response, and the pharmacodynamic index (PDI) response was used to determine the PDI values, resulting in a static effect and 1-log kill reduction. Using R2 as an indicator of the best fit, the area under the concentration-time curve for the unbound fraction of the drug (fAUC)/MIC ratio correlated best with efficacy. The mean AUC required to provide a static effect was 36.83 mg h/liter (fAUC = 8.25 mg h/liter), and that to provide a 1-log reduction was 44.0 mg h/liter (fAUC = 9.86 mg h/liter). The mean static fAUC/MIC was determined to be 27.78, and that for a 1-log reduction was 39.85. These data may serve to determine doses in humans that are likely to be efficacious.