Venturicidin A, A Membrane-active Natural Product Inhibitor of ATP synthase Potentiates Aminoglycoside Antibiotics

Exploring the Darobactin Class of Antibiotics: A Comprehensive Review from Discovery to Recent Advancements

The prevalence of antimicrobial resistance in Gram-negative bacteria poses a greater challenge due to their intrinsic resistance to many antibiotics. Recently, darobactins have emerged as a novel class of antibiotics originating from previously unexplored Gram-negative bacterial species such as Photorhabdus, Vibrio, Pseudoalteromonas and Yersinia. Darobactins belong to the ribosomally synthesized and post-translationally modified peptide (RiPP) class of antibiotics, exhibiting selective activity against Gram-negative bacteria. They target the β-barrel assembly machinery (BAM), which is crucial for the maturation and insertion of outer membrane proteins in Gram-negative bacteria. The dar operon in the producer's genome encodes for the synthesis of darobactins, which are characterized by a fused ring system connected via an alkyl-aryl ether linkage (C-O-C) and a C-C cross-link. The enzyme DarE, using the radical S-adenosyl-l-methionine (rSAM), facilitates the formation of these bonds. Biosynthetic manipulation of the darobactin gene cluster, along with its expression in a surrogate host, has enabled access to diverse darobactin analogues with variable antibiotic activities. Recently, two independent research groups successfully achieved the total synthesis of darobactin, employing Larock heteroannulation to construct the bicyclic structure. This paper presents a comprehensive review of darobactins, encompassing their discovery through to the most recent advancements.

Venturicidin A affects the mitochondrial membrane potential and induces kDNA loss in Trypanosoma brucei

Neglected tropical diseases caused by trypanosomatid parasites have devastating health and economic consequences, especially in tropical areas. New drugs or new combination therapies to fight these parasites are urgently needed. Venturicidin A, a macrolide extracted from Streptomyces, inhibits the ATP synthase complex of fungi and bacteria. However, its effect on trypanosomatids is not fully understood. In this study, we tested venturicidin A on a panel of trypanosomatid parasites using Alamar Blue assays and found it to be highly active against Trypanosoma brucei and Leishmania donovani, but much less so against Trypanosoma evansi. Using fluorescence microscopy, we observed a rapid loss of the mitochondrial membrane potential in T. brucei bloodstream forms upon venturicidin A treatment. Additionally, we report the loss of mitochondrial DNA in approximately 40%-50% of the treated parasites. We conclude that venturicidin A targets the ATP synthase of T. brucei, and we suggest that this macrolide could be a candidate for anti-trypanosomatid drug repurposing, drug combinations, or medicinal chemistry programs.

Antibacterial Activities of the Algal Bromophenol Methylrhodomelol Against Pseudomonas aeruginosa

Methylrhodomelol (1: ) is a bromophenol from the red alga Vertebrata lanosa that has been associated with antimicrobial properties. The aim of the current study was, therefore, to assess the antimicrobial potential of this compound in more detail against the gram-negative pathogen Pseudomonas aeruginosa. 1: exerted weak bacteriostatic activity against different strains when grown in minimal medium, whereas other phenolics were inactive. In addition, 1: (35 and 10 µg/mL) markedly enhanced the susceptibility of multidrug-resistant P. aeruginosa toward the aminoglycoside gentamicin, while it did not affect the viability of Vero kidney cells up to 100 µM. Finally, pyoverdine release was reduced in bacteria treated at sub-inhibitory concentration, but no effect on other virulence factors was observed. Transcriptome analysis of treated versus untreated P. aeruginosa indicated an interference of 1: with bacterial carbon and energy metabolism, which was corroborated by RT-qPCR and decreased ATP-levels in treated bacteria. In summary, the current study characterized the antibacterial properties of methylrhodomelol, revealed its potential as an adjuvant to standard antibiotics, and generated a hypothesis on its mode of action.

Aminoglycoside uptake, stress, and potentiation in Gram-negative bacteria: new therapies with old molecules

SUMMARYAminoglycosides (AGs) are long-known molecules successfully used against Gram-negative pathogens. While their use declined with the discovery of new antibiotics, they are now classified as critically important molecules because of their effectiveness against multidrug-resistant bacteria. While they can efficiently cross the Gram-negative envelope, the mechanism of AG entry is still incompletely understood, although this comprehension is essential for the development of new therapies in the face of the alarming increase in antibiotic resistance. Increasing antibiotic uptake in bacteria is one strategy to enhance effective treatments. This review aims, first, to consolidate old and recent knowledge about AG uptake; second, to explore the connection between AG-dependent bacterial stress and drug uptake; and finally, to present new strategies of potentiation of AG uptake for more efficient antibiotic therapies. In particular, we emphasize on the connection between sugar transport and AG potentiation.

Sensitization of Gram-Negative Bacteria to Aminoglycosides with 2-Aminoimidazole Adjuvants

In 2019, five million deaths associated with antimicrobial resistance were reported by The Centers for Disease Control and Prevention (CDC). Acinetobacter baumannii, a Gram-negative bacterial pathogen, is among the list of urgent threats. Previously, we reported 2-aminoimidazole (2-AI) adjuvants that potentiate macrolide activity against A. baumannii. In this study, we identify several of these adjuvants that sensitize A. baumannii to aminoglycoside antibiotics. Lead compounds 1 and 7 lower the tobramycin (TOB) minimum inhibitory concentration (MIC) against the TOB-resistant strain AB5075 from 128 μg/mL to 2 μg/mL at 30 μM. In addition, the lead compounds lower the TOB MIC against the TOB-susceptible strain AB19606 from 4 μg/mL to 1 μg/mL and 0.5 μg/mL, respectively, at 30 μM and 15 μM. The evolution of resistance to TOB and 1 in AB5075 revealed mutations in genes related to protein synthesis, the survival of bacteria under environmental stressors, bacteriophages, and proteins containing Ig-like domains.

Restoring susceptibility to aminoglycosides: identifying small molecule inhibitors of enzymatic inactivation

Growing resistance to antimicrobial medicines is a critical health problem that must be urgently addressed. Adding to the increasing number of patients that succumb to infections, there are other consequences to the rise in resistance like the compromise of several medical procedures and dental work that are heavily dependent on infection prevention. Since their introduction in the clinics, aminoglycoside antibiotics have been a critical component of the armamentarium to treat infections. Still, the increase in resistance and their side effects led to a decline in their utilization. However, numerous current factors, like the urgent need for antimicrobials and their favorable properties, led to renewed interest in these drugs. While efforts to design new classes of aminoglycosides refractory to resistance mechanisms and with fewer toxic effects are starting to yield new promising molecules, extending the useful life of those already in use is essential. For this, numerous research projects are underway to counter resistance from different angles, like inhibition of expression or activity of resistance components. This review focuses on selected examples of one aspect of this quest, the design or identification of small molecule inhibitors of resistance caused by enzymatic modification of the aminoglycoside. These compounds could be developed as aminoglycoside adjuvants to overcome resistant infections.

Advances in the Discovery of Efflux Pump Inhibitors as Novel Potentiators to Control Antimicrobial-Resistant Pathogens

The excessive use of antibiotics has led to the emergence of multidrug-resistant (MDR) pathogens in clinical settings and food-producing animals, posing significant challenges to clinical management and food control. Over the past few decades, the discovery of antimicrobials has slowed down, leading to a lack of treatment options for clinical infectious diseases and foodborne illnesses. Given the increasing prevalence of antibiotic resistance and the limited availability of effective antibiotics, the discovery of novel antibiotic potentiators may prove useful for the treatment of bacterial infections. The application of antibiotics combined with antibiotic potentiators has demonstrated successful outcomes in bench-scale experiments and clinical settings. For instance, the use of efflux pump inhibitors (EPIs) in combination with antibiotics showed effective inhibition of MDR pathogens. Thus, this review aims to enable the possibility of using novel EPIs as potential adjuvants to effectively control MDR pathogens. Specifically, it provides a comprehensive summary of the advances in novel EPI discovery and the underlying mechanisms that restore antimicrobial activity. In addition, we also characterize plant-derived EPIs as novel potentiators. This review provides insights into current challenges and potential strategies for future advancements in fighting antibiotic resistance.

Venturicidin A Is a Potential Fungicide for Controlling Fusarium Head Blight by Affecting Deoxynivalenol Biosynthesis, Toxisome Formation, and Mitochondrial Structure

Fusarium graminearum, which causes Fusarium head blight (FHB) in cereals, is one of the most devastating fungal diseases by causing great yield losses and mycotoxin contamination. A major bioactive ingredient, venturicidin A (VentA), was isolated from Streptomyces pratensis S10 mycelial extract with an activity-guided approach. No report is available on antifungal activity of VentA against F. graminearum and effects on deoxynivalenol (DON) biosynthesis. Here, VentA showed a high antagonistic activity toward F. graminearum with an EC50 value of 3.69 μg/mL. As observed by scanning electron microscopy, after exposure to VentA, F. graminearum conidia and mycelia appeared abnormal. Different dyes staining revealed that VentA increased cell membrane permeability. In growth chamber and field trials, VentA effectively reduced disease severity of FHB. Moreover, VentA inhibited DON biosynthesis by reducing pyruvic acid, acetyl-CoA production, and accumulation of reactive oxygen species (ROS) and then inhibiting trichothecene (TRI) genes expression and toxisome formation. These results suggest that VentA is a potential fungicide for controlling FHB.

Streptomyces pratensis S10 Controls Fusarium Head Blight by Suppressing Different Stages of the Life Cycle and ATP Production

Fusarium head blight (FHB) of wheat, predominately caused by Fusarium graminearum, is an economically important plant disease worldwide. With increased fungicide resistance, controlling this filamentous fungal disease has become an enormous challenge. Biocontrol agents alone or integrated with other methods could better manage FHB. Streptomyces pratensis S10 has strong antagonistic activity against FHB as reported in our previous study. We now have investigated S10 controls of FHB in more detail by combining microscope observations, biological assays, and transcriptome profiling. S10 culture filtrates (SCF) significantly inhibited essential stages of the life cycle of F. graminearum in the laboratory and under simulated natural conditions. SCF at different concentrations inhibited conidiation of F. graminearum with an inhibition of 57.49 to 83.83% in the medium and 64.04 to 85.89% in plants. Different concentrations of SCF reduced conidia germination by 47.33 to 67.67%. Two percent (vol/vol) SCF suppressed perithecia formation of F. graminearum by 84 and 81% in the laboratory and under simulated natural conditions, respectively. The S10 also reduced the pathogenicity and penetration ability of F. graminearum by suppressing ATP production. Collectively, these findings indicate that S. pratensis S10 should be explored further for efficacy at controlling FHB.

Boosting the Antibacterial Activity of Azithromycin on Multidrug-Resistant Escherichia coli by Efflux Pump Inhibition Coupled with Outer Membrane Permeabilization Induced by Phenylalanine-Arginine β-Naphthylamide

The global spread of multidrug-resistant (MDR) bacteria increases the demand for the discovery of new antibiotics and adjuvants. Phenylalanine-arginine β-naphthylamide (PAβN) is an inhibitor of efflux pumps in Gram-negative bacteria, such as the AcrAB-TolC complex in Escherichia coli. We aimed to explore the synergistic effect and mechanism of action of PAβN combined with azithromycin (AZT) on a group of MDR E. coli strains. Antibiotic susceptibility was tested for 56 strains, which were screened for macrolide resistance genes. Then, 29 strains were tested for synergy using the checkerboard assay. PAβN significantly enhanced AZT activity in a dose-dependent manner in strains expressing the mphA gene and encoding macrolide phosphotransferase, but not in strains carrying the ermB gene and encoding macrolide methylase. Early bacterial killing (6 h) was observed in a colistin-resistant strain with the mcr-1 gene, leading to lipid remodeling, which caused outer membrane (OM) permeability defects. Clear OM damage was revealed by transmission electron microscopy in bacteria exposed to high doses of PAβN. Increased OM permeability was also proven by fluorometric assays, confirming the action of PAβN on OM. PAβN maintained its activity as an efflux pump inhibitor at low doses without permeabilizing OM. A non-significant increase in acrA, acrB, and tolC expression in response to prolonged exposure to PAβN was noted in cells treated with PAβN alone or with AZT, as a reflection of bacterial attempts to counteract pump inhibition. Thus, PAβN was found to be effective in potentiating the antibacterial activity of AZT on E. coli through dose-dependent action. This warrants further investigations of its effect combined with other antibiotics on multiple Gram-negative bacterial species. Synergetic combinations will help in the battle against MDR pathogens, adding new tools to the arsenal of existing medications.

Antibiotic Adjuvants: A Versatile Approach to Combat Antibiotic Resistance

The problem of antibiotic resistance is on the rise, with multidrug-resistant strains emerging even to the last resort antibiotics. The drug discovery process is often stalled by stringent cut-offs required for effective drug design. In such a scenario, it is prudent to delve into the varying mechanisms of resistance to existing antibiotics and target them to improve antibiotic efficacy. Nonantibiotic compounds called antibiotic adjuvants which target bacterial resistance can be used in combination with obsolete drugs for an improved therapeutic regime. The field of "antibiotic adjuvants" has gained significant traction in recent years where mechanisms other than β-lactamase inhibition have been explored. This review discusses the multitude of acquired and inherent resistance mechanisms employed by bacteria to resist antibiotic action. The major focus of this review is how to target these resistance mechanisms by the use of antibiotic adjuvants. Different types of direct acting and indirect resistance breakers are discussed including enzyme inhibitors, efflux pump inhibitors, inhibitors of teichoic acid synthesis, and other cellular processes. The multifaceted class of membrane-targeting compounds with poly pharmacological effects and the potential of host immune-modulating compounds have also been reviewed. We conclude with providing insights about the existing challenges preventing clinical translation of different classes of adjuvants, especially membrane-perturbing compounds, and a framework about the possible directions which can be pursued to fill this gap. Antibiotic-adjuvant combinatorial therapy indeed has immense potential to be used as an upcoming orthogonal strategy to conventional antibiotic discovery.

Inhibitors of ATP Synthase as New Antibacterial Candidates

ATP, the power of all cellular functions, is constantly used and produced by cells. The enzyme called ATP synthase is the energy factory in all cells, which produces ATP by adding inorganic phosphate (Pi) to ADP. It is found in the inner, thylakoid and plasma membranes of mitochondria, chloroplasts and bacteria, respectively. Bacterial ATP synthases have been the subject of multiple studies for decades, since they can be genetically manipulated. With the emergence of antibiotic resistance, many combinations of antibiotics with other compounds that enhance the effect of these antibiotics have been proposed as approaches to limit the spread of antibiotic-resistant bacteria. ATP synthase inhibitors, such as resveratrol, venturicidin A, bedaquiline, tomatidine, piceatannol, oligomycin A and N,N-dicyclohexylcarbodiimide were the starting point of these combinations. However, each of these inhibitors target ATP synthase differently, and their co-administration with antibiotics increases the susceptibility of pathogenic bacteria. After a brief description of the structure and function of ATP synthase, we aim in this review to highlight therapeutic applications of the major bacterial ATP synthase inhibitors, including animal’s venoms, and to emphasize their importance in decreasing the activity of this enzyme and subsequently eradicating resistant bacteria as ATP synthase is their source of energy.

F1·Fo ATP Synthase/ATPase: Contemporary View on Unidirectional Catalysis

F1·Fo-ATP synthases/ATPases (F1·Fo) are molecular machines that couple either ATP synthesis from ADP and phosphate or ATP hydrolysis to the consumption or production of a transmembrane electrochemical gradient of protons. Currently, in view of the spread of drug-resistant disease-causing strains, there is an increasing interest in F1·Fo as new targets for antimicrobial drugs, in particular, anti-tuberculosis drugs, and inhibitors of these membrane proteins are being considered in this capacity. However, the specific drug search is hampered by the complex mechanism of regulation of F1·Fo in bacteria, in particular, in mycobacteria: the enzyme efficiently synthesizes ATP, but is not capable of ATP hydrolysis. In this review, we consider the current state of the problem of “unidirectional” F1·Fo catalysis found in a wide range of bacterial F1·Fo and enzymes from other organisms, the understanding of which will be useful for developing a strategy for the search for new drugs that selectively disrupt the energy production of bacterial cells.

Structural insights into thesubstrate binding sites of O-carbamoyltransferase VtdB from Streptomyces sp. NO1W98

The O-carbamoyltransferase VtdB catalyzes the carbamoylation of venturicidin B, which is essential for the biosynthesis of the antibiotic venturicidin A. Here, the crystal structures of VtdB and VtdB in complex with the intermediate carbamoyladenylate (VtdB^CAO) were determined at resolutions of 2.99 Å and 2.90 Å, respectively. The structures resemble the conserved YrdC-like and specific Kae1-like domains. A magnesium ion and the intermediate carbamoyladenylate were also observed in the Kae1-like domain of VtdB. The structure of VtdB^CAO in complex with the substrate venturicidin B was modeled by a molecular docking method to better understand the substrate binding mode, revealing a novel venturicidin B binding pocket.

Lessons from assembling a microbial natural product and pre-fractionated extract library in an academic laboratory

Microbial natural products are specialized metabolites that are sources of many bioactive compounds including antibiotics, antifungals, antiparasitics, anticancer agents, and probes of biology. The assembly of libraries of producers of natural products has traditionally been the province of the pharmaceutical industry. This sector has gathered significant historical collections of bacteria and fungi to identify new drug leads with outstanding outcomes-upwards of 60% of drug scaffolds originate from such libraries. Despite this success, the repeated rediscovery of known compounds and the resultant diminishing chemical novelty contributed to a pivot from this source of bioactive compounds toward more tractable synthetic compounds in the drug industry. The advent of advanced mass spectrometry tools, along with rapid whole genome sequencing and in silico identification of biosynthetic gene clusters that encode the machinery necessary for the synthesis of specialized metabolites, offers the opportunity to revisit microbial natural product libraries with renewed vigor. Assembling a suitable library of microbes and extracts for screening requires the investment of resources and the development of methods that have customarily been the proprietary purview of large pharmaceutical companies. Here, we report a perspective on our efforts to assemble a library of natural product-producing microbes and the establishment of methods to extract and fractionate bioactive compounds using resources available to most academic labs. We validate the library and approach through a series of screens for antimicrobial and cytotoxic agents. This work serves as a blueprint for establishing libraries of microbial natural product producers and bioactive extract fractions suitable for screens of bioactive compounds.

ONE-SENTENCE SUMMARY

Natural products are key to discovery of novel antimicrobial agents: Here, we describe our experience and lessons learned in constructing a microbial natural product and pre-fractionated extract library.

Natural products, including a new caboxamycin, from Streptomyces and other Actinobacteria isolated in Spain from storm clouds transported by Northern winds of Arctic origin

Actinobacteria, mostly Streptomyces species, are the main source of natural products essential in medicine. While the majority of producer microorganisms of secondary metabolite are reported from terrestrial or marine environments, there are limited reports of their isolation from atmospheric precipitations. Clouds are considered as atmospheric oases for microorganisms and there is a recent paradigm shift whereby atmospheric-derived Actinobacteria emerge as an alternative source for drug discovery. In this context, we studied a total of 18 bioactive Actinobacteria strains, isolated by sampling nine precipitation events with prevailing Northern winds in the Cantabrian Sea coast, Northern Spain. Backward trajectories meteorological analyses indicate that air masses were originated mostly in the Arctic Ocean, and their trajectory to downwind areas involved the Atlantic Ocean and also terrestrial sources from continental Europe, and in some events from Canada, Greenland, Mauritania and Canary Islands. Taxonomic identification of the isolates, by 16S rRNA gene sequencing and phylogenetic analyses, revealed that they are members of three Actinobacteria genera. Fifteen of the isolates are Streptomyces species, thus increasing the number of bioactive species of this genus in the atmosphere to a 6.8% of the total currently validated species. In addition, two of the strains belong to the genus Micromonospora and one to genus Nocardiopsis. These findings reinforce a previous atmospheric dispersal model, extended herein to the genus Micromonospora. Production of bioactive secondary metabolites was screened in ethyl acetate extracts of the strains by LC-UV-MS and a total of 94 secondary metabolites were detected after LC/MS dereplication. Comparative analyses with natural products databases allowed the identification of 69 structurally diverse natural products with contrasted biological activities, mostly as antibiotics and antitumor agents, but also anti-inflammatory, antiviral, antiparasitic, immunosuppressant and neuroprotective among others. The molecular formulae of the 25 remaining compounds were determined by HRMS. None of these molecules had been previously reported in natural product databases indicating potentially novel metabolites. As a proof of concept, a new metabolite caboxamycin B (1) was isolated from the culture broth of Streptomyces sp. A-177 and its structure was determined by various spectrometric methods. To the best of our knowledge, this is the first novel natural product obtained from an atmospheric Streptomyces, thus pointing out precipitations as an innovative source for discovering new pharmaceutical natural products.

Discerning the role of polymyxin B nonapeptide in restoring the antibacterial activity of azithromycin against antibiotic-resistant Escherichia coli

Antimicrobial resistance is a global public health threat. Antibiotic development pipeline has few new drugs; therefore, using antibiotic adjuvants has been envisioned as a successful method to preserve existing medications to fight multidrug-resistant (MDR) pathogens. In this study, we investigated the synergistic effect of a polymyxin derivative known as polymyxin B nonapeptide (PMBN) with azithromycin (AZT). A total of 54 Escherichia coli strains were first characterized for macrolide resistance genes, and susceptibility to different antibiotics, including AZT. A subset of 24 strains was then selected for synergy testing by the checkerboard assay. PMBN was able to re-sensitize the bacteria to AZT, even in strains with high minimum inhibitory concentrations (MIC: 32 to ≥128 μg/ml) for AZT, and in strains resistant to the last resort drugs such as colistin and meropenem. The fractional inhibitory concentration index was lower than 0.5, demonstrating that PMBN and AZT combinations had a synergistic effect. The combinations worked efficiently in strains carrying mphA gene encoding macrolide phosphotransferase which can cause macrolide inactivation. However, the combinations were inactive in strains having an additional ermB gene encoding macrolide methylase which causes ribosomal drug target alteration. Killing kinetics study showed a significant reduction of bacterial growth after 6 h of treatment with complete killing achieved after 24 h. Transmission electron microscopy showed morphological alterations in the bacteria treated with PMBN alone or in combination with AZT, with evidence of damage to the outer membrane. These results suggested that PMBN acted by increasing the permeability of bacterial outer membrane to AZT, which was also evident using a fluorometric assay. Using multiple antimicrobial agents could therefore be a promising strategy in the eradication of MDR bacteria. PMBN is a good candidate for use with other antibiotics to potentiate their activity, but further studies are required in vivo. This will significantly contribute to resolving antimicrobial resistance crisis.

Synthesis and Evaluation of Pseudomonas aeruginosa ATP Synthase Inhibitors

New antibiotics with unique biological targets are desperately needed to combat the growing number of resistant bacterial pathogens. ATP synthase, a critical protein found in all life, has recently become a target of interest for antibiotic development due to the success of the anti-tuberculosis drug bedaquiline, and while many groups have worked on developing drugs to target bacterial ATP synthase, few have been successful at inhibiting Pseudomonas aeruginosa (PA) ATP synthase specifically. PA is one of the leading causes of resistant nosocomial infections across the world and is extremely challenging to treat due to its various antibiotic resistance mechanisms for most commonly used antibiotics. Herein, we detail the synthesis and evaluation of a series of C1/C2 quinoline analogues for their ability to inhibit PA ATP synthase and act as antibiotics against wild-type PA. From this survey, we found six compounds capable of inhibiting PA ATP synthase in vitro showing that bulky/hydrophobic C1/C2 substitutions are preferred. The strongest inhibitor showed an IC₅₀ of 10 μg/mL and decreased activity of PA ATP synthase to 24% relative to the control. While none of the compounds were able to inhibit wild-type PA in cell culture, two showed improved inhibition of PA growth when permeability of the outer membrane was increased or efflux was knocked out, thus demonstrating that these compounds could be further developed into efficacious antibiotics.

Interaction of Venturicidin and Fo·F1-ATPase/ATP Synthase of Tightly Coupled Subbacterial Particles of Paracoccus denitrificans in Energized Membranes

Proton-translocating F_o×F₁-ATPase/synthase that catalyzes synthesis and hydrolysis of ATP is commonly considered to be a reversibly functioning complex. We have previously shown that venturicidin, a specific F_o-directed inhibitor, blocks the synthesis and hydrolysis of ATP with a significant difference in the affinity [Zharova, T. V. and Vinogradov, A. D. (2017) Biochim. Biophys. Acta, 1858, 939-944]. In this paper, we have studied in detail inhibition of F_o×F₁-ATPase/synthase by venturicidin in tightly coupled membranes of Paracoccus denitrificans under conditions of membrane potential generation. ATP hydrolysis was followed by the ATP-dependent succinate-supported NAD+ reduction (potential-dependent reverse electron transfer) catalyzed by the respiratory chain complex I. It has been demonstrated that membrane energization did not affect the affinity of F_o×F₁-ATPase/synthase for venturicidin. The dependence of the residual ATP synthase activity on the concentration of venturicidin approximated a linear function, whereas the dependence of ATP hydrolysis was sigmoidal: at low inhibitor concentrations venturicidin strongly inhibited ATP synthesis without decrease in the rate of ATP hydrolysis. A model is proposed suggesting that ATP synthesis and ATP hydrolysis are catalyzed by two different forms of F_o×F₁.

Antibiotic Potentiators Against Multidrug-Resistant Bacteria: Discovery, Development, and Clinical Relevance

Antimicrobial resistance in clinically important microbes has emerged as an unmet challenge in global health. Extensively drug-resistant bacterial pathogens have cropped up lately defying the action of even the last resort of antibiotics. This has led to a huge burden in the health sectors and increased morbidity and mortality rate across the world. The dwindling antibiotic discovery pipeline and rampant usage of antibiotics has set the alarming bells necessitating immediate actions to combat this looming threat. Various alternatives to discovery of new antibiotics are gaining attention such as reversing the antibiotic resistance and hence reviving the arsenal of antibiotics in hand. Antibiotic resistance reversal is mainly targeted against the antibiotic resistance mechanisms, which potentiates the effective action of the antibiotic. Such compounds are referred to as resistance breakers or antibiotic adjuvants/potentiators that work in conjunction with antibiotics. Many studies have been conducted for the identification of compounds, which decrease the permeability barrier, expression of efflux pumps and the resistance encoding enzymes. Compounds targeting the stability, inheritance and dissemination of the mobile genetic elements linked with the resistance genes are also potential candidates to curb antibiotic resistance. In pursuit of such compounds various natural sources and synthetic compounds have been harnessed. The activities of a considerable number of compounds seem promising and are currently at various phases of clinical trials. This review recapitulates all the studies pertaining to the use of antibiotic potentiators for the reversal of antibiotic resistance and what the future beholds for their usage in clinical settings.

Potentiation of β-Lactams against Methicillin-Resistant Staphylococcus aureus (MRSA) Using Octyl Gallate, a Food-Grade Antioxidant

Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to a number of antibiotics of clinical importance and is a serious threat to public health. Since bacteria rapidly develop resistance even to newly discovered antibiotics, this study aimed to develop drug potentiators to enhance the antibacterial activity of existing antibiotics for the control of MRSA. Based on our previous studies, screening of antimicrobial synergy was conducted with gallic acid and its derivatives using checkerboard assays. Antimicrobial synergy was confirmed with MRSA isolates from clinical cases. Combinations of penicillin, ampicillin, and cephalothin with octyl gallate (OG), an antioxidant approved by the US Food and Drug Administration (FDA), consistently exhibited synergistic bacteriostatic and bactericidal activities against MRSA, rendering MRSA sensitive to β-lactams. The fractional inhibitory concentration (FIC) and fractional bactericidal concentration (FBC) indices exhibited that the antimicrobial effects of OG were synergistic. The results of a permeability assay showed that OG significantly increased the permeability of the bacterial cell wall. Despite the intrinsic resistance of MRSA to β-lactams, the findings in this study demonstrated that OG enhanced the activity of β-lactams in MRSA and sensitized MRSA to β-lactams, suggesting that OG can be used as a drug potentiator to control MRSA using existing antibiotics.

Repurposing Candesartan Cilexetil as Antibacterial Agent for MRSA Infection

Staphylococcus aureus is an important pathogen causing hospital-acquired infections. Methicillin-resistant S. aureus (MRSA), biofilms, and persisters are highly tolerant to traditional antibiotics and make it difficult to treat. Therefore, new antimicrobial agents are urgently needed to treat hard-to-eradicate diseases caused by this bacterium. In this study, candesartan cilexetil (CC), an angiotensin hypertension drug, had strong antimicrobial activity against S. aureus with minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) of 8-16 μg/ml and 16-32 μg/ml. CC exhibited limited cytotoxicity and low potential to induce drug resistance. In addition, it showed a synergistic antibacterial effect when combined with gentamicin and tobramycin. The effective concentrations to inhibit MRSA biofilm formation were 16-64 μg/ml, and intractable persisters were killed at 4-8 × MIC. Through the analysis of its mechanism of action, it was evident that the membrane permeability was disrupted as well as the cell structure was damaged. Furthermore, we demonstrated that CC had antibacterial effects in vivo in MRSA-infected murine skin abscess models. In conclusion, these results imply that CC might be a potential antibacterial agent for the treatment of S. aureus-associated infections.

Inhibition of Aminoglycoside 6'-N-acetyltransferase Type Ib (AAC(6')-Ib): Structure-Activity Relationship of Substituted Pyrrolidine Pentamine Derivatives as Inhibitors

The aminoglycoside 6'-N-acetyltransferase type Ib (AAC(6')-Ib) is a common cause of resistance to amikacin and other aminoglycosides in Gram-negatives. Utilization of mixture-based combinatorial libraries and application of the positional scanning strategy identified an inhibitor of AAC(6')-Ib. This inhibitor's chemical structure consists of a pyrrolidine pentamine scaffold substituted at four locations (R1, R3, R4, and R5). The substituents are two S-phenyl groups (R1 and R4), an S-hydroxymethyl group (R3), and a 3-phenylbutyl group (R5). Another location, R2, does not have a substitution, but it is named because its stereochemistry was modified in some compounds utilized in this study. Structure-activity relationship (SAR) analysis using derivatives with different functionalities, modified stereochemistry, and truncations was carried out by assessing the effect of the addition of each compound at 8 µM to 16 µg/mL amikacin-containing media and performing checkerboard assays varying the concentrations of the inhibitor analogs and the antibiotic. The results show that: (1) the aromatic functionalities at R1 and R4 are essential, but the stereochemistry is essential only at R4; (2) the stereochemical conformation at R2 is critical; (3) the hydroxyl moiety at R3 as well as stereoconformation are required for full inhibitory activity; (4) the phenyl functionality at R5 is not essential and can be replaced by aliphatic groups; (5) the location of the phenyl group on the butyl carbon chain at R5 is not essential; (6) the length of the aliphatic chain at R5 is not critical; and (7) all truncations of the scaffold resulted in inactive compounds. Molecular docking revealed that all compounds preferentially bind to the kanamycin C binding cavity, and binding affinity correlates with the experimental data for most of the compounds evaluated. The SAR results in this study will serve as the basis for the design of new analogs in an effort to improve their ability to induce phenotypic conversion to susceptibility in amikacin-resistant pathogens.

Biology and applications of co-produced, synergistic antimicrobials from environmental bacteria

Environmental bacteria, such as Streptomyces spp., produce specialized metabolites that are potent antibiotics and therapeutics. Selected specialized antimicrobials are co-produced and function together synergistically. Co-produced antimicrobials comprise multiple chemical classes and are produced by a wide variety of bacteria in different environmental niches, suggesting that their combined functions are ecologically important. Here, we highlight the exquisite mechanisms that underlie the simultaneous production and functional synergy of 16 sets of co-produced antimicrobials. To date, antibiotic and antifungal discovery has focused mainly on single molecules, but we propose that methods to target co-produced antimicrobials could widen the scope and applications of discovery programs.

Discovery and Validation of a Novel Target of Molluscicides against Oncomelania hupensis, the Intermediate Host of Schistosoma japonicum

In this study, 196 strains of actinomycetes isolated from marshland soil samples were tested for molluscicidal activity against Oncomelania hupensis. Five strains demonstrated molluscicidal activity, of which the molluscicidal efficiency of Actinomycetes strain A183 was the maximum. After the fermentation supernatant of actinomycetes A183 was extracted with ethyl acetate (EWEA), the LC50 of the EWEA after leaching for 48 h and 72 h were 0.2688 and 0.2195 mg/L, respectively. The effect of EWEA on the key points of energy metabolism was determined. We noted that 1 mg/L of EWEA (A813) significantly reduced the activity of mitochondrial respiratory chain complex I (P < 0.05), while no significant changes were observed in the activities of complexes II, III, and IV. In addition, EWEA (A813) could decrease the membrane potential of O. hupensis purified mitochondria in vitro. The LC50 of the 3 uncoupler (FCCP, DNP, and Tyrphostin A9) after immersion for 24 h were 0.065, 0.135, and 0.110 mg/L, respectively; LC50 after 48 h treatment was 0.064, 0.124, and 0.082 mg/L, respectively; LC50 after 72 h treatment was 0.063, 0.129, and 0.061 mg/L, respectively, and all uncoupler showed strong molluscicidal activities, demonstrating that the mitochondrial membrane potential uncoupling is a potential target for molluscicides against O. hupensis. Moreover, the molluscicidal active substance of strain A183 needs to be further isolated, purified, and structurally characterized considering its promising potential applications.

Complex effects of macrolide venturicidins on bacterial F-ATPases likely contribute to their action as antibiotic adjuvants

Bacterial energy metabolism is now recognized as a critical factor for the efficacy of antibiotics. The F-type ATPase/ATP synthase (F_OF₁) is a central player in cellular bioenergetics of bacteria and eukaryotes, and its potential as a selective antibiotic target has been confirmed by the success of bedaquiline in combatting multidrug-resistant tuberculosis. Venturicidin macrolides were initially identified for their antifungal properties and were found to specifically inhibit F_OF₁ of eukaryotes and bacteria. Venturicidins alone are not effective antibacterials but recently were found to have adjuvant activity, potentiating the efficacy of aminoglycoside antibiotics against several species of resistant bacteria. Here we discovered more complex effects of venturicidins on the ATPase activity of F_OF₁ in bacterial membranes from Escherichia coli and Pseudomonas aeruginosa. Our major finding is that higher concentrations of venturicidin induce time- and ATP-dependent decoupling of F₁-ATPase activity from the venturicidin-inhibited, proton-transporting F_O complex. This dysregulated ATPase activity is likely to be a key factor in the depletion of cellular ATP induced by venturicidins in prior studies with P. aeruginosa and Staphylococcus aureus. Further studies of how this functional decoupling occurs could guide development of new antibiotics and/or adjuvants that target the F-type ATPase/ATP synthase.

Proteomic analysis of serial isolates of Trichosporon asahii identifies host-specific adaptations using the TMT/MRM approach

The opportunistic fungal pathogen Trichosporon asahii (T. asahii) is an important causal agent of mortality in immunocompromised patients and associated with frequent relapses, even with sufficient antifungal treatment. Investigating the proteomes of initial and recurrent isolates may help to identify within-host adaptive changes. In this study, using tandem mass tag (TMT)-labeling combined with liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) technology, we analyzed the proteomes of two T. asahii strains that were isolated 15 years apart from the same patient who suffered initial and recurrent episodes of systemic disseminated trichosporonosis. A total of 597 differentially expressed proteins were identified. Functional analysis showed that the increased proteins were primarily concentrated on peptide/protein/energy/drug metabolism and translation. Most of the results were determined to be consistent with the findings of phenotypic assays, such as tests for drug susceptibility, temperature growth, biofilm formation, melanization and paromomycin assays. Moreover, we performed multiple reaction monitoring (MRM) mass spectrometry to verify 27 candidate proteins, and the results of this experiment were also highly consistent with the results of the TMT analysis. Therefore, to the best of our knowledge, these data provide the first molecular evidence of how the T. asahii proteome changes related to host-specific adaptation during human infection. SIGNIFICANCE: Systemic infection with Trichosporon asahii (T. asahii) has recently been recognized as an important causal agent of mortality in immunocompromised patients. Although triazole treatment usually works efficiently in the early phase of infection, many patients relapse. Hence, comparative analyses of the proteomics of initial and recurrent isolates may reveal evidence of adaptive changes within the host. Our study demonstrates that the recurrent strain has undergone proteomic changes using tandem mass tag (TMT)-labeling combined with liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). Moreover, the results of phenotypic assays, including drug susceptibility, temperature growth, biofilm formation, melanization and paromomycin assays, were highly consistent with the proteomic changes, and multiple reaction monitoring (MRM) verification also showed similar trends to the TMT results. In summary, our study is the first to investigate the adaptation of T. asahii under pressure from antifungal chemotherapy and host immune responses.

Targeting Bacterial Membrane Proteins to Explore the Beneficial Effects of Natural Products: New Antibiotics against Drug Resistance

Antibiotic resistance is currently a world health crisis that urges the development of new antibacterial substances. To this end, natural products, including flavonoids, alkaloids, terpenoids, steroids, peptides and organic acids that play a vital role in the development of medicines and thus constitute a rich source in clinical practices, provide an important source of drugs directly or for the screen of lead compounds for new antibiotic development. Because membrane proteins, which comprise more than 60% of the current clinical drug targets, play crucial roles in signal transduction, transport, bacterial pathogenicity and drug resistance, as well as immunogenicity, it is our aim to summarize those natural products with different structures that target bacterial membrane proteins, such as efflux pumps and enzymes, to provide an overview for the development of new antibiotics to deal with antibiotic resistance.

A Screen of Natural Product Extracts Identifies Moenomycin as a Potent Antigonococcal Agent

Increasing multidrug resistance in Neisseria gonorrheae is a growing public health crisis. Resistance to the last line therapies, cephalosporins and azithromycin, are of particular concern, fueling the need to discover new treatments. Here, we identified the phosphoglycolipid moenomycin from a screen of microbial natural products against drug-resistant N. gonorrheae as a potent antigonococcal agent. Moenomycin demonstrates excellent activity (MIC = 0.004-0.03 μg/mL) against a variety of multidrug-resistant N. gonorrheae. Importantly, moenomycin, thought to be a Gram-positive specific antibiotic, penetrates the Gram-negative gonococcal outer membrane. Moenomycin causes intracellular accumulation of peptidoglycan precursors, cell blebbing, and rupture of the cell envelope, all consistent with cell wall biosynthesis inhibition. Serial bacterial exposure to moenomycin for 14 days revealed slow development of resistance (MIC_Day14 = 0.03-0.06 μg/mL), unlike the clinically used drug azithromycin. Our results offer the potential utility of moenomycin as a lead for antigonococcal therapeutic candidates and warrant further investigation.

Macrolides: From Toxins to Therapeutics

Macrolides are a diverse class of hydrophobic compounds characterized by a macrocyclic lactone ring and distinguished by variable side chains/groups. Some of the most well characterized macrolides are toxins produced by marine bacteria, sea sponges, and other species. Many marine macrolide toxins act as biomimetic molecules to natural actin-binding proteins, affecting actin polymerization, while other toxins act on different cytoskeletal components. The disruption of natural cytoskeletal processes affects cell motility and cytokinesis, and can result in cellular death. While many macrolides are toxic in nature, others have been shown to display therapeutic properties. Indeed, some of the most well known antibiotic compounds, including erythromycin, are macrolides. In addition to antibiotic properties, macrolides have been shown to display antiviral, antiparasitic, antifungal, and immunosuppressive actions. Here, we review each functional class of macrolides for their common structures, mechanisms of action, pharmacology, and human cellular targets.

Targeting the ATP Synthase in Staphylococcus aureus Small Colony Variants, Streptococcus pyogenes and Pathogenic Fungi

The ATP synthase has been validated as a druggable target with the approval of the ATP synthase inhibitor, bedaquiline, for treatment of drug-resistant Mycobacterium tuberculosis, a bacterial species in which the ATP synthase is essential for viability. Gene inactivation studies have also shown that the ATP synthase is essential among Streptococci, and some studies even suggest that inhibition of the ATP synthase is a strategy for the elimination of Staphylococcus aureus small colony variants with deficiencies in the electron transport chain, as well as pathogenic fungi, such as Candida albicans. Here we investigated five structurally diverse ATP synthase inhibitors, namely N,N′-dicyclohexylcarbodiimide (DCCD), oligomycin A, tomatidine, resveratrol and piceatannol, for their growth inhibitory activity against the bacterial strains Streptococcus pyogenes, S. aureus and two isogenic small colony variants, as well as the pathogenic fungal species, C. albicans and Aspergillus niger. DCCD showed broad-spectrum inhibitory activity against all the strains (minimum inhibitory concentration (MIC) 2–16 µg/mL), except for S. aureus, where the ATP synthase is dispensable for growth. Contrarily, oligomycin A selectively inhibited the fungal strains (MIC 1–8 µg/mL), while tomatidine showed very potent, but selective, activity against small colony variants of S. aureus with compromised electron transport chain activity (MIC 0.0625 µg/mL). Small colony variants of S. aureus were also more sensitive to resveratrol and piceatannol than the wild-type strain, and piceatannol inhibited S. pyogenes at 16–32 µg/mL. We previously showed that transposon inactivation of the ATP synthase sensitizes S. aureus towards polymyxin B and colistin, and here we demonstrate that treatment with structurally diverse ATP synthase inhibitors sensitized S. aureus towards polymyxin B. Collectively, our data show that ATP synthase inhibitors can have selective inhibitory activity against pathogenic microorganisms in which the ATP synthase is essential. The data also show that the inhibition of the ATP synthase in Streptococcus pyogenes may be a new strategy for development of a narrow-spectrum antibiotic class. In other major bacterial pathogens, such as S. aureus and potentially Escherichia coli, where the ATP synthase is dispensable, the ATP synthase inhibitors may be applied in combination with antimicrobial peptides to provide new therapeutic options.

Compound Uptake into E. coli Can Be Facilitated by N-Alkyl Guanidiniums and Pyridiniums

Multidrug-resistant Gram-negative bacterial infections are on the rise, and with no FDA approvals for new classes of broad-spectrum antibiotics in over 50 years, these infections constitute a major threat to human health. A significant challenge is the inability of most compounds to accumulate in Gram-negative bacteria. Recently developed predictive guidelines show that appending a primary amine to an appropriately shaped compound can enhance Gram-negative accumulation. Here, we report that other positively charged nitrogen functional groups, namely, N-alkyl guanidiniums and pyridiniums, can also facilitate compound uptake into Gram-negative bacteria. The accumulation of a set of 60 nonantibiotic compounds, consisting of 20 primary amines and their corresponding guanidiniums and pyridiniums, was assessed in Escherichia coli. We also installed these alternate functional groups onto antibiotic scaffolds and assessed their accumulation and antibacterial activity in Gram-negative bacteria. The results suggest that other positively-charged, nitrogen-containing functional groups should be considered when designing antibiotics with Gram-negative activity.

Resistance-Guided Discovery of Elfamycin Antibiotic Producers with Antigonococcal Activity

The rise of bacterial antibiotic resistance coupled with a diminished antibiotic drug pipeline underlines the importance of developing rational strategies to discover new antimicrobials. Microbially derived natural products are the basis for most of the antibiotic arsenal available to modern medicine. Here, we demonstrate a resistance-based approach to identify producers of elfamycins, an under-explored class of natural product antibiotics that target the essential translation factor EF-Tu. Antibiotic producers carry self-resistance genes to avoid suicide. These genes are often found within the same biosynthetic gene cluster (BGC) responsible for making the antibiotic, and we exploited this trait to identify members of the kirromycin class of elfamycin producers. Genome mining of Streptomyces spp. led to the identification of three isolates that harbor kirromycin-resistant EF-Tu (EF-Tu^KirR) within predicted natural product BGCs. Activity-guided purification on extracts of one of the Streptomyces isolates, which was not known to produce an elfamycin, identified it as a producer of phenelfamycin B, a linear polyketide. Phenelfamycin B demonstrates impressive antibacterial activity (MIC ∼ 1 μg/mL) against multidrug-resistant Neisseria gonorrhoeae, a clinically important Gram negative pathogen. The antigonococcal activity of phenelfamycin was shown to be the result of inhibition of protein biosynthesis by binding to EF-Tu. These results indicate that a resistance-based approach of identifying elfamycin producers is translatable to other antibiotic classes that can identify new and overlooked antibiotics necessary to address the antibiotic crisis.

Engineering protein fragments via evolutionary and protein-protein interaction algorithms: de novo design of peptide inhibitors for FO F1 -ATP synthase

Enzyme subunit interfaces have remarkable potential in drug design as both target and scaffold for their own inhibitors. We show an evolution-driven strategy for the de novo design of peptide inhibitors targeting interfaces of the Escherichia coli FoF1-ATP synthase as a case study. The evolutionary algorithm ROSE was applied to generate diversity-oriented peptide libraries by engineering peptide fragments from ATP synthase interfaces. The resulting peptides were scored with PPI-Detect, a sequence-based predictor of protein-protein interactions. Two selected peptides were confirmed by in vitro inhibition and binding tests. The proposed methodology can be widely applied to design peptides targeting relevant interfaces of enzymatic complexes.