Cathelicidin-Derived Synthetic Peptide Improves Therapeutic Potential of Vancomycin Against Pseudomonas aeruginosa

Complementary Activities of Host Defence Peptides and Antibiotics in Combating Antimicrobial Resistant Bacteria

Due to their ability to eliminate antimicrobial resistant (AMR) bacteria and to modulate the immune response, host defence peptides (HDPs) hold great promise for the clinical treatment of bacterial infections. Whereas monotherapy with HDPs is not likely to become an effective first-line treatment, combinations of such peptides with antibiotics can potentially provide a path to future therapies for AMR infections. Therefore, we critically reviewed the recent literature regarding the antibacterial activity of combinations of HDPs and antibiotics against AMR bacteria and the approaches taken in these studies. Of the 86 studies compiled, 56 featured a formal assessment of synergy between agents. Of the combinations assessed, synergistic and additive interactions between HDPs and antibiotics amounted to 84.9% of the records, while indifferent and antagonistic interactions accounted for 15.1%. Penicillin, aminoglycoside, fluoro/quinolone, and glycopeptide antibiotic classes were the most frequently documented as interacting with HDPs, and Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Enterococcus faecium were the most reported bacterial species. Few studies formally evaluated the effects of combinations of HDPs and antibiotics on bacteria, and even fewer assessed such combinations against bacteria within biofilms, in animal models, or in advanced tissue infection models. Despite the biases of the current literature, the studies suggest that effective combinations of HDPs and antibiotics hold promise for the future treatment of infections caused by AMR bacteria.

LL37-Derived Fragments Improve the Antibacterial Potential of Penicillin G and Ampicillin against Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) infections are a severe threat to public health. Antimicrobial peptides (AMPs) are novel and potential antimicrobials with specific antibacterial mechanisms. Our aim was to study the potential of LL37, FK16, and FK13 to enhance the anti-MRSA activity of antibiotics in vitro, particularly penicillin G and ampicillin. Our results showed that FK16 and FK13 have more synergistic inhibitory effects to MRSA strains when combined with penicillin G and ampicillin. In addition, AMPs exhibited strong membrane permeabilizing properties, and membrane permeabilizing effects can provide a possible explanation for the improved antibacterial effects of antibiotics, since permeabilizing AMPs have the potential to increase the access of antibiotics. To further study the electrostatic interactions among cationic AMPs with negatively charged bacteria, we measured the zeta potentials of three MRSA strains and also neutralized three MRSA strains with the addition of cationic AMPs. Further, we demonstrated the connection between membrane permeabilization and zeta potential neutralization. Finally, we treated MRSA strains with AMPs and characterized the MICs of penicillin G and ampicillin. FK16 was the most promising AMP among the three AMPs, since exposure to FK16 decreased the MICs of both penicillin G and ampicillin for all MRSA strains and also demonstrated more synergistic combinations when combined with antibiotics. AMP exposure and subsequent membrane permeabilization provide a possible pathway to re-sensitize drug-resistant bacteria to traditional antibiotics. Re-sensitization may help preserve the effectiveness of traditional antibiotics, thus providing a potential new strategy for fighting MRSA infections.

Flow-Based Fmoc-SPPS Preparation and SAR Study of Cathelicidin-PY Reveals Selective Antimicrobial Activity

Antimicrobial peptides (AMPs) hold promise as novel therapeutics in the fight against multi-drug-resistant pathogens. Cathelicidin-PY (NH₂-RKCNFLCKLKEKLRTVITSHIDKVLRPQG-COOH) is a 29-residue disulfide-cyclised antimicrobial peptide secreted as an innate host defence mechanism by the frog Paa yunnanensis (PY) and reported to possess broad-spectrum antibacterial and antifungal properties, exhibiting low cytotoxic and low hemolytic activity. Herein, we detail the total synthesis of cathelicidin-PY using an entirely on-resin synthesis, including assembly of the linear sequence by rapid flow Fmoc-SPPS and iodine-mediated disulfide bridge formation. By optimising a synthetic strategy to prepare cathelicidin-PY, this strategy was subsequently adapted to prepare a bicyclic head-to-tail cyclised derivative of cathelicidin-PY. The structure-activity relationship (SAR) of cathelicidin-PY with respect to the N-terminally positioned disulfide was further probed by preparing an alanine-substituted linear analogue and a series of lactam-bridged peptidomimetics implementing side chain to side chain cyclisation. The analogues were investigated for antimicrobial activity, secondary structure by circular dichroism (CD), and stability in human serum. Surprisingly, the disulfide bridge emerged as non-essential to antimicrobial activity and secondary structure but was amenable to synthetic modification. Furthermore, the synthetic AMP and multiple analogues demonstrated selective activity towards Gram-negative pathogen E. coli in physiologically relevant concentrations of divalent cations.

Synergy by Perturbing the Gram-Negative Outer Membrane: Opening the Door for Gram-Positive Specific Antibiotics

New approaches to target antibacterial agents toward Gram-negative bacteria are key, given the rise of antibiotic resistance. Since the discovery of polymyxin B nonapeptide as a potent Gram-negative outer membrane (OM)-permeabilizing synergist in the early 1980s, a vast amount of literature on such synergists has been published. This Review addresses a range of peptide-based and small organic compounds that disrupt the OM to elicit a synergistic effect with antibiotics that are otherwise inactive toward Gram-negative bacteria, with synergy defined as a fractional inhibitory concentration index (FICI) of <0.5. Another requirement for the inclusion of the synergists here covered is their potentiation of a specific set of clinically used antibiotics: erythromycin, rifampicin, novobiocin, or vancomycin. In addition, we have focused on those synergists with reported activity against Gram-negative members of the ESKAPE family of pathogens namely, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and/or Acinetobacter baumannii. In cases where the FICI values were not directly reported in the primary literature but could be calculated from the published data, we have done so, allowing for more direct comparison of potency with other synergists. We also address the hemolytic activity of the various OM-disrupting synergists reported in the literature, an effect that is often downplayed but is of key importance in assessing the selectivity of such compounds for Gram-negative bacteria.

Synergistic Activity of Repurposed Peptide Drug Glatiramer Acetate with Tobramycin against Cystic Fibrosis Pseudomonas aeruginosa

Pseudomonas aeruginosa is the most common pathogen infecting the lungs of people with cystic fibrosis (CF), causing both acute and chronic infections. Intrinsic and acquired antibiotic resistance, coupled with the physical barriers resulting from desiccated CF sputum, allow P. aeruginosa to colonize and persist in spite of antibiotic treatment. As well as the specific difficulties in eradicating P. aeruginosa from CF lungs, P. aeruginosa is also subject to the wider, global issue of antimicrobial resistance. Glatiramer acetate (GA) is a peptide drug, used in the treatment of multiple sclerosis (MS), which has been shown to have moderate antipseudomonal activity. Other antimicrobial peptides (AMPs) have been shown to be antibiotic resistance breakers, potentiating the activities of antibiotics when given in combination, restoring and/or enhancing antibiotic efficacy. Growth, viability, MIC determinations, and synergy analysis showed that GA improved the efficacy of tobramycin (TOB) against reference strains of P. aeruginosa, reducing TOB MICs and synergizing with the aminoglycoside. This was also the case for clinical strains from people with CF. GA significantly reduced the MIC50 of TOB for viable cells from 1.69 mg/L (95% confidence interval [CI], 0.26 to 8.97) to 0.62 mg/L (95% CI, 0.15 to 3.94; P = 0.002) and the MIC90 for viable cells from 7.00 mg/L (95% CI, 1.18 to 26.50) to 2.20 mg/L (95% CI, 0.99 to 15.03; P = 0.001), compared to results with TOB only. Investigation of mechanisms of GA activity showed that GA resulted in significant disruption of outer membranes, depolarization of cytoplasmic membranes, and permeabilization of P. aeruginosa and was the only agent tested (including cationic AMPs) to significantly affect all three mechanisms. IMPORTANCE The antimicrobial resistance crisis urgently requires solutions to the lost efficacy of antibiotics. The repurposing of drugs already in clinical use, with strong safety profiles, as antibiotic adjuvants to restore the efficacy of antibiotics is an important avenue to alleviating the resistance crisis. This research shows that a clinically used drug from outside infection treatment, glatiramer acetate, reduces the concentration of tobramycin required to be effective in treating Pseudomonas aeruginosa, based on analyses of both reference and clinical respiratory isolates from people with cystic fibrosis. The two agents acted synergistically against P. aeruginosa, being more effective combined in vitro than predicted for their combination. As a peptide drug, glatiramer acetate functions similarly to many antimicrobial peptides, interacting with and disrupting the P. aeruginosa cell wall and permeabilizing bacterial cells, thereby allowing tobramycin to work. Our findings demonstrate that glatiramer acetate is a strong candidate for repurposing as an antibiotic resistance breaker of pathogenic P. aeruginosa.

Cationic polymer contributes to broaden the spectrum of vancomycin activity achieving eradication of Pseudomonas aeruginosa

Vancomycin (VAN) is unable to penetrate the outer membrane of Gram-negative bacteria and reach the target site. One approach to overcome this limitation is to associate it with compounds with permeabilizing or antimicrobial properties. Eudragit E100^® (Eu) is a cationic polymer insufficiently characterized for its potential antimicrobial action. Eu-VAN combinations were characterized, the antimicrobial efficacy against Pseudomonas aeruginosa was evaluated and previous studies on the effects of Eu on bacterial envelopes were extended. Time-kill assays showed eradication of P. aeruginosa within 3-6 h exposure to Eu-VAN, whilst VAN was ineffective. Eu showed regrowth in 24 h and delayed colony pigmentation. Although permeabilization of bacterial envelopes or morphological alterations observed by TEM and flow cytometry after exposure to Eu were insufficient to cause bacterial death, they allowed access of VAN to the target site, since Eu-VAN/Van-FL-treated cultures showed fluorescent staining in all bacterial cells, indicating Van-FL internalization. Consequently, Eu potentiated the activity of an otherwise inactive antibiotic against P. aeruginosa. Moreover, Eu-VAN combinations exhibited improved physicochemical properties and could be used in the development of therapeutic alternatives in the treatment of bacterial keratitis.

Host Defense Peptides at the Ocular Surface: Roles in Health and Major Diseases, and Therapeutic Potentials

Sight is arguably the most important sense in human. Being constantly exposed to the environmental stress, irritants and pathogens, the ocular surface – a specialized functional and anatomical unit composed of tear film, conjunctival and corneal epithelium, lacrimal glands, meibomian glands, and nasolacrimal drainage apparatus – serves as a crucial front-line defense of the eye. Host defense peptides (HDPs), also known as antimicrobial peptides, are evolutionarily conserved molecular components of innate immunity that are found in all classes of life. Since the first discovery of lysozyme in 1922, a wide range of HDPs have been identified at the ocular surface. In addition to their antimicrobial activity, HDPs are increasingly recognized for their wide array of biological functions, including anti-biofilm, immunomodulation, wound healing, and anti-cancer properties. In this review, we provide an updated review on: (1) spectrum and expression of HDPs at the ocular surface; (2) participation of HDPs in ocular surface diseases/conditions such as infectious keratitis, conjunctivitis, dry eye disease, keratoconus, allergic eye disease, rosacea keratitis, and post-ocular surgery; (3) HDPs that are currently in the development pipeline for treatment of ocular diseases and infections; and (4) future potential of HDP-based clinical pharmacotherapy for ocular diseases.

Atomic-Resolution Structures and Mode of Action of Clinically Relevant Antimicrobial Peptides

Global rise of infections and deaths caused by drug-resistant bacterial pathogens are among the unmet medical needs. In an age of drying pipeline of novel antibiotics to treat bacterial infections, antimicrobial peptides (AMPs) are proven to be valid therapeutics modalities. Direct in vivo applications of many AMPs could be challenging; however, works are demonstrating encouraging results for some of them. In this review article, we discussed 3-D structures of potent AMPs e.g., polymyxin, thanatin, MSI, protegrin, OMPTA in complex with bacterial targets and their mode of actions. Studies on human peptide LL37 and de novo-designed peptides are also discussed. We have focused on AMPs which are effective against drug-resistant Gram-negative bacteria. Since treatment options for the infections caused by super bugs of Gram-negative bacteria are now extremely limited. We also summarize some of the pertinent challenges in the field of clinical trials of AMPs.

Host Defence Peptides: A Potent Alternative to Combat Antimicrobial Resistance in the Era of the COVID-19 Pandemic

One of the greatest challenges facing the medical community today is the ever-increasing trajectory of antimicrobial resistance (AMR), which is being compounded by the decrease in our antimicrobial armamentarium. From their initial discovery to the current day, antibiotics have seen an exponential increase in their usage, from medical to agricultural use. Benefits aside, this has led to an exponential increase in AMR, with the fear that over 10 million lives are predicted to be lost by 2050, according to the World Health Organisation (WHO). As such, medical researchers are turning their focus to discovering novel alternatives to antimicrobials, one being Host Defence Peptides (HDPs). These small cationic peptides have shown great efficacy in being used as an antimicrobial therapy for currently resistant microbial variants. With the sudden emergence of the SARS-CoV-2 variant and the subsequent global pandemic, the great versatility and potential use of HDPs as an alternative to conventional antibiotics in treating as well as preventing the spread of COVID-19 has been reviewed. Thus, to allow the reader to have a full understanding of the multifaceted therapeutic use of HDPs, this literature review shall cover the association between COVID-19 and AMR whilst discussing and evaluating the use of HDPs as an answer to antimicrobial resistance (AMR).

Mammals' humoral immune proteins and peptides targeting the bacterial envelope: from natural protection to therapeutic applications against multidrug‐resistant Gram ‐negatives

Mammalian innate immunity employs several humoral ‘weapons’ that target the bacterial envelope. The threats posed by the multidrug‐resistant ‘ESKAPE’ Gram‐negative pathogens (Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) are forcing researchers to explore new therapeutic options, including the use of these immune elements. Here we review bacterial envelope‐targeting (peptidoglycan and/or membrane‐targeting) proteins/peptides of the mammalian immune system that are most likely to have therapeutic applications. Firstly we discuss their general features and protective activity against ESKAPE Gram‐negatives in the host. We then gather, integrate, and discuss recent research on experimental therapeutics harnessing their bactericidal power, based on their exogenous administration and also on the discovery of bacterial and/or host targets that improve the performance of this endogenous immunity, as a novel therapeutic concept. We identify weak points and knowledge gaps in current research in this field and suggest areas for future work to obtain successful envelope‐targeting therapeutic options to tackle the challenge of antimicrobial resistance.

New potentiators of ineffective antibiotics: Targeting the Gram-negative outer membrane to overcome intrinsic resistance

Because of the rise in antibiotic resistance and the dwindling pipeline of effective antibiotics, it is imperative to explore avenues that breathe new life into existing drugs. This is particularly important for intrinsically resistant Gram-negative bacteria, which are exceedingly difficult to treat. The Gram-negative outer membrane (OM) prevents the entry of a plethora of antibiotics that are effective against Gram-positive bacteria, despite the presence of the targets of these drugs. Uncovering molecules that increase the permeability of the OM to sensitize Gram-negative bacteria to otherwise ineffective antibiotics is an approach that has recently garnered increased attention in the field. In this review, we survey chemical matter which has been shown to potentiate antibiotics against Gram-negative bacteria by perturbing the OM. These include peptides, nanoparticles, macromolecules, antibiotic conjugates, and small molecules.

Gelatin Methacryloyl Hydrogels for the Localized Delivery of Cefazolin

The tuneability of hydrogels renders them promising candidates for local drug delivery to prevent and treat local surgical site infection (SSI) while avoiding the systemic side-effects of intravenous antibiotic injections. Here, we present a newly developed gelatin methacryloyl (GelMA)-based hydrogel drug delivery system (GelMA-DDS) to locally deliver the broad-spectrum antibiotic cefazolin for SSI prophylaxis and treatment. Antibiotic doses from 3 µg to 90 µg were loaded in photocrosslinked GelMA hydrogel discs with 5 to 15% w/v polymer concentration and drug encapsulation efficiencies, mechanical properties, crosslinking and release kinetics, as well as bacterial growth inhibition were assessed. Our results demonstrate that all GelMA groups supported excellent drug encapsulation efficiencies of up to 99%. Mechanical properties of the GelMA-DDS were highly tuneable and unaffected by the loading of small to medium doses of cefazolin. The diffusive and the proteolytic in vitro drug delivery of all investigated cefazolin doses was characterized by a burst release, and the delivered cefazolin amount was directly proportional to the encapsulated dose. Accelerated enzymatic degradation of the GelMA-DDS followed zero-order kinetics and was dependent on both the cefazolin dose and GelMA concentration (3-13 h). Finally, we demonstrate that cefazolin delivered from GelMA induced a dose-dependent antibacterial efficacy against S. aureus, in both a broth and a diffusive assay. The cefazolin-loaded GelMA-DDS presented here provides a highly tuneable and easy-to-use local delivery system for the prophylaxis and treatment of SSI.

Targeted Delivery of Miconazole Employing LL37 Fragment Mutant Peptide CKR12-Poly (Lactic-Co-Glycolic) Acid Polymeric Micelles

We previously reported that conjugates of antimicrobial peptide fragment analogues and poly (lactic-co-glycolic) acid (PLGA) enhance antimicrobial activity and that the conjugated micelle structure is an effective tool for antimicrobial drug delivery. In recent years, the delivery of antimicrobial peptides to targets for antimicrobial activity has attracted attention. In this study, we targeted Candida albicans, a causative organism of catheter-related bloodstream infections, which is refractory to antimicrobial agents and is currently a problem in medical practice. We evaluated the antifungal activity of CKR12 (a mutant fragment of the human cathelicidin peptide, LL-37)-PLGA-miconazole (MCZ) micelles using nanotechnology with MCZ delivery. The prepared CKR12-PLGA-MCZ micelles were characterised by measuring dynamic light scattering, zeta potential, dilution stability, and drug release. CKR12-PLGA-MCZ micelles showed higher antifungal activity than CKR12-PLGA micelles and MCZ solution. Furthermore, scanning and transmission electron microscopy suggested that CKR12-PLGA-MCZ micelles disrupted both cell wall and cell membrane of C. albicans. Our results revealed a synergistic effect of antifungal activity using a combination of antimicrobial peptide fragment analogues and MCZ, and that MCZ is a promising tool for the delivery to target microorganisms.

Evaluation of Host Defense Peptide (CaD23)-Antibiotic Interaction and Mechanism of Action: Insights From Experimental and Molecular Dynamics Simulations Studies

Background/Aim: Host defense peptides (HDPs) have the potential to provide a novel solution to antimicrobial resistance (AMR) in view of their unique and broad-spectrum antimicrobial activities. We had recently developed a novel hybrid HDP based on LL-37 and human beta-defensin-2, named CaD23, which was shown to exhibit good in vivo antimicrobial efficacy against Staphylococcus aureus in a bacterial keratitis murine model. This study aimed to examine the potential CaD23-antibiotic synergism and the secondary structure and underlying mechanism of action of CaD23. Methods: Peptide-antibiotic interaction was evaluated against S. aureus, methicillin-resistant S. aureus (MRSA), and Pseudomonas aeruginosa using established checkerboard and time-kill assays. Fractional inhibitory concentration index (FICI) was calculated and interpreted as synergistic (FIC<0.5), additive (FIC between 0.5-1.0), indifferent (FIC between >1.0 and ≤4), or antagonistic (FIC>4). SYTOX green uptake assay was performed to determine the membrane-permeabilising action of CaD23. Molecular dynamics (MD) simulations were performed to evaluate the interaction of CaD23 with bacterial and mammalian mimetic membranes. Circular dichroism (CD) spectroscopy was also performed to examine the secondary structures of CaD23. Results: CaD23-amikacin and CaD23-levofloxacin combination treatment exhibited a strong additive effect against S. aureus SH1000 (FICI = 0.60-0.69) and MRSA43300 (FICI = 0.56-0.60) but an indifferent effect against P. aeruginosa (FIC = 1.03-1.15). CaD23 (at 25 μg/ml; 2xMIC) completely killed S. aureus within 30 min. When used at sub-MIC concentration (3.1 μg/ml; 0.25xMIC), it was able to expedite the antimicrobial action of amikacin against S. aureus by 50%. The rapid antimicrobial action of CaD23 was attributed to the underlying membrane-permeabilising mechanism of action, evidenced by the SYTOX green uptake assay and MD simulations studies. MD simulations revealed that cationicity, alpha-helicity, amphiphilicity and hydrophobicity (related to the Trp residue at C-terminal) play important roles in the antimicrobial action of CaD23. The secondary structures of CaD23 observed in MD simulations were validated by CD spectroscopy. Conclusion: CaD23 is a novel alpha-helical, membrane-active synthetic HDP that can enhance and expedite the antimicrobial action of antibiotics against Gram-positive bacteria when used in combination. MD simulations serves as a powerful tool in revealing the peptide secondary structure, dissecting the mechanism of action, and guiding the design and optimisation of HDPs.

Interplay between membrane active host defense peptides and heme modulates their assemblies and in vitro activity

In the emerging era of antimicrobial resistance, the susceptibility to co-infections of patients suffering from either acquired or inherited hemolytic disorders can lead to dramatic increase in mortality rates. Closely related, heme liberated during hemolysis is one of the major sources of iron, which is vital for both host and invading microorganisms. While recent intensive research in the field has demonstrated that heme exerts diverse local effects including impairment of immune cells functions, it is almost completely unknown how it may compromise key molecules of our innate immune system, such as antimicrobial host defense peptides (HDPs). Since HDPs hold great promise as natural therapeutic agents against antibiotic-resistant microbes, understanding the effects that may modulate their action in microbial infection is crucial. Here we explore how hemin can interact directly with selected HDPs and influence their structure and membrane activity. It is revealed that induced helical folding, large assembly formation, and altered membrane activity is promoted by hemin. However, these effects showed variations depending mainly on peptide selectivity toward charged lipids, and the affinity of the peptide and hemin to lipid bilayers. Hemin-peptide complexes are sought to form semi-folded co-assemblies, which are present even with model membranes resembling mammalian or bacterial lipid compositions. In vitro cell-based toxicity assays supported that toxic effects of HDPs could be attenuated due to their assembly formation. These results are in line with our previous findings on peptide-lipid-small molecule systems suggesting that small molecules present in the complex in vivo milieu can regulate HDP function. Inversely, diverse effects of endogenous compounds could also be manipulated by HDPs.

Hybrid derivative of cathelicidin and human beta defensin-2 against Gram-positive bacteria: A novel approach for the treatment of bacterial keratitis

Bacterial keratitis (BK) is a major cause of corneal blindness globally. This study aimed to develop a novel class of antimicrobial therapy, based on human-derived hybrid host defense peptides (HyHDPs), for treating BK. HyHDPs were rationally designed through combination of functional amino acids in parent HDPs, including LL-37 and human beta-defensin (HBD)-1 to -3. Minimal inhibitory concentrations (MICs) and time-kill kinetics assay were performed to determine the concentration- and time-dependent antimicrobial activity and cytotoxicity was evaluated against human corneal epithelial cells and erythrocytes. In vivo safety and efficacy of the most promising peptide was examined in the corneal wound healing and Staphylococcus aureus (ATCC SA29213) keratitis murine models, respectively. A second-generation HyHDP (CaD23), based on rational hybridization of the middle residues of LL-37 and C-terminal of HBD-2, was developed and was shown to demonstrate good efficacy against methicillin-sensitive and methicillin-resistant S. aureus [MIC = 12.5-25.0 μg/ml (5.2-10.4 μM)] and S. epidermidis [MIC = 12.5 μg/ml (5.2 μM)], and moderate efficacy against P. aeruginosa [MIC = 25-50 μg/ml (10.4-20.8 μM)]. CaD23 (at 25 μg/ml or 2× MIC) killed all the bacteria within 30 min, which was 8 times faster than amikacin (25 μg/ml or 20× MIC). After 10 consecutive passages, S. aureus (ATCC SA29213) did not develop any antimicrobial resistance (AMR) against CaD23 whereas it developed significant AMR (i.e. a 32-fold increase in MIC) against amikacin, a commonly used treatment for BK. Pre-clinical murine studies showed that CaD23 (0.5 mg/ml) achieved a median reduction of S. aureus bioburden by 94% (or 1.2 log10 CFU/ml) while not impeding corneal epithelial wound healing. In conclusion, rational hybridization of human-derived HDPs has led to generation of a potentially efficacious and safe topical antimicrobial agent for treating Gram-positive BK, with no/minimal risk of developing AMR.

Alternative Therapeutic Interventions: Antimicrobial Peptides and Small Molecules to Treat Microbial Keratitis

Microbial keratitis is a leading cause of blindness worldwide and results in unilateral vision loss in an estimated 2 million people per year. Bacteria and fungus are two main etiological agents that cause corneal ulcers. Although antibiotics and antifungals are commonly used to treat corneal infections, a clear trend with increasing resistance to these antimicrobials is emerging at rapid pace. Extensive research has been carried out to determine alternative therapeutic interventions, and antimicrobial peptides (AMPs) are increasingly recognized for their clinical potential in treating infections. Small molecules targeted against virulence factors of the pathogens and natural compounds are also explored to meet the challenges and growing demand for therapeutic agents. Here we review the potential of AMPs, small molecules, and natural compounds as alternative therapeutic interventions for the treatment of corneal infections to combat antimicrobial resistance. Additionally, we have also discussed about the different formats of drug delivery systems for optimal administration of drugs to treat microbial keratitis.

Membrane Association Modes of Natural Anticancer Peptides: Mechanistic Details on Helicity, Orientation, and Surface Coverage

Anticancer peptides (ACPs) could potentially offer many advantages over other cancer therapies. ACPs often target cell membranes, where their surface mechanism is coupled to a conformational change into helical structures. However, details on their binding are still unclear, which would be crucial to reach progress in connecting structural aspects to ACP action and to therapeutic developments. Here we investigated natural helical ACPs, Lasioglossin LL-III, Macropin 1, Temporin-La, FK-16, and LL-37, on model liposomes, and also on extracellular vesicles (EVs), with an outer leaflet composition similar to cancer cells. The combined simulations and experiments identified three distinct binding modes to the membranes. Firstly, a highly helical structure, lying mainly on the membrane surface; secondly, a similar, yet only partially helical structure with disordered regions; and thirdly, a helical monomeric form with a non-inserted perpendicular orientation relative to the membrane surface. The latter allows large swings of the helix while the N-terminal is anchored to the headgroup region. These results indicate that subtle differences in sequence and charge can result in altered binding modes. The first two modes could be part of the well-known carpet model mechanism, whereas the newly identified third mode could be an intermediate state, existing prior to membrane insertion.

Infectious keratitis: an update on epidemiology, causative microorganisms, risk factors, and antimicrobial resistance

Corneal opacity is the 5th leading cause of blindness and visual impairment globally, affecting ~6 million of the world population. In addition, it is responsible for 1.5-2.0 million new cases of monocular blindness per year, highlighting an ongoing uncurbed burden on human health. Among all aetiologies such as infection, trauma, inflammation, degeneration and nutritional deficiency, infectious keratitis (IK) represents the leading cause of corneal blindness in both developed and developing countries, with an estimated incidence ranging from 2.5 to 799 per 100,000 population-year. IK can be caused by a wide range of microorganisms, including bacteria, fungi, virus, parasites and polymicrobial infection. Subject to the geographical and temporal variations, bacteria and fungi have been shown to be the most common causative microorganisms for corneal infection. Although viral and Acanthamoeba keratitis are less common, they represent important causes for corneal blindness in the developed countries. Contact lens wear, trauma, ocular surface diseases, lid diseases, and post-ocular surgery have been shown to be the major risk factors for IK. Broad-spectrum topical antimicrobial treatment is the current mainstay of treatment for IK, though its effectiveness is being challenged by the emergence of antimicrobial resistance, including multidrug resistance, in some parts of the world. In this review, we aim to provide an updated review on IK, encompassing the epidemiology, causative microorganisms, major risk factors and the impact of antimicrobial resistance.

Antimicrobial resistance of ocular microbes and the role of antimicrobial peptides

Isolation of antimicrobial-resistant microbes from ocular infections may be becoming more frequent. Infections caused by these microbes can be difficult to treat and lead to poor outcomes. However, new therapies are being developed which may help improve clinical outcomes. This review examines recent reports on the isolation of antibiotic-resistant microbes from ocular infections. In addition, an overview of the development of some new antibiotic therapies is given. The recent literature regarding antibiotic use and resistance, isolation of antibiotic-resistant microbes from ocular infections and the development of potential new antibiotics that can be used to treat these infections was reviewed. Ocular microbial infections are a global public health issue as they can result in vision loss which compromises quality of life. Approximately 70 per cent of ocular infections are caused by bacteria including Chlamydia trachomatis, Staphylococcus aureus, and Pseudomonas aeruginosa and fungi such as Candida albicans, Aspergillus spp. and Fusarium spp. Resistance to first-line antibiotics such as fluoroquinolones and azoles has increased, with resistance of S. aureus isolates from the USA to fluoroquinolones reaching 32 per cent of isolates and 35 per cent being methicillin-resistant (MRSA). Lower levels of MRSA (seven per cent) were isolated by an Australian study. Antimicrobial peptides, which are broad-spectrum alternatives to antibiotics, have been tested as possible new drugs. Several have shown promise in animal models of keratitis, especially treating P. aeruginosa, S. aureus or C. albicans infections. Reports of increasing resistance of ocular isolates to mainstay antibiotics are a concern, and there is evidence that for ocular surface disease this resistance translates into worse clinical outcomes. New antibiotics are being developed, but not by large pharmaceutical companies and mostly in university research laboratories and smaller biotech companies. Antimicrobial peptides show promise in treating keratitis.

Systemic diseases and the cornea

There is a number of systemic diseases affecting the cornea. These include endocrine disorders (diabetes, Graves' disease, Addison's disease, hyperparathyroidism), infections with viruses (SARS-CoV-2, herpes simplex, varicella zoster, HTLV-1, Epstein-Barr virus) and bacteria (tuberculosis, syphilis and Pseudomonas aeruginosa), autoimmune and inflammatory diseases (rheumatoid arthritis, Sjögren's syndrome, lupus erythematosus, gout, atopic and vernal keratoconjunctivitis, multiple sclerosis, granulomatosis with polyangiitis, sarcoidosis, Cogan's syndrome, immunobullous diseases), corneal deposit disorders (Wilson's disease, cystinosis, Fabry disease, Meretoja's syndrome, mucopolysaccharidosis, hyperlipoproteinemia), and genetic disorders (aniridia, Ehlers-Danlos syndromes, Marfan syndrome). Corneal manifestations often provide an insight to underlying systemic diseases and can act as the first indicator of an undiagnosed systemic condition. Routine eye exams can bring attention to potentially life-threatening illnesses. In this review, we provide a fairly detailed overview of the pathologic changes in the cornea described in various systemic diseases and also discuss underlying molecular mechanisms, as well as current and emerging treatments.

Nonclassical antagonism between human lysozyme and AMPs against Pseudomonas aeruginosa

Combinations of human lysozyme (hLYS) and antimicrobial peptides (AMPs) are known to exhibit either additive or synergistic activity, and as a result, they have therapeutic potential for persistent and antibiotic‐resistant infections. We examined hLYS activity against Pseudomonas aeruginosa when combined with six different AMPs. In contrast to prior reports, we discovered that some therapeutically relevant AMPs manifest striking antagonistic interactions with hLYS across particular concentration ranges. We further found that the synthetic AMP Tet009 can inhibit hLYS‐mediated bacterial lysis. To the best of our knowledge, these results represent the first observations of antagonism between hLYS and AMPs, and they advise that future development of lytic enzyme and AMP combination therapies considers the potential for antagonistic interactions.

Outer-membrane-acting peptides and lipid II-targeting antibiotics cooperatively kill Gram-negative pathogens

The development and dissemination of antibiotic-resistant bacterial pathogens is a growing global threat to public health. Novel compounds and/or therapeutic strategies are required to face the challenge posed, in particular, by Gram-negative bacteria. Here we assess the combined effect of potent cell-wall synthesis inhibitors with either natural or synthetic peptides that can act on the outer-membrane. Thus, several linear peptides, either alone or combined with vancomycin or nisin, were tested against selected Gram-negative pathogens, and the best one was improved by further engineering. Finally, peptide D-11 and vancomycin displayed a potent antimicrobial activity at low μM concentrations against a panel of relevant Gram-negative pathogens. This combination was highly active in biological fluids like blood, but was non-hemolytic and non-toxic against cell lines. We conclude that vancomycin and D-11 are safe at >50-fold their MICs. Based on the results obtained, and as a proof of concept for the newly observed synergy, a Pseudomonas aeruginosa mouse infection model experiment was also performed, showing a 4 log₁₀ reduction of the pathogen after treatment with the combination. This approach offers a potent alternative strategy to fight (drug-resistant) Gram-negative pathogens in humans and mammals.

Antibiofilm activity of lactoferrin-derived synthetic peptides against Pseudomonas aeruginosa PAO1

Many pathogenic bacteria can protect themselves from the effects of antibiotics and the host immune response system by forming biofilms. Biofilms are polymer-entrapped bacterial cells, which adhere to each other and are often attached to a surface. Eradication of bacterial biofilms typically requires much higher concentrations of antibiotics than are normally needed to kill cultured planktonic cells, raising serious clinical concerns. In an attempt to prevent the formation of biofilms or to break up existing biofilms of pathogenic bacteria, herein we have used the standard crystal violet assay as well as the Calgary biofilm device to test several lactoferrin- and lactoferricin-derived antimicrobial peptides for their antibiofilm activity against Pseudomonas aeruginosa PAO1. Our results revealed that the short bovine lactoferricin-derived RRWQWR-NH₂ (20-25) hexapeptide has no activity against P. aeruginosa PAO1. Moreover, the longer human lactoferricin-derived peptide GRRRRSVQWCA (1-11) and the bovine lactoferrampin (268-284) peptide were also almost devoid of activity. However, several different "mix-and-match" dimeric versions of the two lactoferricin-derived peptides proved quite effective in preventing the formation of biofilms at low concentrations, and in some cases, could even eradicate an existing biofilm. Moreover, the full-length bovine lactoferricinB (17-41) peptide also displayed considerable antimicrobial activity. Some of the longer lactoferricin-derived dimeric peptides acted through a bactericidal mechanism, whereas others seemed to interfere in cell-signalling processes. Taken together, our results indicate that synthetic dimeric peptides comprising short naturally occurring human and bovine lactoferricin constructs could be further developed as antibiofilm agents.

Shorter Antibacterial Peptide Having High Selectivity for E. coli Membranes and Low Potential for Inducing Resistance

Antimicrobial peptides (AMPs) have been recognised as a significant therapeutic option for mitigating resistant microbial infections. It has been found recently that Plasmodium falciparum-derived, 20 residue long, peptide 35409 had antibacterial and haemolytic activity, making it an AMP having reduced selectivity, and suggesting that it should be studied more extensively for obtaining new AMPs having activity solely targeting the bacterial membrane. Peptide 35409 was thus used as template for producing short synthetic peptides (<20 residues long) and evaluating their biological activity and relevant physicochemical characteristics for therapeutic use. Four of the sixteen short peptides evaluated here had activity against E. coli without any associated haemolytic effects. The 35409-1 derivative (17 residues long) had the best therapeutic characteristics as it had high selectivity for bacterial cells, stability in the presence of human sera, activity against E. coli multiresistant clinical isolates and was shorter than the original sequence. It had a powerful membranolytic effect and low potential for inducing resistance in bacteria. This peptide’s characteristics highlighted its potential as an alternative for combating infection caused by E. coli multiresistant bacteria and/or for designing new AMPs.

Strategies in Translating the Therapeutic Potentials of Host Defense Peptides

The golden era of antibiotics, heralded by the discovery of penicillin, has long been challenged by the emergence of antimicrobial resistance (AMR). Host defense peptides (HDPs), previously known as antimicrobial peptides, are emerging as a group of promising antimicrobial candidates for combatting AMR due to their rapid and unique antimicrobial action. Decades of research have advanced our understanding of the relationship between the physicochemical properties of HDPs and their underlying antimicrobial and non-antimicrobial functions, including immunomodulatory, anti-biofilm, and wound healing properties. However, the mission of translating novel HDP-derived molecules from bench to bedside has yet to be fully accomplished, primarily attributed to their intricate structure-activity relationship, toxicity, instability in host and microbial environment, lack of correlation between in vitro and in vivo efficacies, and dwindling interest from large pharmaceutical companies. Based on our previous experience and the expanding knowledge gleaned from the literature, this review aims to summarize the novel strategies that have been employed to enhance the antimicrobial efficacy, proteolytic stability, and cell selectivity, which are all crucial factors for bench-to-bedside translation of HDP-based treatment. Strategies such as residues substitution with natural and/or unnatural amino acids, hybridization, L-to-D heterochiral isomerization, C- and N-terminal modification, cyclization, incorporation with nanoparticles, and "smart design" using artificial intelligence technology, will be discussed. We also provide an overview of HDP-based treatment that are currently in the development pipeline.

Effects of human β-defensin 3 fused with carbohydrate-binding domain on the function of type III secretion system in Pseudomonas aeruginosa PA14

Antimicrobial peptides are considered to be one of the candidate antimicrobial agents for antibiotic-resistant bacterial infection in the future. The effects of antimicrobial peptide hBD3-CBD on Pseudomonas aeruginosa PA14 and PA14 ΔexsA were analyzed by the bactericidal effects, hemolysis assays, pyocyanin pigment productions, and virulence factor expressions (exoU, exoS, hcnA, and lasB). Pyocyanin production and virulence factor expressions are important features of the type III secretion system in Pseudomonas aeruginosa. HBD3-CBD killed PA14 and PA14 ΔexsA with similar efficiency; it lowered the hemolysis levels of PA14 and PA14 ΔexsA and reduced the pyocyanin production, biofilm formation, and exoU, exoS, and lasB expressions in PA14. Compared with PA14, PA14 ΔexsA showed a lower hemolysis effect, pyocyanin production, exoU, and lasB expressions. The effects of hBD3-CBD on the PA14 toxin secretion were similar to the changes in the type III secretion system mutant isolate PA14 ΔexsA. Our results demonstrated that the type III secretion system was involved in the biological functions on PA 14 from hBD3-CBD.