The Mechanisms of Action of Cationic Antimicrobial Peptides Refined by Novel Concepts from Biophysical Investigations

Peptide PaDBS1R6 has potent antibacterial activity on clinical bacterial isolates and integrates an immunomodulatory peptide fragment within its sequence

BACKGROUND

Resistant infectious diseases caused by gram-negative bacteria are among the most serious worldwide health problems. Antimicrobial peptides (AMPs) have been explored as promising antibacterial, antibiofilm, and anti-infective candidates to address these health challenges.

MAJOR CONCLUSIONS

Here we report the potent antibacterial effect of the peptide PaDBS1R6 on clinical bacterial isolates and identify an immunomodulatory peptide fragment incorporated within it. PaDBS1R6 was evaluated against Acinetobacter baumannii and Escherichia coli clinical isolates and had minimal inhibitory concentration (MIC) values from 8 to 32 μmol L-1. It had a rapid bactericidal effect, with eradication showing within 3 min of incubation, depending on the bacterial strain tested. In addition, PaDBS1R6 inhibited biofilm formation for A. baumannii and E. coli and was non-toxic toward healthy mammalian cells. These findings are explained by the preference of PaDBS1R6 for anionic membranes over neutral membranes, as assessed by surface plasmon resonance assays and molecular dynamics simulations. Considering its potent antibacterial activity, PaDBS1R6 was used as a template for sliding-window fr agmentation studies (window size = 10 residues). Among the sliding-window fragments, PaDBS1R6F8, PaDBS1R6F9, and PaDBS1R6F10 were ineffective against any of the bacterial strains tested. Additional biological assays were conducted, including nitric oxide (NO) modulation and wound scratch assays, and the R6F8 peptide fragment was found to be active in modulating NO levels, as well as having strong wound healing properties.

GENERAL SIGNIFICANCE

This study proposes a new concept whereby peptides with different biological properties can be derived by the screening of fragments from within potent AMPs.

A Critical Review of Short Antimicrobial Peptides from Scorpion Venoms, Their Physicochemical Attributes, and Potential for the Development of New Drugs

Scorpion venoms have proven to be excellent sources of antimicrobial agents. However, although many of them have been functionally characterized, they remain underutilized as pharmacological agents, despite their evident therapeutic potential. In this review, we discuss the physicochemical properties of short scorpion venom antimicrobial peptides (ssAMPs). Being generally short (13-25 aa) and amidated, their proven antimicrobial activity is generally explained by parameters such as their net charge, the hydrophobic moment, or the degree of helicity. However, for a complete understanding of their biological activities, also considering the properties of the target membranes is of great relevance. Here, with an extensive analysis of the physicochemical, structural, and thermodynamic parameters associated with these biomolecules, we propose a theoretical framework for the rational design of new antimicrobial drugs. Through a comparison of these physicochemical properties with the bioactivity of ssAMPs in pathogenic bacteria such as Staphylococcus aureus or Acinetobacter baumannii, it is evident that in addition to the net charge, the hydrophobic moment, electrostatic energy, or intrinsic flexibility are determining parameters to understand their performance. Although the correlation between these parameters is very complex, the consensus of our analysis suggests that there is a delicate balance between them and that modifying one affects the rest. Understanding the contribution of lipid composition to their bioactivities is also underestimated, which suggests that for each peptide, there is a physiological context to consider for the rational design of new drugs.

Insect meal in aquafeeds: A sustainable path to enhanced mucosal immunity in fish

The mucosal surfaces of fish, including their intestines, gills, and skin, are constantly exposed to various environmental threats, such as water quality fluctuations, pollutants, and pathogens. However, various cells and microbiota closely associated with these surfaces work in tandem to create a functional protective barrier against these conditions. Recent research has shown that incorporating specific feed ingredients into fish diets can significantly boost their mucosal and general immune response. Among the various ingredients being investigated, insect meal has emerged as one of the most promising options, owing to its high protein content and immunomodulatory properties. By positively influencing the structure and function of mucosal surfaces, insect meal (IM) has the potential to enhance the overall immune status of fish. This review provides a comprehensive overview of the potential benefits of incorporating IM into aquafeed as a feed ingredient for augmenting the mucosal immune response of fish.

Self-assembly of spin-labeled antimicrobial peptides magainin 2 and PGLa in lipid bilayers

The cationic antimicrobial peptides PGLa and magainin 2 (Mag2) are known for their antimicrobial activity and synergistic enhancement in antimicrobial and membrane leakage assays. Further use of peptides in combinatory therapy requires knowledge of the mechanisms of action of both individual peptides and their mixtures. Here, electron paramagnetic resonance (EPR), double electron-electron resonance (DEER, also known as PELDOR) and electron spin echo envelope modulation (ESEEM) spectroscopies were applied to study self-assembly and localization of spin-labeled PGLa and Mag2 in POPE/POPG membranes with a wide range of peptide/lipid ratios (P/L) from ∼1/1500 to 1/50. EPR and DEER data showed that both peptides tend to organize in clusters, which occurs already at the lowest peptide/lipid molar ratio of 1/1500 (0.067 mol%). For individual peptides, these clusters are quite dense with intermolecular distances of the order of ∼2 nm. In the presence of a synergistic peptide partner, these homo-clusters are transformed into lipid-diluted hetero-clusters. These clusters are characterized by a local surface density that is several times higher than expected from a random distribution. ESEEM data indicate a slightly different insertion depth of peptides in hetero-clusters when compared to homo-clusters.

Characterization of the antibacterial and opsonic functions of the antimicrobial peptide LvCrustinVI from Litopenaeus vannamei

Microbial drug resistance is becoming increasingly severe due to antibiotic abuse. The development and utilization of antimicrobial peptides is one of the important ways to solve this difficult problem. Crustins are a family of antimicrobial peptides that play important roles in the innate immune system of crustaceans. Several types of crustins exist in shrimp and their activities vary greatly. In the present study, we studied the immune function of one newly identified crustin and found that the type VI crustin encoding gene in Litopenaeus vannamei (LvCrustinVI) was mainly expressed in gills. Its expression was significantly up-regulated after Vibrio parahaemolyticus infection and knockdown of the gene promoted Vibrio proliferation in the hepatopancreas of shrimp, indicating that LvCrustinVI was involved in pathogens infection. The recombinant LvCrustinVI (rLvCrustinVI) showed strong inhibitory activities against both Gram-negative and Gram-positive bacteria, and exhibited binding activities with the bacteria and bacterial polysaccharides including Glu, LPS and PGN. In the presence of Ca2+, rLvCrustinVI showed a strong agglutination effect on V. parahaemolyticus and could significantly enhance the phagocytic ability of shrimp hemocytes against V. parahaemolyticus. In conclusion, LvCrustinVI played important roles as antimicrobial peptide and opsonin in the innate immune defense of L. vannamei. The study enriched our understanding of the functional activity of Crustin and provides an important basis for the development and utilization of antimicrobial peptides.

The intricate link between membrane lipid structure and composition and membrane structural properties in bacterial membranes

It is now evident that the cell manipulates lipid composition to regulate different processes such as membrane protein insertion, assembly and function. Moreover, changes in membrane structure and properties, lipid homeostasis during growth and differentiation with associated changes in cell size and shape, and responses to external stress have been related to drug resistance across mammalian species and a range of microorganisms. While it is well known that the biomembrane is a fluid self-assembled nanostructure, the link between the lipid components and the structural properties of the lipid bilayer are not well understood. This perspective aims to address this topic with a view to a more detailed understanding of the factors that regulate bilayer structure and flexibility. We describe a selection of recent studies that address the dynamic nature of bacterial lipid diversity and membrane properties in response to stress conditions. This emerging area has important implications for a broad range of cellular processes and may open new avenues of drug design for selective cell targeting.

Anti-Microbial Activities of Mussel-Derived Recombinant Proteins against Gram-Negative Bacteria

Many anti-microbial peptides (AMPs) and pro-apoptotic peptides are considered as novel anti-microbial agents, distinguished by their different characteristics. Nevertheless, AMPs exhibit certain limitations, including poor stability and potential toxicity, which hinder their suitability for applications in pharmaceutics and medical devices. In this study, we used recombinant mussel adhesive protein (MAP) as a robust scaffold to overcome these limitations associated with AMPs. Mussel adhesive protein fused with functional peptides (MAP-FPs) was used to evaluate anti-microbial activities, minimal inhibitory concentration (MIC), and time-kill kinetics (TKK) assays against six of bacteria strains. MAP and MAP-FPs were proved to have an anti-microbial effect with MIC of 4 or 8 µM against only Gram-negative bacteria strains. All tested MAP-FPs killed four different Gram-negative bacteria strains within 180 min. Especially, MAP-FP-2 and -5 killed three Gram-negative bacteria strain, including E. coli, S. typhimurium, and K. pneumoniae, within 10 min. A cytotoxicity study using Vero and HEK293T cells indicated the safety of MAP and MAP-FP-2 and -3. Thermal stability of MAP-FP-2 was also validated by HPLC analysis at an accelerated condition for 4 weeks. This study identified that MAP-FPs have novel anti-microbial activity, inhibiting the growth and rapidly killing Gram-negative bacteria strains with high thermal stability and safety.

Structural analysis of the NK-lysin-derived peptide NK-2 upon interaction with bacterial membrane mimetics consisting of phosphatidylethanolamine and phosphatidylglycerol

NK-2 is an antimicrobial peptide derived from helices 3 and 4 of the pore-forming protein of natural killer cells, NK-lysin. It has potent activities against Gram-negative and Gram-positive bacteria, fungi and protozoan parasites without being toxic to healthy human cells. In biophysical assays its membrane activities were found to require phosphatidylglycerol (PG) and phosphatidylethanolamine (PE), lipids which dominate the composition of bacterial membranes. Here the structure and activities of NK-2 in binary mixtures of different PE/PG composition were investigated. CD spectroscopy reveals that a threshold concentration of 50 % PG is needed for efficient membrane association of NK-2 concomitant with a random coil - helix transition. Association with PE occurs but is qualitatively different when compared to PG membranes. Oriented solid-state NMR spectroscopy of NK-2 specifically labelled with 15N indicates that the NK-2 helices are oriented parallel to the PG bilayer surface. Upon reduction of the PG content to 20 mol% interactions are weaker and/or an in average more tilted orientation is observed. Fluorescence spectroscopy of differently labelled lipids is in agreement of an interfacial localisation of both helices where the C-terminal end is in a less hydrophobic environment. By inserting into the membrane interface and interacting differently with PE and PG the peptides probably induce high curvature strain which result in membrane openings and rupture.

Effects of Antimicrobial Peptides on Antioxidant Properties, Non-specific Immune Response and Gut Microbes of Tsinling Lenok Trout (Brachymystax lenok tsinlingensis)

Antimicrobial peptides (AMPs) are an important part of non-specific immunity and play a key role in the cellular host defense against pathogens and tissue injury infections. We investigated the effects of AMP supplementation on the antioxidant capacity, non-specific immunity, and gut microbiota of tsinling lenok trout. 240 fish were fed diets (CT, A120, A240 and A480) containing different amounts of AMP peptides (0, 120 mg kg-1, 240 mg kg-1, 480 mg kg-1) for 8 weeks. Our results showed that the activity of total antioxidant capacity (T-SOD) and glutathione peroxidase (GSH-Px), lysozyme (LZM), catalase (CAT) and acid phosphatase (ACP) in the A240 and A480 group were higher than that in the CT group (P < 0.05). The content of malondialdehyde (MDA) in AMP group was significantly lower than that in CT group (P < 0.05). Furthermore, we harvested the mid-gut and applied next-generation sequencing of 16S rDNA. The results showed that the abundance of Halomonas in AMP group was significantly lower than that in CT group. Functional analysis showed that the abundance of chloroalkane and chloroalkene degradation pathway increased significantly in AMP group. In conclusion, AMP enhanced the antioxidant capacity, non-specific immunity, and intestinal health of tsinling lenok trout.

Peptide Flexibility and the Hydrophobic Moment are Determinants to Evaluate the Clinical Potential of Magainins

Using a flexibility prediction algorithm and in silico structural modeling, we have calculated the intrinsic flexibility of several magainin derivatives. In the case of magainin-2 (Mag-2) and magainin H2 (MAG-H2) we have found that MAG-2 is more flexible than its hydrophobic analog, Mag-H2. This affects the degree of bending of both peptides, with a kink around two central residues (R10, R11), whereas, in Mag-H2, W10 stiffens the peptide. Moreover, this increases the hydrophobic moment of Mag-H2, which could explain its propensity to form pores in POPC model membranes, which exhibit near-to-zero spontaneous curvatures. Likewise, the protective effect described in DOPC membranes for this peptide regarding its facilitation in pore formation would be related to the propensity of this lipid to form membranes with negative spontaneous curvature. The flexibility of another magainin analog (MSI-78) is even greater than that of Mag-2. This facilitates the peptide to present a kind of hinge around the central F12 as well as a C-terminal end prone to be disordered. Such characteristics are key to understanding the broad-spectrum antimicrobial actions exhibited by this peptide. These data reinforce the hypothesis on the determinant role of spontaneous membrane curvature, intrinsic peptide flexibility, and specific hydrophobic moment in assessing the bioactivity of membrane-active antimicrobial peptides.

Peptide-Based Nanoparticles for Systemic Extrahepatic Delivery of Therapeutic Nucleotides

Peptide-based nanoparticles (PBN) for nucleotide complexation and targeting of extrahepatic diseases are gaining recognition as potent pharmaceutical vehicles for fine-tuned control of protein production (up- and/or down-regulation) and for gene delivery. Herein, we review the principles and mechanisms underpinning self-assembled formation of PBN, cellular uptake, endosomal release, and delivery to extrahepatic disease sites after systemic administration. Selected examples of PBN that have demonstrated recent proof of concept in disease models in vivo are summarized to offer the reader a comparative view of the field and the possibilities for clinical application.

The biological role of charge distribution in linear antimicrobial peptides

INTRODUCTION

Antimicrobial peptides (AMP) have received particular attention due to their capacity to kill bacteria. Although much is known about them, peptides are currently being further researched. A large number of AMPs have been discovered, but only a few have been approved for topical use, due to their promiscuity and other challenges, which need to be overcome.

AREAS COVERED

AMPs are diverse in structure. Consequently, they have varied action mechanisms when targeting microorganisms or eukaryotic cells. Herein, the authors focus on linear peptides, particularly those that are alpha-helical structured, and examine how their charge distribution and hydrophobic amino acids could modulate their biological activity.

EXPERT OPINION

The world currently needs urgent solutions to the infective problems caused by resistant pathogens. In order to start the race for antimicrobial development from the charge distribution viewpoint, bioinformatic tools will be necessary. Currently, there is no software available that allows to discriminate charge distribution in AMPs and predicts the biological effects of this event. Furthermore, there is no software available that predicts the side-chain length of residues and its role in biological functions. More specialized software is necessary.

The Mechanism of Action of SAAP-148 Antimicrobial Peptide as Studied with NMR and Molecular Dynamics Simulations

Background: SAAP-148 is an antimicrobial peptide derived from LL-37. It exhibits excellent activity against drug-resistant bacteria and biofilms while resisting degradation in physiological conditions. Despite its optimal pharmacological properties, its mechanism of action at the molecular level has not been explored. Methods: The structural properties of SAAP-148 and its interaction with phospholipid membranes mimicking mammalian and bacterial cells were studied using liquid and solid-state NMR spectroscopy as well as molecular dynamics simulations. Results: SAAP-148 is partially structured in solution and stabilizes its helical conformation when interacting with DPC micelles. The orientation of the helix within the micelles was defined by paramagnetic relaxation enhancements and found similar to that obtained using solid-state NMR, where the tilt and pitch angles were determined based on 15N chemical shift in oriented models of bacterial membranes (POPE/POPG). Molecular dynamic simulations revealed that SAAP-148 approaches the bacterial membrane by forming salt bridges between lysine and arginine residues and lipid phosphate groups while interacting minimally with mammalian models containing POPC and cholesterol. Conclusions: SAAP-148 stabilizes its helical fold onto bacterial-like membranes, placing its helix axis almost perpendicular to the surface normal, thus probably acting by a carpet-like mechanism on the bacterial membrane rather than forming well-defined pores.

The potential of antifungal peptide Sesquin as natural food preservative

Sesquin is a wide spectrum antimicrobial peptide displaying a remarkable activity on fungi. Contrarily to most antimicrobial peptides, it presents an overall negative charge. In the present study, we elucidate the molecular basis of its mode of action towards biomimetic membranes by NMR and MD experiments. While a specific recognition of phosphatidylethanolamine (PE) might explain its activity in a variety of different organisms (including bacteria), a further interaction with ergosterol accounts for its strong antifungal activity. NMR data reveal a charge gradient along its amide protons allowing the peptide to reach the membrane phosphate groups despite its negative charge. Subsequently, the peptide gets structured inside the bilayer, reducing its order. MD simulations predict that its activity is retained in conditions commonly used for food preservation: low temperatures, high pressure, or the presence of electric field pulses, making Sesquin a good candidate as food preservative.

Effect of lipid saturation on the topology and oligomeric state of helical membrane polypeptides

Natural liquid crystalline membranes are made up of many different lipids carrying a mixture of saturated and unsaturated fatty acyl chains. Whereas in the past considerable attention has been paid to cholesterol content, the phospholipid head groups and the membrane surface charge the detailed fatty acyl composition was often considered less important. However, recent investigations indicate that the detailed fatty acyl chain composition has pronounced effects on the oligomerization of the transmembrane helical anchoring domains of the MHC II receptor or the membrane alignment of the cationic antimicrobial peptide PGLa. In contrast the antimicrobial peptides magainin 2 and alamethicin are less susceptible to lipid saturation. Using histidine-rich LAH4 designer peptides the high energetic contributions of lipid saturation in stabilizing transmembrane helical alignments are quantitatively evaluated. These observations can have important implications for the biological regulation of membrane proteins and should be taken into considerations during biophysical or structural experiments.

Antibacterial Photodynamic Therapy in the Near-Infrared Region with a Targeting Antimicrobial Peptide Connected to a π-Extended Porphyrin

The increase of antimicrobial resistance to conventional antibiotics is worldwide a major health problem that requires the development of new bactericidal strategies. Antimicrobial photodynamic therapy (a-PDT) that generates reactive oxygen species acting on multiple cellular targets is unlikely to induce bacterial resistance. This localized treatment requires, for safe and efficient treatment of nonsuperficial infections, a targeting photosensitizer excited in the near IR. To this end, a new conjugate consisting of an antimicrobial peptide linked to a π-extended porphyrin photosensitizer was designed for a-PDT. Upon irradiation at 720 nm, the conjugate has shown at micromolar concentration strong bactericidal action on both Gram-positive and Gram-negative bacteria. Moreover, this conjugate allows one to reach a low minimum bactericidal concentration with near IR excitation without inducing toxicity to skin cells.

Rescuing humanity by antimicrobial peptides against colistin-resistant bacteria

Abstract

It has been about a century since the discovery of the first antibiotic, and during this period, several antibiotics were produced and marketed. The production of high-potency antibiotics against infections led to victories, but these victories were temporary. Overuse and misuse of antibiotics have continued to the point that humanity today is almost helpless in the fight against infection. Researchers have predicted that by the middle of the new century, there will be a dark period after the production of antibiotics that doctors will encounter antibiotic-resistant infections for which there is no cure. Accordingly, researchers are looking for new materials with antimicrobial properties that will strengthen their ammunition to fight antibiotic-resistant infections. One of the most important alternatives to antibiotics introduced in the last three decades is antimicrobial peptides (AMPs), which affect a wide range of microbes. Due to their different antimicrobial properties from antibiotics, AMPs can fight and kill MDR, XDR, and colistin-resistant bacteria through a variety of mechanisms. Therefore, in this study, we intend to use the latest studies to give a complete description of AMPs, the importance of colistin-resistant bacteria, and their resistance mechanisms, and represent impact of AMPs on colistin-resistant bacteria.

Key points

• AMPs as limited options to kill colistin-resistant bacteria.

• Challenge of antibiotics resistance, colistin resistance, and mechanisms.

• What is AMPs in the war with colistin-resistant bacteria?

Lipid saturation and head group composition have a pronounced influence on the membrane insertion equilibrium of amphipathic helical polypeptides

The histidine-rich peptides of the LAH4 family were designed using cationic antimicrobial peptides such as magainin and PGLa as templates. The LAH4 amphipathic helical sequences exhibit a multitude of interesting biological properties such as antimicrobial activity, cell penetration of a large variety of cargo and lentiviral transduction enhancement. The parent peptide associates with lipid bilayers where it changes from an orientation along the membrane interface into a transmembrane configuration in a pH-dependent manner. Here we show that LAH4 adopts a transmembrane configuration in fully saturated DMPC membranes already at pH 3.5, i.e. much below the pK_a of the histidines whereas the transition pH in POPC correlates closely with histidine neutralization. In contrast in POPG membranes the in-planar configuration is stabilized by about one pH unit. The differences in pH can be converted into energetic contributions for the in-plane to transmembrane transition equilibrium, where the shift in the transition pH due to lipid saturation corresponds to energies which are otherwise obtained by the exchange of several cationic with hydrophobic residues. A similar dependence on lipid saturation has also been observed when the PGLa and magainin antimicrobial peptides interact within lipid bilayers suggesting that the quantitative evaluation presented in this paper also applies to other membrane polypeptides.

Far-UV circular dichroism signatures indicate fluorophore labeling induced conformational changes of penetratin

Fluorescent labeling is a broadly utilized approach to assess in vitro and in vivo behavior of biologically active, especially cell-penetrating and antimicrobial peptides. In this communication, far-UV circular dichroism (CD) spectra of penetratin (PEN) fluorophore conjugates reported previously have been re-evaluated. Compared to the intrinsically disordered native peptide, rhodamine B and carboxyfluorescein derivatives of free and membrane-bound PEN exhibit extrinsic CD features. Potential sources of these signals displayed above 220 nm are discussed suggesting the contributions of both intra- and intermolecular chiral exciton coupling mechanisms. Careful evaluation of the CD spectra of fluorophore-labeled peptides is a valuable tool for early detection of labeling-provoked structural alterations which in turn may modify the membrane binding and cellular uptake compared to the unconjugated form.

An oomycete NLP cytolysin forms transient small pores in lipid membranes

Microbial plant pathogens secrete a range of effector proteins that damage host plants and consequently constrain global food production. Necrosis and ethylene-inducing peptide 1-like proteins (NLPs) are produced by numerous phytopathogenic microbes that cause important crop diseases. Many NLPs are cytolytic, causing cell death and tissue necrosis by disrupting the plant plasma membrane. Here, we reveal the unique molecular mechanism underlying the membrane damage induced by the cytotoxic model NLP. This membrane disruption is a multistep process that includes electrostatic-driven, plant-specific lipid recognition, shallow membrane binding, protein aggregation, and transient pore formation. The NLP-induced damage is not caused by membrane reorganization or large-scale defects but by small membrane ruptures. This distinct mechanism of lipid membrane disruption is highly adapted to effectively damage plant cells.

Antimicrobial Peptide Mechanisms Studied by Whole-Cell Deuterium NMR

Much of the work probing antimicrobial peptide (AMP) mechanisms has focussed on how these molecules permeabilize lipid bilayers. However, AMPs must also traverse a variety of non-lipid cell envelope components before they reach the lipid bilayer. Additionally, there is a growing list of AMPs with non-lipid targets inside the cell. It is thus useful to extend the biophysical methods that have been traditionally applied to study AMP mechanisms in liposomes to the full bacteria, where the lipids are present along with the full complexity of the rest of the bacterium. This review focusses on what can be learned about AMP mechanisms from solid-state NMR of AMP-treated intact bacteria. It also touches on flow cytometry as a complementary method for measuring permeabilization of bacterial lipid membranes in whole bacteria.

IN SITU SOLID-STATE NMR STUDY OF ANTIMICROBIAL PEPTIDE INTERACTIONs WITH ERYTHROCYTE MEMBRANES

Antimicrobial peptides are promising therapeutic agents to mitigate the global rise of antibiotic resistance. They generally act by perturbing the bacterial cell membrane and are thus less likely to induce resistance. Because they are membrane-active molecules, it is critical to verify and understand their potential action toward eukaryotic cells to help design effective and safe drugs. In this work, we studied the interaction of two antimicrobial peptides, aurein 1.2 and caerin 1.1, with red blood cell (RBC) membranes using in situ ³¹P and ²H solid-state NMR (SS-NMR). We established a protocol to integrate up to 25% of deuterated fatty acids in the membranes of ghosts, which are obtained when hemoglobin is removed from RBCs. Fatty acid incorporation and the integrity of the lipid bilayer were confirmed by SS-NMR and fluorescence confocal microscopy. Leakage assays were performed to assess the lytic power of the antimicrobial peptides. The in situ perturbation of the ghost membranes by aurein 1.2 and caerin 1.1 revealed by ³¹P and ²H SS-NMR is consistent with membrane perturbation through a carpet mechanism for aurein 1.2, whereas caerin 1.1 acts on RBCs via pore formation. These results are compatible with fluorescence microscopy images of the ghosts. The peptides interact with eukaryotic membranes following similar mechanisms that take place in bacteria, highlighting the importance of hydrophobicity when determining such interactions. Our work bridges model membranes and in vitro studies and provides an analytical toolbox to assess drug toxicity toward eukaryotic cells.

Design and Characterization of Myristoylated and Non-Myristoylated Peptides Effective against Candida spp. Clinical Isolates

The increasing resistance of fungi to antibiotics is a severe challenge in public health, and newly effective drugs are required. Promising potential medications are lipopeptides, linear antimicrobial peptides (AMPs) conjugated to a lipid tail, usually at the N-terminus. In this paper, we investigated the in vitro and in vivo antifungal activity of three short myristoylated and non-myristoylated peptides derived from a mutant of the AMP Chionodracine. We determined their interaction with anionic and zwitterionic membrane-mimicking vesicles and their structure during this interaction. We then investigated their cytotoxic and hemolytic activity against mammalian cells. Lipidated peptides showed a broad spectrum of activity against a relevant panel of pathogen fungi belonging to Candida spp., including the multidrug-resistant C. auris. The antifungal activity was also observed vs. biofilms of C. albicans, C. tropicalis, and C. auris. Finally, a pilot efficacy study was conducted on the in vivo model consisting of Galleria mellonella larvae. Treatment with the most-promising myristoylated peptide was effective in counteracting the infection from C. auris and C. albicans and the death of the larvae. Therefore, this myristoylated peptide is a potential candidate to develop antifungal agents against human fungal pathogens.

Discovery and Characterization of a New Crustin Antimicrobial Peptide from Amphibalanus amphitrite

Crustins are an antimicrobial peptide (AMP) family that plays an important role in innate immunity in crustaceans. It is important to discover new AMPs from natural sources to expand the current database. Here, we identified and characterized a new crustin family member, named AaCrus1, from Amphibalanus amphitrite. AaCrus1 shares high identity (48.10%) with PvCrus, a Type I crustin of Penaeus vannamei that possesses a whey acidic protein (WAP) domain. AaCrus1 contains 237 amino acids and eight cysteine residues forming conserved 'four-disulfide core' structure. Our recombinant AaCrus1 (rAaCrus 1) could inhibit the growth of two Gram-positive bacteria (Staphylococcus aureus, Bacillus sp. T2) and four Gram-negative bacteria (Vibrio parahaemolyticus, Vibrio harveyi, Vibrio anguillarum, Vibrio alginolyticus) with a minimum inhibitory concentration of 3.5-28 μM. It can further induce agglutination of both Gram-positive and Gram-negative bacteria. rAaCrus1 can bind to bacteria and damage bacterial cell membranes. Furthermore, rAaCrus1 disrupted biofilm development of S. aureus and V. parahaemolyticus. Our discovery and characterization of this new crustin can be further optimized as a good alternative to antibiotics.

Binding of cationic analogues of α-MSH to lipopolysaccharide and disruption of the cytoplasmic membranes caused bactericidal action against Escherichia coli

In earlier reports, we have shown the antimicrobial activity of a host neuropeptide, alpha-melanocyte stimulating hormone (α-MSH) and its cationic analogues against Staphylococcus aureus. These analogues of α-MSH showed enhanced staphylocidal activity without any significant mammalian cell toxicity. Therefore, here, we explored the antimicrobial activity of α-MSH and its cationic analogues against Escherichia coli. Though the presence of lipopolysaccharide (LPS) in Gram-negative bacteria enables them to resist most conventional antibiotics, encouragingly α-MSH and its four analogues showed killing of both logarithmic and stationary phase E. coli cells in a time, dose and cationicity-dependent manner. In fact, the most cationic analogue, KKK-MSH with a + 5 charge, demonstrated successful eradication of 10⁵ CFU/mL of E. coli cells within 15 min at a concentration as low as 1 µM. BC displacement experiment revealed that cationicity of the peptides was directly related to the killing efficacy of these α-MSH analogues against E. coli cells via initial LPS-binding, leading to rapid disruption of the LPS-outer membrane complex followed by inner bacterial membrane damage and eventual cell death. Here, we propose α-MSH based cationic peptides as promising future agents with broad-spectrum antibacterial efficacy against both Gram-negative and Gram-positive pathogens.

Deuterium Solid State NMR Studies of Intact Bacteria Treated With Antimicrobial Peptides

Solid state NMR has been tremendously useful in characterizing the structure and dynamics of model membranes composed of simple lipid mixtures. Model lipid studies employing solid state NMR have included important work revealing how membrane bilayer structure and dynamics are affected by molecules such as antimicrobial peptides (AMPs). However, solid state NMR need not be applied only to model membranes, but can also be used with living, intact cells. NMR of whole cells holds promise for helping resolve some unsolved mysteries about how bacteria interact with AMPs. This mini-review will focus on recent studies using ²H NMR to study how treatment with AMPs affect membranes in intact bacteria.

Molecular basis of the anticancer, apoptotic and antibacterial activities of Bombyx mori Cecropin A

As Cecropin XJ, Cecropin A from Bombyx mori is one of the very few antimicrobial peptides having shown activity against esophageal cancer cells. It displays remarkable sequence-similarity to Cecropin XJ but slightly enhanced activity. In this work we show by NMR that both peptides are unstructured in solution but get structured in the presence of DPC micelles, mimicking the surface of biological membranes. In order to get insight into the molecular basis of its anticancer, antimicrobial and antifungal activity, we have investigated by MD simulations their interaction with a large variety of lipid bilayers mimicking cancer, mitochondrial, bacterial and fungal membranes. At variance with CecXJ, organized in two main helices, CecA tends to form a three helix bundle resulting in enhanced adaptability to its membrane targets. A specificity for the headgroup of phosphatidylserine and affinity for phosphatidylglycerol and cardiolipin may account for its selective targeting of cancer, bacterial and mitochondrial membranes, respectively.

Antimicrobial peptides: mechanism of action and lipid-mediated synergistic interactions within membranes

Biophysical and structural studies of peptide-lipid interactions, peptide topology and dynamics have changed our view of how antimicrobial peptides insert and interact with membranes. Clearly, both peptides and lipids are highly dynamic, and change and mutually adapt their conformation, membrane penetration and detailed morphology on a local and a global level. As a consequence, peptides and lipids can form a wide variety of supramolecular assemblies in which the more hydrophobic sequences preferentially, but not exclusively, adopt transmembrane alignments and have the potential to form oligomeric structures similar to those suggested by the transmembrane helical bundle model. In contrast, charged amphipathic sequences tend to stay intercalated at the membrane interface. Although the membranes are soft and can adapt, at increasing peptide density they cause pronounced disruptions of the phospholipid fatty acyl packing. At even higher local or global concentrations the peptides cause transient membrane openings, rupture and ultimately lysis. Interestingly, mixtures of peptides such as magainin 2 and PGLa, which are stored and secreted naturally as a cocktail, exhibit considerably enhanced antimicrobial activities when investigated together in antimicrobial assays and also in pore forming experiments applied to biophysical model systems. Our most recent investigations reveal that these peptides do not form stable complexes but act by specific lipid-mediated interactions and the nanoscale properties of phospholipid bilayers.

Chemical modifications to increase the therapeutic potential of antimicrobial peptides

The continued use of antibiotics has been accompanied by the rapid emergence and spread of antibiotic-resistant strains of bacteria. Antimicrobial peptides (AMPs), also known as host defense peptides, show multiple features as an ideal antimicrobial agent, including potent, rapid, and broad-spectrum antimicrobial activity, low promotion of antimicrobial resistance, potent anti-biofilm activity, and lethality against metabolically inactive microorganisms. However, several crucial drawbacks constrain the use of AMPs as clinical drugs, e.g., liability in vivo, toxicity when used systemically, and high production costs. Based on recent findings and our own experiences, here we summarize some chemical modifications and key design strategies to increase the therapeutic potential of AMPs, including 1) enhancing antimicrobial activities, 2) improving in vivo effectiveness, and 3) reduction in toxicity, which may facilitate the design and optimization of AMPs for the development of drug candidates. We also discuss the present challenges in the optimization of AMPs and future concerns about the resistance and cross-resistance to AMPs in the development of AMPs as therapeutic drugs.

The impact of phosphatidylserine exposure on cancer cell membranes on the activity of the anticancer peptide HB43

HB43 (FAKLLAKLAKKLL) is a synthetic peptide active against cell lines derived from breast, colon, melanoma, lung, prostate, and cervical cancers. Despite its remarkable spectrum of activity, the mechanism of action at the molecular level has never been investigated, preventing further optimization of its selectivity. The alternation of charged and hydrophobic residues suggests amphipathicity, but the formation of alpha-helical structure seems discouraged by its short length and the large number of positively charged residues. Using different biophysical and in silico approaches we show that HB43 is completely unstructured in solution but assumes alpha-helical conformation in the presence of DPC micelles and liposomes exposing phosphatidylserine (PS) used as mimics of cancer cell membranes. Membrane permeabilization assays demonstrate that the interaction leads to the preferential destabilization of PS-containing vesicles with respect to PC-containing ones, here used as noncancerous cell mimics. ssNMR reveals that HB43 is able to fluidify the internal structure of cancer-cell mimicking liposomes while MD simulations show its internalization in such bilayers. This is achieved by the formation of specific interactions between the lysine side chains and the carboxylate group of phosphatidylserine and/or the phosphate oxygen atoms of targeted phospholipids, which could catalyze the formation of the alpha helix required for internalization. With the aim of better understanding the peptide biocompatibility and the additional antibacterial activity, the interaction with noncancerous cell mimicking liposomes exposing phosphatidylcholine (PC) and bacterial mimicking bilayers exposing phosphatidylglycerol (PG) is also described.

In-Cell FRET Indicates Magainin Peptide Induced Permeabilization of Bacterial Cell Membranes at Lower Peptide-to-Lipid Ratios Relevant to Liposomal Studies

Antimicrobial peptides (AMPs) are promising candidates for anti-infective drugs. The majority of AMPs are considered to disrupt the lipid matrix of bacterial membranes, exerting bactericidal activity. A number of biophysical studies have been carried out to elucidate the underlying molecular mechanisms. However, the fact that the number of peptide molecules bound to a bacterial cell under bactericidal conditions is much larger than that expected from liposomal studies raises the question of whether membrane permeabilization mechanisms proposed by liposomal studies are relevant to bacteria. In this study, the peptide-to-lipid molar ratio needed for an antimicrobial magainin peptide to permeabilize the cell membrane of the Gram-positive bacterium Bacillus megaterium was estimated by random fluorescence resonance energy transfer from a BODIPY FL-labeled lipid to a Texas Red-labeled peptide. The comparison of the observed energy transfer efficiency with the two-dimensional energy transfer theory estimated that the leakage of the calcein dye from bacterial cells occurred at a peptide-to-lipid molar ratio of 0.025. At this ratio, the peptide induced dye leakage from liposomes mimicking the bacterial membrane, indicating that the lipid matrix is a target of membrane-acting AMPs and that liposomes are a useful model system to investigate their mechanisms of action. Furthermore, a binding assay suggested that most peptide molecules were bound to cellular components other than cell membranes.

Investigation of the Action of Peptides on Lipid Membranes

Calorimetric and incoherent neutron scattering methods were employed to investigate the action of magainin 2 and PGLa peptides on the phase behavior and molecular dynamics of lipids mimicking cytoplasmic membranes of Gram-negative bacteria. The impact of the peptides, tested individually and cooperatively by differential scanning calorimetry, presented a broadened peak, sometimes with a second shoulder, depicting the phase transition temperature around 21 °C. Neutron scattering revealed a small but significant variation of the membrane dynamics due to the peptides in both in-plane and out-of-plane directions. Although we did not find a clear hint for synergy in the interplay of the two peptides, the calorimetric and neutron data give compatible results in terms of a decrease of the enthalpy due to the presence of the peptides, which destabilize the membrane. The dynamics in the two directions was differentiated when the individual peptides were added to the membranes, but the impact was smaller when both peptides were added together.

Antimicrobial Peptides: A Potent Alternative to Antibiotics

Antimicrobial peptides constitute one of the most promising alternatives to antibiotics since they could be used to treat bacterial infections, especially those caused by multidrug-resistant pathogens. Many antimicrobial peptides, with various activity spectra and mechanisms of actions, have been described. This review focuses on their use against ESKAPE bacteria, especially in biofilm treatments, their synergistic activity, and their application as prophylactic agents. Limitations and challenges restricting therapeutic applications are highlighted, and solutions for each challenge are evaluated to analyze whether antimicrobial peptides could replace antibiotics in the near future.

Advances in Molecular Understanding of α-Helical Membrane-Active Peptides

Biological membranes separate the interior of cells or cellular compartments from their outer environments. This barrier function of membranes can be disrupted by membrane-active peptides, some of which can spontaneously penetrate through the membranes or open leaky transmembrane pores. However, the origin of their activity/toxicity is not sufficiently understood for the development of more potent peptides. To this day, there are no design rules that would be generally valid, and the role of individual amino acids tends to be sequence-specific.In this Account, we describe recent progress in understanding the design principles that govern the activity of membrane-active peptides. We focus on α-helical amphiphilic peptides and their ability to (1) translocate across phospholipid bilayers, (2) form transmembrane pores, or (3) act synergistically, i.e., to produce a significantly more potent effect in a mixture than the individual components.We refined the description of peptide translocation using computer simulations and demonstrated the effect of selected residues. Our simulations showed the necessity to explicitly include charged residues in the translocation description to correctly sample the membrane perturbations they can cause. Using this description, we calculated the translocation of helical peptides with and without the kink induced by the proline/glycine residue. The presence of the kink had no effect on the translocation barrier, but it decreased the peptide affinity to the membrane and reduced the peptide stability inside the membrane. Interestingly, the effects were mainly caused by the peptide's increased polarity, not the higher flexibility of the kink.Flexibility plays a crucial role in pore formation and affects distinct pore structures in different ways. The presence of a kink destabilizes barrel-stave pores, because the kink prevents the tight packing of peptides in the bundle, which is characteristic of the barrel-stave structure. In contrast, the kink facilitates the formation of toroidal pores, where the peptides are only loosely arranged and do not need to closely assemble. The exact position of the kink in the sequence further determines the preferred arrangement of peptides in the pore, i.e., an hourglass or U-shaped structure. In addition, we demonstrated that two self-associated (via termini) helical peptides could mimic the behavior of peptides with a helix-kink-helix motif.Finally, we review the recent findings on the peptide synergism of the archetypal mixture of Magainin 2 and PGLa peptides. We focused on a bacterial plasma membrane mimic that contains negatively charged lipids and lipids with negative intrinsic curvature. We showed that the synergistic action of peptides was highly dependent on the lipid composition. When the lipid composition and peptide/lipid ratios were changed, the systems exhibited more complex behavior than just the previously reported pore formation. We observed membrane adhesion, fusion, and even the formation of the sponge phase in this regime. Furthermore, enhanced adhesion/partitioning to the membrane was reported to be caused by lipid-induced peptide aggregation.In conclusion, the provided molecular insight into the complex behavior of membrane-active peptides provides clues for the design and modification of antimicrobial peptides or toxins.

A Synergic Potential of Antimicrobial Peptides against Pseudomonas syringae pv. actinidiae

Pseudomonas syringae pv. actinidiae (Psa) is the pathogenic agent responsible for the bacterial canker of kiwifruit (BCK) leading to major losses in kiwifruit productions. No effective treatments and measures have yet been found to control this disease. Despite antimicrobial peptides (AMPs) having been successfully used for the control of several pathogenic bacteria, few studies have focused on the use of AMPs against Psa. In this study, the potential of six AMPs (BP100, RW-BP100, CA-M, 3.1, D4E1, and Dhvar-5) to control Psa was investigated. The minimal inhibitory and bactericidal concentrations (MIC and MBC) were determined and membrane damaging capacity was evaluated by flow cytometry analysis. Among the tested AMPs, the higher inhibitory and bactericidal capacity was observed for BP100 and CA-M with MIC of 3.4 and 3.4-6.2 µM, respectively and MBC 3.4-10 µM for both. Flow cytometry assays suggested a faster membrane permeation for peptide 3.1, in comparison with the other AMPs studied. Peptide mixtures were also tested, disclosing the high efficiency of BP100:3.1 at low concentration to reduce Psa viability. These results highlight the potential interest of AMP mixtures against Psa, and 3.1 as an antimicrobial molecule that can improve other treatments in synergic action.

Antimicrobial Peptides and their Multiple Effects at Sub-Inhibitory Concentrations

The frequent occurrence of multidrug-resistant strains to conventional antimicrobials has led to a clear decline in antibiotic therapies. Therefore, new molecules with different mechanisms of action are extremely necessary. Due to their unique properties, antimicrobial peptides (AMPs) represent a valid alternative to conventional antibiotics and many of them have been characterized for their activity and cytotoxicity. However, the effects that these peptides cause at concentrations below the minimum growth inhibitory concentration (MIC) have yet to be fully analyzed along with the underlying molecular mechanism. In this mini-review, the ability of AMPs to synergize with different antibiotic classes or different natural compounds is examined. Furthermore, data on microbial resistance induction are reported to highlight the importance of antibiotic resistance in the fight against infections. Finally, the effects that sub-MIC levels of AMPs can have on the bacterial pathogenicity are summarized while showing how signaling pathways can be valid therapeutic targets for the treatment of infectious diseases. All these aspects support the high potential of AMPs as lead compounds for the development of new drugs with antibacterial and immunomodulatory activities.

Molecular Basis of the Anticancer and Antibacterial Properties of CecropinXJ Peptide: An In Silico Study

Esophageal cancer is an aggressive lethal malignancy causing thousands of deaths every year. While current treatments have poor outcomes, cecropinXJ (CXJ) is one of the very few peptides with demonstrated in vivo activity. The great interest in CXJ stems from its low toxicity and additional activity against most ESKAPE bacteria and fungi. Here, we present the first study of its mechanism of action based on molecular dynamics (MD) simulations and sequence-property alignment. Although unstructured in solution, predictions highlight the presence of two helices separated by a flexible hinge containing P24 and stabilized by the interaction of W2 with target biomembranes: an amphipathic helix-I and a poorly structured helix-II. Both MD and sequence-property alignment point to the important role of helix I in both the activity and the interaction with biomembranes. MD reveals that CXJ interacts mainly with phosphatidylserine (PS) but also with phosphatidylethanolamine (PE) headgroups, both found in the outer leaflet of cancer cells, while salt bridges with phosphate moieties are prevalent in bacterial biomimetic membranes composed of PE, phosphatidylglycerol (PG) and cardiolipin (CL). The antibacterial activity of CXJ might also explain its interaction with mitochondria, whose phospholipid composition recalls that of bacteria and its capability to induce apoptosis in cancer cells.

Antimicrobial Peptide K11 Selectively Recognizes Bacterial Biomimetic Membranes and Acts by Twisting Their Bilayers

K11 is a synthetic peptide originating from the introduction of a lysine residue in position 11 within the sequence of a rationally designed antibacterial scaffold. Despite its remarkable antibacterial properties towards many ESKAPE bacteria and its optimal therapeutic index (320), a detailed description of its mechanism of action is missing. As most antimicrobial peptides act by destabilizing the membranes of the target organisms, we investigated the interaction of K11 with biomimetic membranes of various phospholipid compositions by liquid and solid-state NMR. Our data show that K11 can selectively destabilize bacterial biomimetic membranes and torque the surface of their bilayers. The same is observed for membranes containing other negatively charged phospholipids which might suggest additional biological activities. Molecular dynamic simulations reveal that K11 can penetrate the membrane in four steps: after binding to phosphate groups by means of the lysine residue at the N-terminus (anchoring), three couples of lysine residues act subsequently to exert a torque in the membrane (twisting) which allows the insertion of aromatic side chains at both termini (insertion) eventually leading to the flip of the amphipathic helix inside the bilayer core (helix flip and internalization).

Revealing the Mechanisms of Synergistic Action of Two Magainin Antimicrobial Peptides

The study of peptide-lipid and peptide-peptide interactions as well as their topology and dynamics using biophysical and structural approaches have changed our view how antimicrobial peptides work and function. It has become obvious that both the peptides and the lipids arrange in soft supramolecular arrangements which are highly dynamic and able to change and mutually adapt their conformation, membrane penetration, and detailed morphology. This can occur on a local and a global level. This review focuses on cationic amphipathic peptides of the magainin family which were studied extensively by biophysical approaches. They are found intercalated at the membrane interface where they cause membrane thinning and ultimately lysis. Interestingly, mixtures of two of those peptides namely magainin 2 and PGLa which occur naturally as a cocktail in the frog skin exhibit synergistic enhancement of antimicrobial activities when investigated together in antimicrobial assays but also in biophysical experiments with model membranes. Detailed dose-response curves, presented here for the first time, show a cooperative behavior for the individual peptides which is much increased when PGLa and magainin are added as equimolar mixture. This has important consequences for their bacterial killing activities and resistance development. In membranes that carry unsaturations both peptides align parallel to the membrane surface where they have been shown to arrange into mesophases involving the peptides and the lipids. This supramolecular structuration comes along with much-increased membrane affinities for the peptide mixture. Because this synergism is most pronounced in membranes representing the bacterial lipid composition it can potentially be used to increase the therapeutic window of pharmaceutical formulations.

Structure, membrane topology and influence of cholesterol of the membrane proximal region: transmembrane helical anchor sequence of gp41 from HIV

During the first steps of HIV infection the Env subunit gp41 is thought to establish contact between the membranes and to be the main driver of fusion. Here we investigated in liquid crystalline membranes the structure and cholesterol recognition of constructs made of a gp41 external region carrying a cholesterol recognition amino acid consensus (CRAC) motif and a hydrophobic membrane anchoring sequence. CD- und ATR-FTIR spectroscopies indicate that the constructs adopt a high degree of helical secondary structure in membrane environments. Furthermore, 15N and 2H solid-state NMR spectra of gp41 polypeptides reconstituted into uniaxially oriented bilayers agree with the CRAC domain being an extension of the transmembrane helix. Upon addition of cholesterol the CRAC NMR spectra remain largely unaffected when being associated with the native gp41 transmembrane sequence but its topology changes when anchored in the membrane by a hydrophobic model sequence. The 2H solid-state NMR spectra of deuterated cholesterol are indicative of a stronger influence of the model sequence on this lipid when compared to the native gp41 sequence. These observations are suggestive of a strong coupling between the transmembrane and the membrane proximal region of gp41 possibly enforced by oligomerization of the transmembrane helical region.

β-Amyloid (1-42) peptide adsorbs but does not insert into ganglioside-containing phospholipid membranes in the liquid-disordered state: modelling and experimental studies

β-Amyloid (Aβ) is a 39-43 residue peptide involved in the pathogenesis of Alzheimer's disease. Aβ deposits onto the cells and gives rise to the plaques that are characteristic of the disease. In an effort to understand the molecular mechanism of plaque formation, we have examined the interaction of Aβ42, considered to be the most pathogenic of the peptides, with lipid bilayers consisting of 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) to which small amounts of GM1 ganglioside (1-5 mol%) were incorporated. POPC bilayers exist in the fluid, or liquid-disordered state at room temperature, mimicking the fluidity of cell membranes. An Aβ42 preparation consisting essentially of peptide monomers was used. A combination of molecular dynamics (MD), isothermal calorimetry and Langmuir balance measurements was applied. Our results show that Aβ binds POPC bilayers, and that binding increases (ΔG of binding decreases) with GM1, but only up to 3 mol% of the ganglioside, larger concentrations appearing to have a lower effect. MD and Langmuir balance measurements concur in showing that the peptide adsorbs onto the bilayer surface, but does not become inserted into it at surface pressures compatible with the cell membrane conditions. Thioflavin T measurements agree with MD in revealing a very low degree of peptide oligomerization/aggregation under our conditions. This is in contrast with previous studies showing peptide aggregation and insertion when interacting with membranes in the liquid-ordered state. The present contribution underlines the importance of bilayer lipid composition and properties for Aβ plaque formation.

Roles of histidine charge and cardiolipin in membrane disruption by antimicrobial peptides Gaduscidin-1 and Gaduscidin-2

Gad-1 and Gad-2 are helical, histidine-rich antimicrobial peptides (AMPs) from paralogous genes in cod. ¹⁵N and ²H solid state nuclear magnetic resonance (NMR) were used to characterize their lipid-bound structures and lipid interactions. Gad-1 was found to position in-plane in POPC: POPG bilayers. Gad-1 displayed greater effects than Gad-2 on lipid acyl chain order of POPE: POPG and POPE: POPG: CL bilayers, in keeping with its greater activity against E. coli. The effect of Gad-1 and Gad-2 on lipid bilayer order was only weakly affected by changes in pH, and hence changes in histidine charge. This was somewhat surprising for Gad-2 as this peptide's biological activity has been shown to be greater at low pH and thus the finding may point to the existence of functional interactions with non-lipid components of bacteria. The incorporation of cardiolipin into POPE: POPG bilayers in such a way as to preserve the overall charge of the bilayers did not alter Gad-1's effects on lipid acyl chain order parameters, which report on motions on the 10^-5 s timescale. When cardiolipin and Gad-1 were both present, there were subtle changes on membrane dynamics at other timescales.

NoPv1: a synthetic antimicrobial peptide aptamer targeting the causal agents of grapevine downy mildew and potato late blight

Grapevine (Vitis vinifera L.) is a crop of major economic importance. However, grapevine yield is guaranteed by the massive use of pesticides to counteract pathogen infections. Under temperate-humid climate conditions, downy mildew is a primary threat for viticulture. Downy mildew is caused by the biotrophic oomycete Plasmopara viticola Berl. & de Toni, which can attack grapevine green tissues. In lack of treatments and with favourable weather conditions, downy mildew can devastate up to 75% of grape cultivation in one season and weaken newly born shoots, causing serious economic losses. Nevertheless, the repeated and massive use of some fungicides can lead to environmental pollution, negative impact on non-targeted organisms, development of resistance, residual toxicity and can foster human health concerns. In this manuscript, we provide an innovative approach to obtain specific pathogen protection for plants. By using the yeast two-hybrid approach and the P. viticola cellulose synthase 2 (PvCesA2), as target enzyme, we screened a combinatorial 8 amino acid peptide library with the aim to identify interacting peptides, potentially able to inhibit PvCesa2. Here, we demonstrate that the NoPv1 peptide aptamer prevents P. viticola germ tube formation and grapevine leaf infection without affecting the growth of non-target organisms and without being toxic for human cells. Furthermore, NoPv1 is also able to counteract Phytophthora infestans growth, the causal agent of late blight in potato and tomato, possibly as a consequence of the high amino acid sequence similarity between P. viticola and P. infestans cellulose synthase enzymes.

The Best Peptidomimetic Strategies to Undercover Antibacterial Peptides

Health-care systems that develop rapidly and efficiently may increase the lifespan of humans. Nevertheless, the older population is more fragile, and is at an increased risk of disease development. A concurrently growing number of surgeries and transplantations have caused antibiotics to be used much more frequently, and for much longer periods of time, which in turn increases microbial resistance. In 1945, Fleming warned against the abuse of antibiotics in his Nobel lecture: "The time may come when penicillin can be bought by anyone in the shops. Then there is the danger that the ignorant man may easily underdose himself and by exposing his microbes to non-lethal quantities of the drug make them resistant". After 70 years, we are witnessing the fulfilment of Fleming's prophecy, as more than 700,000 people die each year due to drug-resistant diseases. Naturally occurring antimicrobial peptides protect all living matter against bacteria, and now different peptidomimetic strategies to engineer innovative antibiotics are being developed to defend humans against bacterial infections.

The Reversible Non-covalent Aggregation Into Fibers of PGLa and Magainin 2 Preserves Their Antimicrobial Activity and Synergism

Magainin 2 and PGLa are antimicrobial peptides found together in frog skin secretions. When added as a mixture they show an order of magnitude increase in antibacterial activity and in model membrane permeation assays. Here we demonstrate that both peptides can form fibers with beta-sheet/turn signature in ATR-FTIR- and CD-spectroscopic analyses, but with different morphologies in EM images. Whereas, fiber formation results in acute reduction of the antimicrobial activity of the individual peptides, the synergistic enhancement of activity remains for the equimolar mixture of PGLa and magainin 2 also after fibril formation. The biological significance and potential applications of such supramolecular aggregates are discussed.

Antimicrobial Activities of Different Fractions from Mucus of the Garden Snail Cornu aspersum

Natural products have long played a major role in medicine and science. The garden snail Cornu aspersum is a rich source of biologically active natural substances that might be an important source for new drugs to treat human disease. Based on our previous studies, nine fractions containing compounds with Mw <3 kDa; <10 kDa; <20 kDa; >20 kDa; >30 kDa>50 kDa and between 3 and 5 kDa; 5 and 10 kDa; and 10 and 30 kDa were purified from the mucus of C. aspersum and analyzed by tandem mass spectrometry (MALDI-TOF/TOF). Seventeen novel peptides with potential antibacterial activity were identified by de novo MS/MS sequencing using tandem mass spectrometry. The different fractions were tested for antibacterial activity against Gram^─ (Pseudomonas aureofaciens and Escherichia coli) and Gram+ (Brevibacillus laterosporus) bacterial strains as well the anaerobic bacterium Clostridium perfringens. These results revealed that the peptide fractions exhibit a predominant antibacterial activity against B. laterosporus; the fraction with Mw 10–30 kDa against E. coli; another peptide fraction <20 kDa against P. aureofaciens; and the protein fraction >20 kDa against the bacterial strain C. perfringens. The discovery of new antimicrobial peptides (AMPs) from natural sources is of great importance for public health due to the AMPs’ effective antimicrobial activities and low resistance rates.

Role of Charge in Lipid Vesicle Binding and Vesicle Surface Saturation by Gaduscidin-1 and Gaduscidin-2

The histidine-rich antimicrobial peptides Gad-1 and Gad-2, from paralogous genes in cod, provide an opportunity to examine the effect of charge and nonelectrostatic factors on peptide-vesicle interaction and on peptide antimicrobial activity. In this study, the dependence of vesicle ζ-potential on peptide concentration has been used to examine the binding of these peptides to model vesicle surfaces at pH = 5.0, for which the charges of Gad-1 and Gad-2 are +8 and +5, respectively, and at pH = 7.0, where their charges are +3 and +1, respectively. Interpreting the observed ζ-potential behaviors as examples of Langmuir adsorption isotherms, it is possible to infer the equilibrium constant for peptide-vesicle binding, the fraction of the peptide bound at low peptide concentration, and the maximum peptide-to-lipid ratio when the vesicle surface is saturated at high peptide concentration. For both peptides, higher peptide charge is found to be correlated with a lower fraction of the peptide being bound to vesicle surfaces at low peptide concentration and with a smaller maximum bound peptide-to-lipid ratio at high peptide concentration. The equilibrium binding constant, on the other hand, is more strongly correlated with the peptide sequence than with the charge. Gad-1, which has been shown to be more biologically active than Gad-2, displayed a significantly higher equilibrium binding constant. These observations suggest that while the maximum peptide density on the vesicle surface is limited by electrostatic interactions, the free energy of peptide binding, like the observed antimicrobial activities of the Gad peptides, is also sensitive to other peptide factors which might, for example, influence hydrophobic interactions.

Antimicrobial Peptide Cec4 Eradicates the Bacteria of Clinical Carbapenem-Resistant Acinetobacter baumannii Biofilm

The drug resistance rate of Acinetobacter baumannii increases year on year, and the drugs available for the treatment of carbapenem-resistant A. baumannii (CRAB) infection are extremely limited. A. baumannii, which forms biofilms, protects itself by secreting substrates such as exopolysaccharides, allowing it to survive under adverse conditions and increasing drug resistance. Antimicrobial peptides are small molecular peptides with broad-spectrum antibacterial activity and immunomodulatory function. Previous studies have shown that the antimicrobial peptide Cec4 has a strong effect on A. baumannii, but the antibacterial and biofilm inhibition of this antimicrobial peptide on clinical carbapenem resistance A. baumannii is not thoroughly understood. In this study, it was indicated that most of the 200 strains of CRAB were susceptible to Cec4 with a MIC of 4 μg/ml. Cec4 has a strong inhibitory and eradication effect on the CRAB biofilm; the minimum biofilm inhibition concentration (MBIC) was 64–128 μg/ml, and the minimum biofilm eradication concentration (MBEC) was 256–512 μg/ml. It was observed that Cec4 disrupted the structure of the biofilm using scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). A comparative transcriptome analysis of the effects of the antimicrobial peptide Cec4 on CRAB biofilm, identified 185 differentially expressed genes, including membrane proteins, bacterial resistance genes, and pilus-related genes. The results show that multiple metabolic pathways, two-component regulation systems, quorum sensing, and antibiotic synthesis-related pathways in A. baumannii biofilms were affected after Cec4 treatment. In conclusion, Cec4 may represent a new choice for the prevention and treatment of clinical infections, and may also provide a theoretical basis for the development of antimicrobial peptide drugs.

Highly synergistic antimicrobial activity of magainin 2 and PGLa peptides is rooted in the formation of supramolecular complexes with lipids

Magainin 2 and PGLa are cationic, amphipathic antimicrobial peptides which when added as equimolar mixture exhibit a pronounced synergism in both their antibacterial and pore-forming activities. Here we show for the first time that the peptides assemble into defined supramolecular structures along the membrane interface. The resulting mesophases are quantitatively described by state-of-the art fluorescence self-quenching and correlation spectroscopies. Notably, the synergistic behavior of magainin 2 and PGLa correlates with the formation of hetero-domains and an order-of-magnitude increased membrane affinity of both peptides. Enhanced membrane association of the peptide mixture is only observed in the presence of phophatidylethanolamines but not of phosphatidylcholines, lipids that dominate bacterial and eukaryotic membranes, respectively. Thereby the increased membrane-affinity of the peptide mixtures not only explains their synergistic antimicrobial activity, but at the same time provides a new concept to increase the therapeutic window of combinatorial drugs.