Properties and applications of antimicrobial peptides in biodefense against biological warfare threat agents

Antimicrobial activity of novel symmetrical antimicrobial peptides centered on a hydrophilic motif against resistant clinical isolates: in vitro and in vivo analyses

Antibiotic resistance poses a significant public health threat worldwide. The rise in antibiotic resistance and the sharp decline in effective antibiotics necessitate the development of innovative antibacterial agents. Based on the central symmetric structure of glycine-serine-glycine, combined with tryptophan and arginine, we designed a range of antimicrobial peptides (AMPs) that exhibited broad-spectrum antibacterial activity. Notably, AMP W5 demonstrated a rapid and effective sterilization against methicillin-resistant Staphylococcus aureus (MRSA), displaying both a minimum inhibitory concentration and a minimum bactericidal concentration of 8 µM. Mechanistic studies revealed that AMP W5 killed bacterial cells by disrupting the cytoplasmic membrane integrity, triggering leakage of cell contents. AMP W5 also exhibited excellent biocompatibility in both in vitro and in vivo safety evaluations. AMP W5 treatment significantly reduced skin bacterial load in our murine skin infection model. In conclusion, we designed a novel centrosymmetric AMP representing a promising medical alternative to conventional antibiotics for treating MRSA infections.

IMPORTANCE

Increasing antibiotic resistance and the paucity of effective antibiotics necessitate innovative antibacterial agents. Methicillin-resistant Staphylococcus aureus (MRSA) is a major pathogen causing bacterial infections with high incidence and mortality rates, showing increasing resistance to clinical drugs. Antimicrobial peptides (AMPs) exhibit significant potential as alternatives to traditional antibiotics. This study designed a novel series of AMPs, characterized by a glycine-serine-glycine-centered symmetrical structure, and our results indicated that AMP W5 exhibited a rapid and effective bactericidal effect against MRSA. AMP W5 also demonstrated excellent biocompatibility and a bactericidal mechanism that disrupted membrane integrity, leading to leakage of cellular contents. The notable reduction in skin bacterial load observed in mouse models reinforced the clinical applicability of AMP W5. This study provides a promising solution for addressing the increasing threat of antibiotic-resistant bacteria and heralds new prospects for clinical applications.

Resistance is futile: targeting multidrug-resistant bacteria with de novo Cys-rich cyclic polypeptides

The search for novel antimicrobial agents to combat microbial pathogens is intensifying in response to rapid drug resistance development to current antibiotic therapeutics. The use of disulfide-rich head-to-tail cyclized polypeptides as molecular frameworks for designing a new type of peptide antibiotics is gaining increasing attention among the scientific community and the pharmaceutical industry. The use of macrocyclic peptides, further constrained by the presence of several disulfide bonds, makes these peptide frameworks remarkably more stable to thermal, biological, and chemical degradation showing better activities when compared to their linear analogs. Many of these novel peptide scaffolds have been shown to have a high tolerance to sequence variability in those residues not involved in disulfide bonds, able to cross biological membranes, and efficiently target complex biomolecular interactions. Hence, these unique properties make the use of these scaffolds ideal for many biotechnological applications, including the design of novel peptide antibiotics. This article provides an overview of the new developments in the use of several disulfide-rich cyclic polypeptides, including cyclotides, θ-defensins, and sunflower trypsin inhibitor peptides, among others, in the development of novel antimicrobial peptides against multidrug-resistant bacteria.

Antibacterial and Antifungal Properties of a Novel Antimicrobial Peptide GK-19 and Its Application in Skin and Soft Tissue Infections Induced by MRSA or Candida albicans

The increasing resistance of human pathogens promotes the development of novel antimicrobial agents. Due to the physical bactericidal mechanism of membrane disruption, antimicrobial peptides are considered as potential therapeutic candidates without inducing microbial resistance. Scorpion venom-derived peptide, Androctonus amoreuxi Antimicrobial Peptide 1 (AamAP1), has been proved to have broad-spectrum antimicrobial properties. However, AamAP1 can induce hemolysis and shows strong toxicity against mammalian cells. Herein, the antimicrobial activity and mechanism of a novel synthetic antimicrobial peptide, GK-19, derived from AamAP1 and its derivatives, was evaluated. Five bacteria and three fungi were used to evaluate the antimicrobial effects of GK-19 in vitro. Scalded mice models combined with skin and soft tissue infections (SSTIs) were used to evaluate its applicability. The results indicated that GK-19 could not only inhibit Gram-positive and Gram-negative bacterial growth, but also kill fungi by disrupting the microbial cell membrane. Meanwhile, GK-19 showed negligible toxicity to mammalian cells, low hemolytic activity and high stability in plasma. Furthermore, in scalded mice models combined with SSTIs induced by either Methicillin-Resistant Staphylococcus aureus (MRSA) or Candida albicans, GK-19 showed significant antimicrobial and healing effects. Overall, it was demonstrated that GK-19 might be a promising drug candidate in the battle against drug-resistant bacterial and fungal infections.

Antibiofilm peptides as a promising strategy: comparative research

Biofilms lead to approximately 65% of infections, and these infections are hard to treat. Thus, it is crucial to identify effective antibiofilm agents with low cytotoxicity. Peptides with antibiofilm activity have been regarded as promising solutions, and peptides with MBICs (minimal biofilm inhibitory concentrations) that are lower than their minimal inhibitory concentration (MICs) (minimal inhibitory concentrations) are appealing. Therefore, we systematically summarized and classified previously reported peptides with antibiofilm activity. A total of 51 peptides with antibiofilm activity were classified into 14 categories. The MICs and MBICs of these fourteen representative peptides, one selected from each category, were compared against the Gram-positive bacterium Streptococcus mutans, the Gram-negative bacterium Pseudomonas aeruginosa, and the fungus Candida albicans. Six representative peptides (C5-pleurocidin, C6-Pac-525, C9-protegrin-1, C11-TetraF2W-RR, C13-WLBU2, and C14-melittin) showed antibiofilm activity against both bacteria and fungi, and among these 6 representative peptides, 4 peptides (C9-protegrin-1, C11-TetraF2W-RR, C13-WLBU2, and C14-melittin) could prevent biofilm formation with lower MBIC values than their MICs. CLSM (confocal laser scanning microscopy), SEM (scanning electron microscopy), and TEM (transmission electron microscopy) were further used to observe the morphologies of the biofilms after treatment with the peptides. Among the above 4 peptides, WLBU2 and melittin sparsely scattered the biofilms without destroying the bacteria. In conclusion, the currently reported peptides with antibiofilm activity are limited in number, but peptides with lower MBICs than MICs exist as promising candidates against biofilm-related infections and need further study. KEY POINTS: • Antibiofilm peptides could inhibit biofilm formation with MBICs that are lower than MICs. • The mechanism of antibiofilm peptides is not only due to antimicrobial activity.

HAPPENN is a novel tool for hemolytic activity prediction for therapeutic peptides which employs neural networks

The growing prevalence of resistance to antibiotics motivates the search for new antibacterial agents. Antimicrobial peptides are a diverse class of well-studied membrane-active peptides which function as part of the innate host defence system, and form a promising avenue in antibiotic drug research. Some antimicrobial peptides exhibit toxicity against eukaryotic membranes, typically characterised by hemolytic activity assays, but currently, the understanding of what differentiates hemolytic and non-hemolytic peptides is limited. This study leverages advances in machine learning research to produce a novel artificial neural network classifier for the prediction of hemolytic activity from a peptide's primary sequence. The classifier achieves best-in-class performance, with cross-validated accuracy of [Formula: see text] and Matthews correlation coefficient of 0.71. This innovative classifier is available as a web server at https://research.timmons.eu/happenn , allowing the research community to utilise it for in silico screening of peptide drug candidates for high therapeutic efficacies.

Cell Membrane-Interrupting Antimicrobial Peptides from Isatis indigotica Fortune Isolated by a Bacillus subtilis Expression System

The situation of drug resistance has become more complicated due to the scarcity of plant resistance genes, and overcoming this challenge is imperative. Isatis indigotica has been used for the treatment of wounds, viral infections, and inflammation for centuries. Antimicrobial peptides (AMPs) are found in all classes of life ranging from prokaryotes to eukaryotes. To identify AMPs, I. indigotica was explored using a novel, sensitive, and high-throughput Bacillus subtilis screening system. We found that IiR515 and IiR915 exhibited significant antimicrobial activities against a variety of bacterial (Xanthomonas oryzae, Ralstonia solanacearum, Clavibacter michiganensis, and C. fangii) and fungal (Phytophthora capsici and Botrytis cinerea) pathogens. Scanning electron microscope and cytometric analysis revealed the possible mechanism of these peptides, which was to target and disrupt the bacterial cell membrane. This model was also supported by membrane fluidity and electrical potential analyses. Hemolytic activity assays revealed that these peptides may act as a potential source for clinical medicine development. In conclusion, the plant-derived novel AMPs IiR515 and IiR915 are effective biocontrol agents and can be used as raw materials in the drug discovery field.

LFchimera protects HeLa cells from invasion by Yersinia spp. in vitro

Yersinia pestis is the causative agent of plague. As adequate antibiotic treatment falls short and currently no effective vaccine is available, alternative therapeutic strategies are needed. In order to contribute to solving this problem we investigated the therapeutic potential of the peptide construct LFchimera against the safer-to-handle Y. pestis simulants Yersinia enterocolitica and Yersinia pseudotuberculosis in vitro. LFchimera is a heterodimeric peptide construct mimicking two antimicrobial domains of bovine lactoferrin, i.e. lactoferrampin and lactoferricin. LFchimera has been shown to be a potent antimicrobial peptide against a variety of bacteria in vitro and in vivo. Also Y. enterocolitica and Y. pseudotuberculosis have been shown to be susceptible for LFchimera in vitro. As Yersiniae spp. adhere to and invade host cells upon infection, we here investigated the effects of LFchimera on these processes. It was found that LFchimera has the capacity to inhibit host-cell invasion by Yersiniae spp. in vitro. This effect appeared to be host-cell mediated, not bacteria-mediated. Furthermore it was found that exposure of human HeLa epithelial cells to both LFchimera and the bacterial strains evoked a pro-inflammatory cytokine release from the cells in vitro.

LyeTxI-b, a Synthetic Peptide Derived From Lycosa erythrognatha Spider Venom, Shows Potent Antibiotic Activity in Vitro and in Vivo

The antimicrobial peptide LyeTxI isolated from the venom of the spider Lycosa erythrognatha is a potential model to develop new antibiotics against bacteria and fungi. In this work, we studied a peptide derived from LyeTxI, named LyeTxI-b, and characterized its structural profile and its in vitro and in vivo antimicrobial activities. Compared to LyeTxI, LyeTxI-b has an acetylated N-terminal and a deletion of a His residue, as structural modifications. The secondary structure of LyeTxI-b is a well-defined helical segment, from the second amino acid to the amidated C-terminal, with no clear partition between hydrophobic and hydrophilic faces. Moreover, LyeTxI-b shows a potent antimicrobial activity against Gram-positive and Gram-negative planktonic bacteria, being 10-fold more active than the native peptide against Escherichia coli. LyeTxI-b was also active in an in vivo model of septic arthritis, reducing the number of bacteria load, the migration of immune cells, the level of IL-1β cytokine and CXCL1 chemokine, as well as preventing cartilage damage. Our results show that LyeTxI-b is a potential therapeutic model for the development of new antibiotics against Gram-positive and Gram-negative bacteria.

Perspectives for clinical use of engineered human host defense antimicrobial peptides

Infectious diseases caused by bacteria, viruses or fungi are among the leading causes of death worldwide. The emergence of drug-resistance mechanisms, especially among bacteria, threatens the efficacy of all current antimicrobial agents, some of them already ineffective. As a result, there is an urgent need for new antimicrobial drugs. Host defense antimicrobial peptides (HDPs) are natural occurring and well-conserved peptides of innate immunity, broadly active against Gram-negative and Gram-positive bacteria, viruses and fungi. They also are able to exert immunomodulatory and adjuvant functions by acting as chemotactic for immune cells, and inducing cytokines and chemokines secretion. Moreover, they show low propensity to elicit microbial adaptation, probably because of their non-specific mechanism of action, and are able to neutralize exotoxins and endotoxins. HDPs have the potential to be a great source of novel antimicrobial agents. The goal of this review is to provide an overview of the advances made in the development of human defensins as well as the cathelicidin LL-37 and their derivatives as antimicrobial agents against bacteria, viruses and fungi for clinical use.

Effects of lactoferrin derived peptides on simulants of biological warfare agents

Lactoferrin (LF) is an important immune protein in neutrophils and secretory fluids of mammals. Bovine LF (bLF) harbours two antimicrobial stretches, lactoferricin and lactoferampin, situated in close proximity in the N1 domain. To mimic these antimicrobial domain parts a chimeric peptide (LFchimera) has been constructed comprising parts of both stretches (LFcin17–30 and LFampin265–284). To investigate the potency of this construct to combat a set of Gram positive and Gram negative bacteria which are regarded as simulants for biological warfare agents, the effect on bacterial killing, membrane permeability and membrane polarity were determined in comparison to the constituent peptides and the native bLF. Furthermore we aimed to increase the antimicrobial potency of the bLF derived peptides by cationic amino acid substitutions. Overall, the bactericidal activity of the peptides could be related to membrane disturbing effects, i.e. membrane permeabilization and depolarization. Those effects were most prominent for the LFchimera. Arginine residues were found to be crucial for displaying antimicrobial activity, as lysine to arginine substitutions resulted in an increased antimicrobial activity, affecting mostly LFampin265–284 whereas arginine to lysine substitutions resulted in a decreased bactericidal activity, predominantly in case of LFcin17–30.

Piscidin: Antimicrobial peptide of rock bream, Oplegnathus fasciatus

The piscidin family consists of antimicrobial peptides (AMPs) that are mainly found in fish and are crucial effectors of fish innate immune responses. The piscidin family typically has broad-spectrum antimicrobial activity and can modulate immune responses. In this study, we cloned rock bream piscidin (Rbpisc) and investigated its gene expression and biological activity (including antimicrobial and cytotoxic activities). The coding region of Rbpisc consisted of 213 base pairs (bp) encoding 70 amino acid residues. The tertiary structure predicted for Rbpisc includes an amphipathic helix-loop-helix structure. The Rbpisc gene was highly expressed in the gills of healthy fish. The gene expression of Rbpisc increased in the gills after pathogen infection, while the expression was down-regulated in other tissues. A synthetic peptide based on the AMP 12 domain amino acid sequence of Rbpisc appeared to have broad-spectrum antimicrobial activity against various bacteria. However, the synthetic peptide exhibited weak haemolytic activity against fish erythrocytes. These results suggest that Rbpisc might play an important role in the innate immune responses of rock bream.

AaeAP1 and AaeAP2: novel antimicrobial peptides from the venom of the scorpion, Androctonus aeneas: structural characterisation, molecular cloning of biosynthetic precursor-encoding cDNAs and engineering of analogues with enhanced antimicrobial and anticancer activities

The main functions of the abundant polypeptide toxins present in scorpion venoms are the debilitation of arthropod prey or defence against predators. These effects are achieved mainly through the blocking of an array of ion channel types within the membranes of excitable cells. However, while these ion channel-blocking toxins are tightly-folded by multiple disulphide bridges between cysteine residues, there are additional groups of peptides in the venoms that are devoid of cysteine residues. These non-disulphide bridged peptides are the subject of much research interest, and among these are peptides that exhibit antimicrobial activity. Here, we describe two novel non-disulphide-bridged antimicrobial peptides that are present in the venom of the North African scorpion, Androctonus aeneas. The cDNAs encoding the biosynthetic precursors of both peptides were cloned from a venom-derived cDNA library using 3'- and 5'-RACE strategies. Both translated precursors contained open-reading frames of 74 amino acid residues, each encoding one copy of a putative novel nonadecapeptide, whose primary structures were FLFSLIPSVIAGLVSAIRN and FLFSLIPSAIAGLVSAIRN, respectively. Both peptides were C-terminally amidated. Synthetic versions of each natural peptide displayed broad-spectrum antimicrobial activities, but were devoid of antiproliferative activity against human cancer cell lines. However, synthetic analogues of each peptide, engineered for enhanced cationicity and amphipathicity, exhibited increases in antimicrobial potency and acquired antiproliferative activity against a range of human cancer cell lines. These data clearly illustrate the potential that natural peptide templates provide towards the design of synthetic analogues for therapeutic exploitation.

Expression analysis and biological activity of moronecidin from rock bream, Oplegnathus fasciatus

The piscidin-family, one of antimicrobial peptides (AMPs) mainly distributed in fish, is crucial effectors of fish innate immune response. Piscidin-family typically has broad-spectrum antimicrobial activity and the ability to modulate the immune response. In this study, we identified moronecidin (Rbmoro) included in piscidin-family from rock bream and investigated its gene expression using quantitative real-time PCR and biological activity (including antimicrobial and cytotoxic activity). The coding region of Rbmoro was 204 bp encoding 67 amino acid residues. Tertiary structure prediction of Rbmoro showed an amphipathic α-helical structure. Rbmoro gene was widely expressed in different tissues of healthy fish. Additionally, Rbmoro gene expression was induced in all tested tissues after infection with Edwardsiella tarda, Streptococcus iniae and red seabream iridovirus. We synthesized mature peptide of Rbmoro based on amino acid sequence of its AMP 12 domain, and the synthetic peptide appeared broad-spectrum antimicrobial activity to various bacteria. However, the synthetic peptide has weak haemolytic activity against fish erythrocytes. These results suggest that Rbmoro might play an important role in innate immune response of rock bream.

Optical and dielectric sensors based on antimicrobial peptides for microorganism diagnosis

Antimicrobial peptides (AMPs) are natural compounds isolated from a wide variety of organisms that include microorganisms, insects, amphibians, plants, and humans. These biomolecules are considered as part of the innate immune system and are known as natural antibiotics, presenting a broad spectrum of activities against bacteria, fungi, and/or viruses. Technological innovations have enabled AMPs to be utilized for the development of novel biodetection devices. Advances in nanotechnology, such as the synthesis of nanocomposites, nanoparticles, and nanotubes have permitted the development of nanostructured platforms with biocompatibility and greater surface areas for the immobilization of biocomponents, arising as additional tools for obtaining more efficient biosensors. Diverse AMPs have been used as biological recognition elements for obtaining biosensors with more specificity and lower detection limits, whose analytical response can be evaluated through electrochemical impedance and fluorescence spectroscopies. AMP-based biosensors have shown potential for applications such as supplementary tools for conventional diagnosis methods of microorganisms. In this review, conventional methods for microorganism diagnosis as well new strategies using AMPs for the development of impedimetric and fluorescent biosensors are highlighted. AMP-based biosensors show promise as methods for diagnosing infections and bacterial contaminations as well as applications in quality control for clinical analyses and microbiological laboratories.

Design of short membrane selective antimicrobial peptides containing tryptophan and arginine residues for improved activity, salt-resistance, and biocompatibility

Antimicrobial peptides (AMPs) kill microbes by non-specific membrane permeabilization, making them ideal templates for designing novel peptide-based antibiotics that can combat multi-drug resistant pathogens. For maximum efficacy in vivo and in vitro, AMPs must be biocompatible, salt-tolerant and possess broad-spectrum antimicrobial activity. These attributes can be obtained by rational design of peptides guided by good understanding of peptide structure-function. Toward this end, this study investigates the influence of charge and hydrophobicity on the activity of tryptophan and arginine rich decamer peptides engineered from a salt resistant human β-defensin-28 variant. Mechanistic investigations of the decamers with detergents mimicking the composition of bacterial and mammalian membrane, reveal a correlation between improved antibacterial activity and the increase in tryptophan and positive residue content, while keeping hemolysis low. The potent antimicrobial activity and high cell membrane selective behavior of the two most active decamers, D5 and D6, are attributed to an optimum peptide charge to hydrophobic ratio bestowed by systematic arginine and tryptophan substitution. D5 and D6 show surface localization behavior with binding constants of 1.86 × 10(8) and 2.6 × 10(8)  M(-1) , respectively, as determined by isothermal calorimetry measurements. NMR derived structures of D5 and D6 in SDS detergent micelles revealed proximity of Trp and Arg residues in an extended structural scaffold. Such potential cation-π interactions may be critical in cell permeabilization of the AMPs. The fundamental characterization of the engineered decamers provided in this study improves the understanding of structure-activity relationship of short arginine tryptophan rich AMPs, which will pave the way for future de novo design of potent AMPs for therapeutic and biomedical applications.

Transcriptomic and phenotypic analysis of paralogous spx gene function in Bacillus anthracis Sterne

Spx of Bacillus subtilis is a redox-sensitive protein, which, under disulfide stress, interacts with RNA polymerase to activate genes required for maintaining thiol homeostasis. Spx orthologs are highly conserved among low %GC Gram-positive bacteria, and often exist in multiple paralogous forms. In this study, we used B. anthracis Sterne, which harbors two paralogous spx genes, spxA1 and spxA2, to examine the phenotypes of spx null mutations and to identify the genes regulated by each Spx paralog. Cells devoid of spxA1 were sensitive to diamide and hydrogen peroxide, while the spxA1 spoxA2 double mutant was hypersensitive to the thiol-specific oxidant, diamide. Bacillus anthracis Sterne strains expressing spxA1DD or spxA2DD alleles encoding protease-resistant products were used in microarray and quantitative real-time polymerase chain reaction (RT-qPCR) analyses in order to uncover genes under SpxA1, SpxA2, or SpxA1/SpxA2 control. Comparison of transcriptomes identified many genes that were upregulated when either SpxA1DD or SpxA2DD was produced, but several genes were uncovered whose transcript levels increased in only one of the two SpxADD-expression strains, suggesting that each Spx paralog governs a unique regulon. Among genes that were upregulated were those encoding orthologs of proteins that are specifically involved in maintaining intracellular thiol homeostasis or alleviating oxidative stress. Some of these genes have important roles in B. anthracis pathogenesis, and a large number of upregulated hypothetical genes have no homology outside of the B. cereus/thuringiensis group. Microarray and RT-qPCR analyses also unveiled a regulatory link that exists between the two spx paralogous genes. The data indicate that spxA1 and spxA2 are transcriptional regulators involved in relieving disulfide stress but also control a set of genes whose products function in other cellular processes.

Bacillus anthracis harbors two paralogs of the global transcriptional regulator of stress response, SpxA. SpxA1 and SpxA2 contribute to disulfide stress tolerance, but only SpxA1 functions in resistance to peroxide. Transcriptome analysis uncovered potential SpxA1 and SpxA2 regulon members, which include genes activated by both paralogs. However, paralog-specific gene activation was also observed. Genes encoding glutamate racemase, CoA disulfide reductase, and products functioning in bacillithiol biosynthesis, are among the genes activated by the SpxA paralogs.

Cloning, expression and characterization of antimicrobial porcine β defensin 1 in Escherichia coli

Porcine β defensin 1 (pBD1) is a cationic antimicrobial peptide with three pairs of disulfide bonds. When expressed in insect cells, two polypeptides of different length (pBD1(38) and pBD1(42)) accumulated, which differed by N-terminal truncation. However, only pBD1(42) was found in pigs. pBD1(42) had stronger antimicrobial activity than pBD1(38), and thus could be a good candidate as a bactericidal agent for pigs. In this study, pBD1(42) gene, obtained by RT-PCR using the tongue total RNA as a template, was cloned into pET30a expression vector and transformed into Escherichia coli BL21 (DE3) plysS. The recombinant pBD1(42) was expressed after induction by IPTG and purified by His tag affinity column with 90% purity. The recombinant pBD1(42) exhibited antimicrobial activity against both Gram-positive Staphylococcus aureus and Gram-negative E. coli including the multi-resistant E. coli. The minimum inhibitory concentrations (MICs) of recombinant pBD1(42) against tested bacteria were 100 μg/mL for E. coli and 80 μg/mL for S. aureus. In addition, pBD1(42) showed low hemolytic activity and high thermal stability. These properties are relevant for the biotechnological applications of the peptide.

Panning of a phage display library against a synthetic capsule for peptide ligands that bind to the native capsule of Bacillus anthracis

Bacillus anthracis is the causative agent of anthrax with the ability to not only produce a tripartite toxin, but also an enveloping capsule comprised primarily of γ-D-glutamic acid residues. The purpose of this study was to isolate peptide ligands capable of binding to the native capsule of B. anthracis from a commercial phage display peptide library using a synthetic form of the capsule consisting of 12 γ-D-glutamic acid residues. Following four rounds of selection, 80 clones were selected randomly and analysed by DNA sequencing. Four clones, each containing a unique consensus sequence, were identified by sequence alignment analysis. Phage particles were prepared and their derived 12-mer peptides were also chemically synthesized and conjugated to BSA. Both the phage particles and free peptide-BSA conjugates were evaluated by ELISA for binding to encapsulated cells of B. anthracis as well as a B. anthracis capsule extract. All the phage particles tested except one were able to bind to both the encapsulated cells and the capsule extract. However, the peptide-BSA conjugates could only bind to the encapsulated cells. One of the peptide-BSA conjugates, with the sequence DSSRIPMQWHPQ (termed G1), was fluorescently labelled and its binding to the encapsulated cells was further confirmed by confocal microscopy. The results demonstrated that the synthetic capsule was effective in isolating phage-displayed peptides with binding affinity for the native capsule of B. anthracis.

Bacterial resistance to cationic antimicrobial peptides

Naturally occurring cationic antimicrobial peptides (CAMPs) have been considered as promising candidates to treat infections caused by pathogenic bacteria to animals and humans. This assumption is based on their mechanism of action, which is mainly performed through electrostatic membrane interactions. Unfortunately, the rise in the reports that describe bacterial resistance to CAMPs has redefined their role as therapeutic agents. In this review, we describe the state of the art of the most common resistance mechanisms developed by bacteria to CAMPs, making special emphasis on resistance selection. Considering most of the resistance mechanisms here reviewed, the emergence of resistance is unlikely in the short term, however we also described evidences that show the evolution of resistance to CAMPs, reevaluating their use as good antibacterial agents. Finally, the knowledge related to the description of CAMP resistance mechanisms may provide useful information for improving strategies to control infections.

Chemical synthesis and biological evaluation of an antimicrobial peptide gonococcal growth inhibitor

Gonococcal growth inhibitor 1 (GGI-1) is a 44-residue peptide with potent anti-Legionella activity. It has been isolated from Staphylococcus haemolyticus but, to date, its chemical synthesis has not been reported. Acquisition of this peptide via this means would enable a more detailed examination of its antimicrobial properties. However, its synthesis represents a significant challenge because of two predicted "difficult sequences" within the peptide. Its successful solid-phase assembly is reported in this paper, and was accomplished by use of simple palliative measures including the introduction of a single pseudo-proline isostere in order to counteract on-resin aggregation. The peptide had moderate antimicrobial activity against Escherichia coli but was inactive against another Gram-negative bacterium and two Gram-positive bacteria (Bacillus species). It had significant haemolytic activity, with a H(50) (concentration of peptide that causes 50 % haemolysis) of 20 and 125 μM for two blood samples from different donors. An alternative therapeutic index to that proposed for GGI-1 in a recent publication is proposed.

State-of-the-art therapeutic medical countermeasures for viral threat agents

In recent years, there has been an increase in the perceived threat of biological agents being used against civilian populations. This has prompted an urgent need for the development and procurement of medical countermeasures (MCMs) against highly pathogenic viruses that can prevent morbidity and mortality from infections caused by these agents. To date, antiviral drug development has been largely focused on clinically prevalent chronic infections due to their commercial viability. This has left a huge gap in the drug development path for acute infections of biodefense importance. In this review, we discuss the antiviral research and development initiatives focusing specifically on poxviruses, filoviruses, and equine encephalitis viruses (EEV). We discuss the benefits and technical challenges in the current development strategies and the hurdles in the licensure path for MCMs against these highly pathogenic viruses under the FDA Animal Rule, and we provide recommendations for the path forward.

C- and N-truncated antimicrobial peptides from LFampin 265 - 284: Biophysical versus microbiology results

Lactoferrin is a glycoprotein with two globular lobes, each having two domains. Since the discovery of its antimicrobial properties, efforts have been made to find peptides derived from this protein showing antimicrobial properties. Most peptides initially studied were derived from Lactoferricin B, obtained from the protein by digestion with pepsin. More recently, a new family of antimicrobial peptides (AMPs) derived from Lactoferrin was discovered by Bolcher et al, and named Lactoferrampin (LFampin). The original sequence of LFampin contained residues 268 – 284 from the N1 domain of Lactoferrin. From this peptide, the Bolscher’s group synthesized a collection of peptides obtained by extension and / or truncation at the C or N-terminal sides, in order to unravel the main structural features responsible for antimicrobial action. Here, we present results for three of these peptides, namely LFampin 265 – 284, LFampin 265 — 280, and LFampin 270 – 284. The peptides were tested against bacteria (E. coli and S. sanguinis), fungi (C. albicans), and model membranes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPG), and their mixtures at a ratio of 3 : 1 (DMPC : DMPG (3 : 1)). The ability to adopt a helical conformation was followed by a circular dichroism (CD), and the perturbation of the gel to the liquid-crystalline phase transition of the membrane was characterized by differential scanning calorimetry (DSC). Distinct behavior was observed in the three peptides, both from the microbiology and model membrane studies, with the biophysical results showing excellent correlation with the microbiology activity studies. LFampin 265 – 284 was the most active peptide toward the tested microorganisms, and in the biophysical studies it showed the highest ability to form an α-helix and the strongest interaction with model membranes, followed by LFampin 265 – 280. LFampin 270 – 284 was inactive, showing marginal secondary structure and no interaction with the pathogen model membranes.

Potent antimicrobial peptides with selectivity for Bacillus anthracis over human erythrocytes

In this study, 39 antimicrobial peptides, most with documented low haemolytic activity and potent efficacy against Gram-negative and Gram-positive bacteria, were evaluated for their haemolytic activity against human red blood cells as well as their antimicrobial activity against Escherichia coli, Burkholderia thailandensis, Bacillus globigii and Bacillus anthracis. The majority of the peptides had a minimum inhibitory concentration (MIC) of <10 μM against B. globigii. However, only eight of these (CaLL, Ci-MAM-A24, LLaMA, Ltc2a, OV-5, papillosin, smapspin and smapspin-G) had a MIC<10 μM against B. anthracis. All except one (papillosin) were ineffective at 100 μM against B. thailandensis and none had potent activity against E. coli. Potent activity against B. anthracis was associated with significant haemolytic activity, but the ratio of the concentration of peptide that caused 50% haemolysis to the concentration that inhibited growth of B. anthracis by 50% (the therapeutic index) varied from 0.8 to 34.2. Two peptides (papillosin and Ltc2a) had a therapeutic index >30 and could be considered as candidates for further development for potential medical countermeasures against anthrax. Although B. globigii has often been used as a non-pathogenic simulant for B. anthracis, in this study it was found that the sensitivity of B. globigii to peptides was not a reliable predictor of the sensitivity of B. anthracis to the same peptides.

Using zeta-potential measurements to quantify peptide partition to lipid membranes

Many cellular phenomena occur on the biomembranes. There are plenty of molecules (natural or xenobiotics) that interact directly or partially with the cell membrane. Biomolecules, such as several peptides (e.g., antimicrobial peptides) and proteins, exert their effects at the cell membrane level. This feature makes necessary investigating their interactions with lipids to clarify their mechanisms of action and side effects necessary. The determination of molecular lipid/water partition constants (K_p) is frequently used to quantify the extension of the interaction. The determination of this parameter has been achieved by using different methodologies, such as UV-Vis absorption spectrophotometry, fluorescence spectroscopy and ζ-potential measurements. In this work, we derived and tested a mathematical model to determine the K_p from ζ-potential data. The values obtained with this method were compared with those obtained by fluorescence spectroscopy, which is a regular technique used to quantify the interaction of intrinsically fluorescent peptides with selected biomembrane model systems. Two antimicrobial peptides (BP100 and pepR) were evaluated by this new method. The results obtained by this new methodology show that ζ-potential is a powerful technique to quantify peptide/lipid interactions of a wide variety of charged molecules, overcoming some of the limitations inherent to other techniques, such as the need for fluorescent labeling.

rBPI(21) promotes lipopolysaccharide aggregation and exerts its antimicrobial effects by (hemi)fusion of PG-containing membranes

Antimicrobial peptides (AMPs) are important potential alternatives to conventional therapies against bacterial infections. rBPI(21) is a 21 kDa peptide based on the N-terminal region of the neutrophil bactericidal/permeability-increasing protein (BPI). This AMP possesses highly selective bactericidal effects on Gram-negative bacteria and have affinity for lipopolysaccharide (LPS) which is believed to be at the origin of its neutralizing effect of the LPS segregated into the bloodstream. We aim at understanding the molecular bases of rBPI(21) bactericidal and LPS neutralization actions, using biomembrane model systems. Using dynamic light scattering spectroscopy we demonstrate that rBPI(21) promotes aggregation of negatively charged large unilamellar vesicles (LUV), even in the absence of LPS, and LPS aggregates, while for zwitterionic phosphatidylcholine (POPC) LUV the size remains unchanged. The peptide also promotes the fusion (or hemifusion) of membranes containing phosphatidylglycerol (POPG). The aggregation and fusion of negatively charged LUV are peptide concentration-dependent until massive aggregation is reached, followed by sample flocculation/precipitation. Concomitantly, there is a progressive change in the zeta-potential of the LUV systems and LPS aggregates. LUV systems composed of phosphatidylglycerol (POPG) and lipid mixtures with POPG have higher zeta-potential variations than in the absence of POPG. The interaction of rBPI(21) with lipid vesicles is followed by leakage, with higher effect in POPG-containing membranes. LPS aggregation can be related with a decreased toxicity, possibly by facilitating its clearance by macrophage phagocytosis and/or blocking of LPS specific receptor recognition. Our data indicate that rBPI(21) mechanism of action at the molecular level involves the interaction with the LPS of the outer membrane of Gram-negative bacteria, followed by internalization and leakage induction through the (hemi)fusion of the bacterial outer and inner membranes, both enriched in phosphatidylglycerol.

Design of antimicrobially active small amphiphilic peptide dendrimers

Novel polyfunctional small amphiphilic peptide dendrimers characterized by incorporation of a new core compounds – tris-amino acids or tetrakis-amino alcohols that originated from a series of basic amino acids – were efficiently synthesized. These new core elements yielded molecules with multiple branching and (+5)/(+6) charge at the 1-st dendrimer generation. Dendrimers exhibited significant antimicrobial potency against Gram(+) and Gram(-) strains involving also multiresistant reference strains (S. aureus ATCC 43300 and E. coli ATCC BAA-198). In addition, high activity against fungi from the Candida genus was detected. More charged and more hydrophobic peptide dendrimers expressed hemolytic properties.

On the physiology and pathophysiology of antimicrobial peptides

Antimicrobial peptides (AMP) are a heterogeneous group of molecules involved in the nonspecific immune responses of a variety of organisms ranging from prokaryotes to mammals, including humans. AMP have various physical and biological properties, yet the most common feature is their antimicrobial effect. The majority of AMP disrupt the integrity of microbial cells by 1 of 3 known mechanisms--the barrel-stave pore model, the thoroidal pore model, or the carpet model. Results of growing numbers of descriptive and experimental studies show that altered expression of AMP in various tissues is important in the pathogenesis of several gastrointestinal, respiratory, and other diseases. We discuss novel approaches and strategies to further improve the promising future of therapeutic applications of AMP. The spread of antibiotic resistance increases the importance of developing a clinical role for AMP.