An exceptional salt-tolerant antimicrobial peptide derived from a novel gene family of haemocytes of the marine invertebrate Ciona intestinalis

Modes of action and potential as a peptide-based biofungicide of a plant defensin MtDef4

Due to rapidly emerging resistance to single-site fungicides in fungal pathogens of plants, there is a burgeoning need for safe and multisite fungicides. Plant antifungal peptides with multisite modes of action (MoA) have potential as bioinspired fungicides. Medicago truncatula defensin MtDef4 was previously reported to exhibit potent antifungal activity against fungal pathogens. Its MoA involves plasma membrane disruption and binding to intracellular targets. However, specific biochemical processes inhibited by this defensin and causing cell death have not been determined. Here, we show that MtDef4 exhibited potent antifungal activity against Botrytis cinerea. It induced severe plasma membrane and organelle irregularities in the germlings of this pathogen. It bound to fungal ribosomes and inhibited protein translation in vitro. A MtDef4 variant lacking antifungal activity exhibited greatly reduced protein translation inhibitory activity. A cation-tolerant MtDef4 variant was generated that bound to β-glucan of the fungal cell wall with higher affinity than MtDef4. It also conferred a greater reduction in the grey mould disease symptoms than MtDef4 when applied exogenously on Nicotiana benthamiana plants, tomato fruits and rose petals. Our findings revealed inhibition of protein synthesis as a likely target of MtDef4 and the potential of its cation-tolerant variant as a peptide-based fungicide.

Embracing the era of antimicrobial peptides with marine organisms

Covering: 2018 to Jun of 2023The efficiency of traditional antibiotics has been undermined by the proliferation of antibiotic-resistant pathogenic microorganisms, necessitating the pursuit of innovative therapeutic agents. Antimicrobial peptides (AMPs), which are part of host defence peptides found ubiquitously in nature, exhibiting a wide range of activity towards bacteria, fungi, and viruses, offer a highly promising candidate solution. The efficacy of AMPs can frequently be augmented via alterations to their amino acid sequences or structural adjustments. Given the vast reservoir of marine life forms and their distinctive ecosystems, marine AMPs stand as a burgeoning focal point in the quest for alternative peptide templates extracted from natural sources. Advances in identification and characterization techniques have accelerated the discoveries of marine AMPs, thereby stimulating AMP customization, optimization, and synthesis research endeavours. This review presents an overview of recent discoveries related to the intriguing qualities of marine AMPs. Emphasis will be placed upon post-translational modifications (PTMs) of marine AMPs and how they may impact functionality and potency. Additionally, this review considers ways in which marine PTM might support larger-scale, heterologous AMP manufacturing initiatives, providing insights into translational applications of these important biomolecules.

A Novel Hemocyte-Derived Peptide and Its Possible Roles in Immune Response of Ciona intestinalis Type A

A wide variety of bioactive peptides have been identified in the central nervous system and several peripheral tissues in the ascidian Ciona intestinalis type A (Ciona robusta). However, hemocyte endocrine peptides have yet to be explored. Here, we report a novel 14-amino-acid peptide, CiEMa, that is predominant in the granular hemocytes and unilocular refractile granulocytes of Ciona. RNA-seq and qRT-PCR revealed the high CiEma expression in the adult pharynx and stomach. Immunohistochemistry further revealed the highly concentrated CiEMa in the hemolymph of the pharynx and epithelial cells of the stomach, suggesting biological roles in the immune response. Notably, bacterial lipopolysaccharide stimulation of isolated hemocytes for 1-4 h resulted in 1.9- to 2.4-fold increased CiEMa secretion. Furthermore, CiEMa-stimulated pharynx exhibited mRNA upregulation of the growth factor (Fgf3/7/10/22), vanadium binding proteins (CiVanabin1 and CiVanabin3), and forkhead and homeobox transcription factors (Foxl2, Hox3, and Dbx) but not antimicrobial peptides (CrPap-a and CrMam-a) or immune-related genes (Tgfbtun3, Tnfa, and Il17-2). Collectively, these results suggest that CiEMa plays roles in signal transduction involving tissue development or repair in the immune response, rather than in the direct regulation of immune response genes. The present study identified a novel Ciona hemocyte peptide, CiEMa, which paves the way for research on the biological roles of hemocyte peptides in chordates.

Bioactive Molecules from the Innate Immunity of Ascidians and Innovative Methods of Drug Discovery: A Computational Approach Based on Artificial Intelligence

The study of bioactive molecules of marine origin has created an important bridge between biological knowledge and its applications in biotechnology and biomedicine. Current studies in different research fields, such as biomedicine, aim to discover marine molecules characterized by biological activities that can be used to produce potential drugs for human use. In recent decades, increasing attention has been paid to a particular group of marine invertebrates, the Ascidians, as they are a source of bioactive products. We describe omics data and computational methods relevant to identifying the mechanisms and processes of innate immunity underlying the biosynthesis of bioactive molecules, focusing on innovative computational approaches based on Artificial Intelligence. Since there is increasing attention on finding new solutions for a sustainable supply of bioactive compounds, we propose that a possible improvement in the biodiscovery pipeline might also come from the study and utilization of marine invertebrates' innate immunity.

Tunicates as Sources of High-Quality Nutrients and Bioactive Compounds for Food/Feed and Pharmaceutical Applications: A Review

Tunicates are widely distributed worldwide and are recognized as abundant marine bioresources with many potential applications. In this review, state-of-the-art studies on chemical composition analyses of various tunicate species were summarized; these studies confirmed that tunicates contain nutrients similar to fish (such as abundant cellulose, protein, and ω-3 fatty acid (FA)-rich lipids), indicating their practical and feasible uses for food or animal feed exploration. However, the presence of certain toxic elements should be evaluated in terms of safety. Moreover, recent studies on bioactive substances extracted from tunicates (such as toxins, sphingomyelins, and tunichromes) were analyzed, and their biological properties were comprehensively reviewed, including antimicrobial, anticancer, antioxidant, antidiabetic, and anti-inflammatory activities. In addition, some insights and prospects for the future exploration of tunicates are provided which are expected to guide their further application in the food, animal feed, and pharmaceutical industries. This review is critical to providing a new pathway for converting the common pollution issues of hydroponic nutrients into valuable marine bioresources.

Computer-aided design of proline-rich antimicrobial peptides based on the chemophysical properties of a peptide isolated from Olivancillaria hiatula

The chemophysical properties of a peptide isolated from Olivancillaria hiatula were combined with computational tools to design new antimicrobial peptides (AMPs). The in silico peptide design utilized arbitrary sequence shuffling, AMP sequence prediction and alignments such that putative sequences mimicked those of proline-rich AMPs (PrAMPs) and were potentially active against bacteria. Molecular modelling and docking experiments were used to monitor peptide binding to some intracellular targets like bacteria ribosome, DnaK and LasR. Peptide candidates were tested in vitro for antibacterial and antivirulence activities. Chemophysical studies of peptide extract suggested hydrophobic, acidic and proline-rich peptide properties. The amino acid signature of the extract matched that of AMPs that inhibit intracellular targets. Two of the designed PrAMP peptides (OhPrP-3 and OhPrP-5) had high affinity for the ribosome and DnaK. OhPrP-1, 2 and 4 also had favorable interactions with the biomolecular targets investigated. Peptides had bactericidal activity at the minimum inhibitory concentration against Pseudomonas aeruginosa. The designed peptides docked strongly to LasR suggesting possible interference with quorum sensing, and this was corroborated by in vitro data where sub-inhibitory doses of all peptides reduced pyocyanin and pyoverdine expression. The designed peptides can be further studied for the development of new anti-infective agents.

Transcriptional and proteomic analysis of the innate immune response to microbial stimuli in a model invertebrate chordate

Inflammatory response triggered by innate immunity can act to protect against microorganisms that behave as pathogens, with the aim to restore the homeostatic state between host and beneficial microbes. As a filter-feeder organism, the ascidian Ciona robusta is continuously exposed to external microbes that may be harmful under some conditions. In this work, we used transcriptional and proteomic approaches to investigate the inflammatory response induced by stimuli of bacterial (lipopolysaccharide -LPS- and diacylated lipopeptide - Pam2CSK4) and fungal (zymosan) origin, in Ciona juveniles at stage 4 of metamorphosis. We focused on receptors, co-interactors, transcription factors and cytokines belonging to the TLR and Dectin-1 pathways and on immune factors identified by homology approach (i.e. immunoglobulin (Ig) or C-type lectin domain containing molecules). While LPS did not induce a significant response in juvenile ascidians, Pam2CSK4 and zymosan exposure triggered the activation of specific inflammatory mechanisms. In particular, Pam2CSK4-induced inflammation was characterized by modulation of TLR and Dectin-1 pathway molecules, including receptors, transcription factors, and cytokines, while immune response to zymosan primarily involved C-type lectin receptors, co-interactors, Ig-containing molecules, and cytokines. A targeted proteomic analysis enabled to confirm transcriptional data, also highlighting a temporal delay between transcriptional induction and protein level changes. Finally, a protein-protein interaction network of Ciona immune molecules was rendered to provide a wide visualization and analysis platform of innate immunity. The in vivo inflammatory model described here reveals interconnections of innate immune pathways in specific responses to selected microbial stimuli. It also represents the starting point for studying ontogeny and regulation of inflammatory disorders in different physiological conditions.

Anti-Candidal Marine Natural Products: A Review

Candida spp. are common opportunistic microorganisms in the human body and can cause mucosal, cutaneous, and systemic infections, mainly in individuals with weakened immune systems. Candida albicans is the most isolated and pathogenic species; however, multi-drug-resistant yeasts like Candida auris have recently been found in many different regions of the world. The increasing development of resistance to common antifungals by Candida species limits the therapeutic options. In light of this, the present review attempts to discuss the significance of marine natural products in controlling the proliferation and metabolism of C. albicans and non-albicans species. Natural compounds produced by sponges, algae, sea cucumber, bacteria, fungi, and other marine organisms have been the subject of numerous studies since the 1980s, with the discovery of several products with different chemical frameworks that can inhibit Candida spp., including antifungal drug-resistant strains. Sponges fall under the topmost category when compared to all other organisms investigated. Terpenoids, sterols, and alkaloids from this group exhibit a wide array of inhibitory activity against different Candida species. Especially, hippolide J, a pair of enantiomeric sesterterpenoids isolated from the marine sponge Hippospongia lachne, exhibited strong activity against Candida albicans, Candida parapsilosis, and Candida glabrata. In addition, a comprehensive analysis was performed to unveil the mechanisms of action and synergistic activity of marine products with conventional antifungals. In general, the results of this review show that the majority of chemicals derived from the marine environment are able to control particular functions of microorganisms belonging to the Candida genus, which can provide insights into designing new anti-candidal therapies.

Botryllin, a Novel Antimicrobial Peptide from the Colonial Ascidian Botryllus schlosseri

By mining the transcriptome of the colonial ascidian Botryllus schlosseri, we identified a transcript for a novel styelin-like antimicrobial peptide, which we named botryllin. The gene is constitutively transcribed by circulating cytotoxic morula cells (MCs) as a pre-propeptide that is then cleaved to mature peptide. The synthetic peptide, obtained from in silico translation of the transcript, shows robust killing activity of bacterial and unicellular yeast cells, causing breakages of both the plasma membrane and the cell wall. Specific monoclonal antibodies were raised against the epitopes of the putative amino acid sequence of the propeptide and the mature peptide; in both cases, they label the MC granular content. Upon MC degranulation induced by the presence of nonself, the antibodies recognise the extracellular nets with entrapped bacteria nearby MC remains. The obtained results suggest that the botryllin gene carries the information for the synthesis of an AMP involved in the protection of B. schlosseri from invading foreign cells.

Short, mirror-symmetric antimicrobial peptides centered on "RRR" have broad-spectrum antibacterial activity with low drug resistance and toxicity

The increasingly severe bacterial resistance worldwide pushes people to discover and design potential antibacterial drugs unavoidably. In this work, a series of short, mirror-symmetric peptides were designed and successfully synthesized, centered on "RRR" and labeled with hydrophobic amino acids at both ends. Based on the structure-activity relationship analysis, LWWR (LWWRRRWWL-NH₂) was screened as a desirable mirror-symmetric peptide for further study. As expected, LWWR displayed broad-spectrum antibacterial activity against the standard bacteria and antibiotic-resistant strains. Undoubtedly, the high stability of LWWR in a complex physiological environment was an essential guarantee to maximizing its antibacterial activity. Indeed, LWWR also exhibited a rapid bactericidal speed and a low tendency to develop bacterial resistance, based on the multiple actions of non-receptor-mediated membrane actions and intra-cellular mechanisms. Surprisingly, although LWWR showed similar in vivo antibacterial activity compared with Polymyxin B and Melittin, the in vivo safety of LWWR was far higher than that of them, so LWWR had better therapeutic potential. In summary, the desirable mirror-symmetric peptide LWWR was promised as a potential antibacterial agent to confront the antibiotics resistance crisis. STATEMENT OF SIGNIFICANCE: Witnessing the growing problem of antibiotic resistance, a series of short, mirror-symmetric peptides based on the symmetric center "RRR" and hydrophobic terminals were designed and synthesized in this study. Among, LWWR (LWWRRRWWL-NH₂) presented broad-spectrum antibacterial activity both in vitro and in vivo due to its multiple mechanisms and good stability. Meanwhile, the low drug resistance and toxicity of LWWR also suggested its potential for clinical application. The findings of this study will provide some inspiration for the design and development of potential antibacterial agents, and contribute to the elimination of bacterial infections worldwide as soon as possible.

Assessing the Activity of Antimicrobial Peptides Against Common Marine Bacteria Located in Rotifer (Brachionus plicatilis) Cultures

Rotifers are used as the first feed for marine fish larvae and are grown in large cultures that have high loads of organic matter and heterotrophic bacteria; these bacteria are passed on to the developing fish larvae and can potentially lead to bacterial infections. A modified minimum inhibitory concentration (MIC) protocol for antimicrobial peptides was used to determine the potency of ten antimicrobial peptides (AMPs) in artificial seawater relevant to a rotifer culture (salinity of 25‰) against common marine pathogens. All of the AMPs had antimicrobial activity against the bacterial isolates when the salt concentration was approximately zero. However, in high salt concentrations, the majority of the AMPs had an MIC value greater than 65 µg mL⁻¹ in artificial seawater (25‰). The only exceptions were 2009 (32.5 µg mL⁻¹) and 3002 (32.5 µg mL⁻¹) against Vibrio rotiferianus and Tenacibaculum discolor, respectively. The selected synthetic AMPs were not effective at reducing the bacterial load in brackish salt concentrations of a typical commercial rotifer culture (25‰).

Biomaterials and Bioactive Natural Products from Marine Invertebrates: From Basic Research to Innovative Applications

Aquatic invertebrates are a major source of biomaterials and bioactive natural products that can find applications as pharmaceutics, nutraceutics, cosmetics, antibiotics, antifouling products and biomaterials. Symbiotic microorganisms are often the real producers of many secondary metabolites initially isolated from marine invertebrates; however, a certain number of them are actually synthesized by the macro-organisms. In this review, we analysed the literature of the years 2010–2019 on natural products (bioactive molecules and biomaterials) from the main phyla of marine invertebrates explored so far, including sponges, cnidarians, molluscs, echinoderms and ascidians, and present relevant examples of natural products of interest to public and private stakeholders. We also describe omics tools that have been more relevant in identifying and understanding mechanisms and processes underlying the biosynthesis of secondary metabolites in marine invertebrates. Since there is increasing attention on finding new solutions for a sustainable large-scale supply of bioactive compounds, we propose that a possible improvement in the biodiscovery pipeline might also come from the study and utilization of aquatic invertebrate stem cells.

Biomaterial-Interrelated Bacterial Sweeper: Simplified Self-Assembled Octapeptides with Double-Layered Trp Zipper Induces Membrane Destabilization and Bacterial Apoptosis-Like Death

Treatment of microbial-associated infections continues to be hampered by impaired antibacterial efficiency and the variability in nanomedicines. Herein, an octapeptide library with a double-layered zipper, constructed via a systematic arrangement, simplifying the sequence and optimizing the structure (diverse motifs including surfactant-like, central-bola, and end-bola), is assessed in terms of biological efficiency and self-assembly properties. The results indicate that peptides with double-layered Trp zipper exhibit significant antimicrobial activity. Extracellularly, affinity interactions between micelles and bacteria induce the lateral flow of the membrane and electric potential perturbation. Intracellularly, lead molecules cause apoptosis-like death, as indicated by excessive accumulation of reactive oxygen species, generation of a DNA ladder, and upregulation of mazEF expression. Among them, RW-1 performs the best in vivo and in vitro. The intersecting combination of Trp zipper and surfactants possesses overwhelming superiority with respect to bacterial sweepers (therapeutic index [TI] = 52.89), nanostructures (micelles), and bacterial damage compared to RW-2 (central-bola) and RW-3 (end-bola). These findings confirm that the combination of double-layered Trp zipper and surfactants has potential for application as a combined motif for combating microbial infection and connects the vast gap between antimicrobial peptides and self-assembly, such as Jacob's ladder.

The conservation and diversity of ascidian cells and molecules involved in the inflammatory reaction: The Ciona robusta model

Ascidians are marine invertebrate chordates belonging to the earliest branch (Tunicata) in the chordate phylum, therefore, they are of interest for studying the evolution of immune systems. Due to the known genome, the non-colonial Ciona robusta, previously considered to be C. intestinalis type A, is a model species for the study of inflammatory response. The internal defense of ascidians mainly relies on hemocytes circulating in the hemolymph and pharynx. Hemocytes can be in vivo challenged by LPS injection and various granulocyte and vacuolated cell populations differentiated to produce and release inflammatory factors. Molecular biology and gene expression studies revealed complex defense mechanisms involving different inflammatory hemocytes. Furthermore, cloning procedures allowed sequence analyses and molecular studies disclose immune-related gene families including TOLL-like receptors, galectins, C-type lectins, collectins, interlectins, pentraxine-like, peroxinectins, complement factors-like, TNFα-like, IL-17-like, TGF-like, MIF-like. These genes are promptly upregulated by the inflammatory stimulus and show a time course of transcription similar to each other. Domains sequence similarity and phylogenetic relationships with the vertebrate counterparts are shedding some light on immune-related gene evolution. Selective bioassays as well as bioinformatic approaches have allowed the characterization of antimicrobial peptides and the identification of post transcriptional molecular mechanisms able of influencing dynamics of gene regulation are described. In synthesis, the purpose of this article is to further explore the topic of hemocyte and molecules related to internal defence of ascidians involved in the inflammatory reaction, as well as to discuss current and future study options through a detailed literature review.

Designed Trp-Cage Proteins with Antimicrobial Activity and Enhanced Stability

α-Helical antimicrobial peptides (αAMPs) are among the potential candidates for new anti-infectives to tackle the global crisis in antibiotic resistance, but they suffer from low bioavailability due to high susceptibility to enzymatic degradation. Here, we describe a strategy to increase the resistance of αAMPs against proteases. Fusing the 12-residue αAMP KR-12 with a Trp-cage domain induces an α-helical structure in the otherwise unfolded KR-12 moiety in solution. The resulting antimicrobial Trp-cage exhibits higher proteolytic resistance due to its stable fold as evidenced by correlating sequence-resolved digest data with structural analyses. In addition, the antimicrobial Trp-cage displays increased activity against bacteria in the presence of physiologically relevant concentrations of NaCl, while the hemolytic activity remains negligible. In contrast to previous strategies, the presented approach is not reliant on artificial amino acids and is therefore applicable to biosynthetic procedures. Our study aims to improve the pharmacokinetics of αAMPs to facilitate their use as therapeutics.

The Trp-rich Antimicrobial Amphiphiles With Intramolecular Aromatic Interactions for the Treatment of Bacterial Infection

Antibiotic resistance is emerging as a hot issue with the abuse and overuse of antibiotics, and the shortage of effective antimicrobial agents against multidrug resistant bacteria creates a huge problem to treat the threatening nosocomial skin and soft tissue infection. Antimicrobial peptides (AMPs) exhibite enormous potential as one of the most promising candidates of antibiotic to fight against pathogenic infections because of its unique membrane penetration mechanism to kill pathogens, whereas the clinical application of AMPs still faces the challenges of production cost, stability, safety, and design strategy. Herein, a series of Trp-rich peptides was designed following the principle of paired Trp plated at the ith and ith+4 position on the backbone of peptides, based on the template (VKKX)4, where X represents W, A, or L, to study the effect of intramolecular aromatic interactions on the bioactivity of AMPs. Through comparing the antimicrobial performance, hemolysis, cytotoxicity, and stability, VW5 which is equipped with the characters of direct antimicrobial efficacy (GM=1.68μM) and physical destruction of bacterial membrane (SEM and electron microscopy) stood out from the engineering peptides. VW5 also performed well in mice models, which could significantly decrease the bacterial colony (VW5 vs infection group, 12.72±2.26 vs 5.52±2.01×109CFU/abscess), the area of dermo-necrosis (VW5 vs infection group, 0.74±0.29 vs 1.86±0.98mm2) and the inflammation cytokine levels at the abscess site without causing toxicity to the skin. Overall, this study provides a strategy and template to diminish the randomness in the exploration and design of novel peptides.

Marine Natural Products from Tunicates and Their Associated Microbes

Marine tunicates are identified as a potential source of marine natural products (MNPs), demonstrating a wide range of biological properties, like antimicrobial and anticancer activities. The symbiotic relationship between tunicates and specific microbial groups has revealed the acquisition of microbial compounds by tunicates for defensive purpose. For instance, yellow pigmented compounds, "tambjamines", produced by the tunicate, Sigillina signifera (Sluiter, 1909), primarily originated from their bacterial symbionts, which are involved in their chemical defense function, indicating the ecological role of symbiotic microbial association with tunicates. This review has garnered comprehensive literature on MNPs produced by tunicates and their symbiotic microbionts. Various sections covered in this review include tunicates' ecological functions, biological activities, such as antimicrobial, antitumor, and anticancer activities, metabolic origins, utilization of invasive tunicates, and research gaps. Apart from the literature content, 20 different chemical databases were explored to identify tunicates-derived MNPs. In addition, the management and exploitation of tunicate resources in the global oceans are detailed for their ecological and biotechnological implications.

Membrane-Interacting Antifungal Peptides

The incidence of invasive fungal infections is increasing worldwide, resulting in more than 1.6 million deaths every year. Due to growing antifungal drug resistance and the limited number of currently used antimycotics, there is a clear need for novel antifungal strategies. In this context, great potential is attributed to antimicrobial peptides (AMPs) that are part of the innate immune system of organisms. These peptides are known for their broad-spectrum activity that can be directed toward bacteria, fungi, viruses, and/or even cancer cells. Some AMPs act via rapid physical disruption of microbial cell membranes at high concentrations causing cell leakage and cell death. However, more complex mechanisms are also observed, such as interaction with specific lipids, production of reactive oxygen species, programmed cell death, and autophagy. This review summarizes the structure and mode of action of antifungal AMPs, thereby focusing on their interaction with fungal membranes.

Antimicrobial peptides with symmetric structures against multidrug-resistant bacteria while alleviating antimicrobial resistance

In response to the dramatically increasing antimicrobial resistance, a series of new symmetric peptides were designed and synthesized in this study by a "WWW" motif as the symmetric center, arginine as the positive charge amino acid and the terminus symmetrically tagged with hydrophobic amino acids. Amongst the new symmetric peptide FRRW (FRRWWWRRF-NH₂) presented the highest cell selectivity for bacteria over mammalian cell and exerted excellent antimicrobial potential against a broad of bacteria, especially difficult-to-kill multidrug-resistant strains clinical isolates. FRRW also displayed perfect stability in physiological salt ions and rapid killing speed as well as acted on multiple mechanisms including non-receptor mediated membrane and intra-molecular mechanisms. Importantly, FRRW emerged a low tendency of resistance in contrast to traditional antibiotics ciprofloxacin and gentamicin. What's more, FRRW could resist or alleviate or even reverse the ciprofloxacin- and gentamicin-resistance by changing the permeability of bacterial membrane and inhibiting the efflux pumps of bacteria. Furthermore, FRRW exhibited remarkable effectiveness and higher safety in vivo than polymyxin B. In summary, the new symmetric peptide FRRW was promised to be as a new antimicrobial candidate for overcoming the increasing bacterial resistance.

Using Bacillus subtilis as a Host Cell to Express an Antimicrobial Peptide from the Marine Chordate Ciona intestinalis

Ciona molecule against microbes-A24 (CiMAM) isolated from the marine chordate Ciona intestinalis is an antimicrobial peptide. To generate CiMAM-expressing transgenic Bacillus subtilis, we constructed a plasmid expressing recombinant CiMAM (rCiMAM) and introduced it into B. subtilis. Transgenic strains C117 and C166 were selected since they were able to highly and stably express rCiMAM. We studied the bactericidal activity of pepsin-digested extracts from rCiMAM-expressing strains against freshwater and euryhaline pathogens that commonly occur in aquaculture ponds and found no difference from that of lactoferricin-expressing strains. The bactericidal activity of 1-μL aliquot from a total 5.5 mL extracted from 5 mL of cultured C117 (1.45 × 10⁸ CFU·mL⁻¹) and C166 (2.17 × 10⁸ CFU·mL⁻¹) against halophilic bacteria was equivalent to the efficacy of 57.06 and 32.35 ng of Tetracycline against Vibrio natriegens, 47.07 and 25.2 ng against V. parahaemolyticus, and 58.17 and 36.55 ng against V. alginolyticus, respectively, indicating higher bactericidal activity of pepsin-extracts from rCiMAM-containing strains against halophilic bacteria compared to that from lactoferricin-containing strains. Since the antibacterial activity of rCiMAM-expressing B. subtilis strains shows higher competence against halophilic pathogens compared to that against freshwater and euryhaline pathogens, these strains are promising candidates to protect marine fish and shellfish from halophilic bacterial infection.

Systematically Studying the Optimal Amino Acid Distribution Patterns of the Amphiphilic Structure by Using the Ultrashort Amphiphiles

Amphipathicity has traditionally been considered to be essential for the de novo design or systematic optimization of antimicrobial peptides (AMPs). However, the current research methods to study the relationship between amphiphilicity and antimicrobial activity are inappropriate, because the key parameters (hydrophobicity, positive charge, etc.) and secondary structure of AMPs are changed. To systematically and accurately study the effects of amphiphilicity on antimicrobial properties of AMPs, we designed parallel series of AMPs with a different order of amino acids in a sequence composed only of Arg and either Trp (WR series) or Leu (LR series), under conditions in which other vital parameters were fixed. Furthermore, based on the WR and LR peptides that can form stable amphiphilic β-sheet structures in the anionic membrane-mimetic environment, we found that high β-sheet amphipathic was accompanied by strong antimicrobial activity. Of such peptides, W5 ([RW]₄W) and L5 ([RL]₄L) with a nicely amphipathic β-sheet structure possessed the optimal therapeutic index. W5 and L5 also exhibited high stability in vitro and a potent membrane-disruptive mechanism. These results suggest that the alternate arrangement of hydrophobic and hydrophilic residues to form a stable amphipathic β-sheet structure is an essential factor that significantly affects the antimicrobial properties.

Culture-Dependent Microbiome of the Ciona intestinalis Tunic: Isolation, Bioactivity Profiling and Untargeted Metabolomics

Ascidians and their associated microbiota are prolific producers of bioactive marine natural products. Recent culture-independent studies have revealed that the tunic of the solitary ascidian Cionaintestinalis (sea vase) is colonized by a diverse bacterial community, however, the biotechnological potential of this community has remained largely unexplored. In this study, we aimed at isolating the culturable microbiota associated with the tunic of C.intestinalis collected from the North and Baltic Seas, to investigate their antimicrobial and anticancer activities, and to gain first insights into their metabolite repertoire. The tunic of the sea vase was found to harbor a rich microbial community, from which 89 bacterial and 22 fungal strains were isolated. The diversity of the tunic-associated microbiota differed from that of the ambient seawater samples, but also between sampling sites. Fungi were isolated for the first time from the tunic of Ciona. The proportion of bioactive extracts was high, since 45% of the microbial extracts inhibited the growth of human pathogenic bacteria, fungi or cancer cell lines. In a subsequent bioactivity- and metabolite profiling-based approach, seven microbial extracts were prioritized for in-depth chemical investigations. Untargeted metabolomics analyses of the selected extracts by a UPLC-MS/MS-based molecular networking approach revealed a vast chemical diversity with compounds assigned to 22 natural product families, plus many metabolites that remained unidentified. This initial study indicates that bacteria and fungi associated with the tunic of C.intestinalis represent an untapped source of putatively new marine natural products with pharmacological relevance.

In silico identification and functional validation of linear cationic α-helical antimicrobial peptides in the ascidian Ciona intestinalis

We developed a computing method to identify linear cationic α-helical antimicrobial peptides (LCAMPs) in the genome of Ciona intestinalis based on its structural and physicochemical features. Using this method, 22 candidates of Ciona LCAMPs, including well-known antimicrobial peptides, were identified from 21,975 non-redundant amino acid sequences in Ciona genome database, Ghost database. We also experimentally confirmed the antimicrobial activities of five LCAMP candidates, and three of them were found to be active in the presence of 500 mM NaCl, nearly equivalent to the salt concentration of seawater. Membrane topology prediction suggested that salt resistance of Ciona LCAMPs might be influenced by hydrophobic interactions between the peptide and membrane. Further, we applied our method to Xenopus tropicalis genome and found 11 LCAMP candidates. Thus, our method may serve as an effective and powerful tool for searching LCAMPs that are difficult to find using conventional homology-based methods.

Peptides With Triplet-Tryptophan-Pivot Promoted Pathogenic Bacteria Membrane Defects

Development of probiotic-ineffective antimicrobial peptides (AMPs)-based coatings that can kill pathogenic bacteria at low concentrations but are essentially harmless (even high concentrations) to probiotic organisms is a relatively new trend for therapy against GI tract infections. In this study, a series of triplet-tryptophan-pivot peptides with various hydrophilic amino acids was constructed. One AMP in particular, S7, showed bactericidal activity against Staphylococcus epidermidis, Pseudomonas aeruginosa, Escherichia coli and antibiotic-resistant Staphylococcus aureus, yet was shown to be harmless to Lactobacillus rhamnosus, a key GI tract commensal. Furthermore, antibacterial mechanism assays, drug resistance assays, and mouse model tests suggested that S7 was useful in a clinical setting as it proved to significantly reduce bacterial load and cytokines (TNF-α, IL-6; P < 0.05) with a low probability of resistance via bacterial membrane physical destruction and formation of intracellular ROS. Combined, the results show that a triplet-tryptophan-pivot peptide containing a pair of serine residues was an excellent pathogen-selective candidate for medical devices and was potentially useful in food preservation, crop protection, and human health.

Influence of disulfide bonds in human beta defensin-3 on its strain specific activity against Gram-negative bacteria

Antimicrobial peptides (AMPs) play an important role in the host defense against various microbes. One of the most efficient human AMPs is the human beta defensin-3 (hBD-3) which is produced by, e.g. keratinocytes and lung epithelial cells. However, the structure-function relationship for AMPs and in particular for defensins with their typical three disulfide bonds is still poorly understood. In this study the importance of the three disulfide bonds for the activity of the AMPs is investigated with biological assays and with biophysical experiments utilizing different membrane reconstitution systems. The activities of natural hBD-3, hBD-3-c (cyclic variant with one disulfide bond), and hBD-3-l (linear variant without disulfide bonds) and fragments thereof were tested against specific Gram-negative bacteria. Furthermore, hemolytic and cytotoxic activities were analyzed as well as the potency to neutralize immune cell stimulation of lipopolysaccharide (LPS). Experiments using reconstituted lipid matrices composed of phospholipids or LPS purified from the respective Gram-negative bacteria, showed that the membrane activity of all three hBD-3 peptides is decisive for their capability to kill bacteria and to neutralize LPS. In most of the test systems the linear hBD-3-l showed the highest activity. It was also the only peptide significantly active against polymyxin B-resistant Proteus mirabilis R45. However, the stability of hBD-3 against protease activity decreases with decreasing number of disulfide bonds. This study demonstrates that the refining of AMP structures can generate more active compounds against certain strains.

Design of Heptad Repeat Amphiphiles Based on Database Filtering and Structure-Function Relationships to Combat Drug-Resistant Fungi and Biofilms

Due to the emergence of reports of multidrug-resistant fungi, infections caused by multidrug-resistant fungi and biofilms are considered to be a global threat to human health due to the lack of effective broad-spectrum drugs. Here, we developed a series heptad repeat sequences based on an antimicrobial peptide database (APD) and structure-function relationships. Among the developed peptides, the target peptide ACR3 exhibited good activity against all fungi and bacteria tested, including fluconazole-resistant Candida albicans (C. albicans) and methicillin-resistant Staphylococcu saureus (S. aureus), while demonstrating relatively low toxicity and good salt tolerance. The peptide ACR3 inhibits the formation of C. albicans biofilms and has a therapeutic effect on mature biofilms in vitro and in vivo. Moreover, we did not observe any resistance of C. albicans and E. coli against the peptide ACR3. A series of assays and microscopy were used to analyze the antimicrobial mechanism. These results showed that the antimicrobial activity of the peptide ACR3 utilizes a multimodal mechanism that degrades the cell wall barrier, alters the cytoplasmic membrane electrical potential, and induces intracellular reactive oxygen species (ROS) production. In general, the peptide ACR3 is a potent antibacterial agent that shows great potential for use in biomedical coatings and healthcare formulas to combat the growing threat of fungal and bacterial infection.

Isolation and Characterization of Antimicrobial Peptides with Unusual Disulfide Connectivity from the Colonial Ascidian Synoicum turgens

This study reports the isolation of two novel cysteine-rich antibacterial peptides, turgencin A and turgencin B, along with their oxidized derivatives, from the Arctic marine colonial ascidian Synoicum turgens. The peptides are post-translationally modified, containing six cysteines with an unusual disulfide connectivity of Cys¹-Cys⁶, Cys²-Cys⁵, and Cys³-Cys⁴ and an amidated C-terminus. Furthermore, the peptides contain methionine residues resulting in the isolation of peptides with different degrees of oxidation. The most potent peptide, turgencin A_Mox1 with one oxidized methionine, displayed antimicrobial activity against both Gram-negative and Gram-positive bacteria with a minimum inhibitory concentration (MIC) as low as 0.4 µM against selected bacterial strains. In addition, the peptide inhibited the growth of the melanoma cancer cell line A2058 (IC₅₀ = 1.4 µM) and the human fibroblast cell line MRC-5 (IC₅₀ = 4.8 µM). The results from this study show that natural peptides isolated from marine tunicates have the potential to be promising drug leads.

The interaction with fungal cell wall polysaccharides determines the salt tolerance of antifungal plant defensins

The fungal cell wall is the first point of contact between fungal pathogens and host organisms. It serves as a protective barrier against biotic and abiotic stresses and as a signal to the host that a fungal pathogen is present. The fungal cell wall is made predominantly of carbohydrates and glycoproteins, many of which serve as binding receptors for host defence molecules or activate host immune responses through interactions with membrane-bound receptors. Plant defensins are a large family of cationic antifungal peptides that protect plants against fungal disease. Binding of the plant defensin NaD1 to the fungal cell wall has been described but the specific component of the cell wall with which this interaction occurred was unknown. The effect of binding was also unclear, that is whether the plant defensin used fungal cell wall components as a recognition motif for the plant to identify potential pathogens or if the cell wall acted to protect the fungus against the defensin. Here we describe the interaction between the fungal cell wall polysaccharides chitin and β-glucan with NaD1 and other plant defensins. We discovered that the β-glucan layer protects the fungus against plant defensins and the loss of activity experienced by many cationic antifungal peptides at elevated salt concentrations is due to sequestration by fungal cell wall polysaccharides. This has limited the development of cationic antifungal peptides for the treatment of systemic fungal diseases in humans as the level of salt in serum is enough to inactivate most cationic peptides.

Peptide receptors and immune-related proteins expressed in the digestive system of a urochordate, Ciona intestinalis

The digestive system is responsible for nutrient intake and defense against pathogenic microbes. Thus, identification of regulatory factors for digestive functions and immune systems is a key step to the verification of the life cycle, homeostasis, survival strategy and evolutionary aspects of an organism. Over the past decade, there have been increasing reports on neuropeptides, their receptors, variable region-containing chitin-binding proteins (VCBPs) and Toll-like receptors (TLRs) in the ascidian, Ciona intestinalis. Mass spectrometry-based peptidomes and genome database-searching detected not only Ciona orthologs or prototypes of vertebrate peptides and their receptors, including cholecystokinin, gonadotropin-releasing hormones, tachykinin, calcitonin and vasopressin but also Ciona-specific neuropeptides including Ci-LFs and Ci-YFVs. The species-specific regulation of GnRHergic signaling including unique signaling control via heterodimerization among multiple GnRH receptors has also been revealed. These findings shed light on the remarkable significance of ascidians in investigations of the evolution and diversification of the peptidergic systems in chordates. In the defensive systems of C. intestinalis, VCBPs and TLRs have been shown to play major roles in the recognition of exogenous microbes in the innate immune system. These findings indicate both common and species-specific functions of the innate immunity-related molecules between C. intestinalis and vertebrates. In this review article, we present recent advances in molecular and functional features and evolutionary aspects of major neuropeptides, their receptors, VCBPs and TLRs in C. intestinalis.

Rational Design of Short Peptide Variants by Using Kunitzin-RE, an Amphibian-Derived Bioactivity Peptide, for Acquired Potent Broad-Spectrum Antimicrobial and Improved Therapeutic Potential of Commensalism Coinfection of Pathogens

Commensalism coinfection of pathogens has seriously jeopardized human health. Currently, Kunitzin-RE, as an amphibian-derived bioactivity peptide, is regarded as a potential antimicrobial candidate. However, its antimicrobial properties were unsatisfactory. In this study, a set of shortened variants of Kunitzin-RE was developed by the interception of a peptide fragment and single-site mutation to investigate the effect of chain length, positive charge, hydrophobicity, amphipathicity, and secondary structure on antimicrobial properties. Among them, W8 (AARIILRWRFR) significantly broadened the antimicrobial spectrum and showed the highest antimicrobial activity (GM_all = 2.48 μM) against all the fungi and bacteria tested. Additionally, W8 showed high cell selectivity and salt tolerance in vitro, whereas it showed high effectiveness against mice keratitis cause by infection by C. albicans 2.2086. Additionally, it also had obviously lipopolysaccharide-binding ability and a potent membrane-disruptive mechanism. Overall, these findings contributed to the design of short antimicrobial peptides and to combat the serious threat of commensalism coinfection of pathogens.

Salt-Tolerant Antifungal and Antibacterial Activities of the Corn Defensin ZmD32

Pathogenic microbes are developing resistance to established antibiotics, making the development of novel antimicrobial molecules paramount. One major resource for discovery of antimicrobials is the arsenal of innate immunity molecules that are part of the first line of pathogen defense in many organisms. Gene encoded cationic antimicrobial peptides are a major constituent of innate immune arsenals. Many of these peptides exhibit potent antimicrobial activity in vitro. However, a major hurdle that has impeded their development for use in the clinic is the loss of activity at physiological salt concentrations, attributed to weakening of the electrostatic interactions between the cationic peptide and anionic surfaces of the microbial cells in the presence of salt. Using plant defensins we have investigated the relationship between the charge of an antimicrobial peptide and its activity in media with elevated salt concentrations. Plant defensins are a large class of antifungal peptides that have remarkable stability at extremes of pH and temperature as well as resistance to protease digestion. A search of a database of over 1200 plant defensins identified ZmD32, a defensin from Zea mays, with a predicted charge of +10.1 at pH 7, the highest of any defensin in the database. Recombinant ZmD32 retained activity against a range of fungal species in media containing elevated concentrations of salt. In addition, ZmD32 was active against Candida albicans biofilms as well as both Gram negative and Gram-positive bacteria. This broad spectrum antimicrobial activity, combined with a low toxicity on human cells make ZmD32 an attractive lead for development of future antimicrobial molecules.

Identification and Characterization of Novel Antimicrobial Peptide from Hippocampus comes by In Silico and Experimental Studies

Antimicrobial peptides (AMPs) have attracted attentions as a novel antimicrobial agent because of their unique activity against microbes. In the present study, we described a new, previously unreported AMP, moronecidin-like peptide, from Hippocampus comes and compared its antimicrobial activity with moronecidin from hybrid striped bass. Antibacterial assay indicated that gram-positive bacteria were more sensitive to moronecidin and moronecidin-like compared with gram-negative bacteria. Furthermore, both AMPs were found to exhibit effective antifungal activity. Comparative analysis of the antimicrobial activity revealed that moronecidin-like peptide has higher activity against Acinetobacter baumannii and Staphylococcus epidermidis relative to moronecidin. Both moronecidin-like and moronecidin peptides retained their antibacterial activity in physiological pH and salt concentration. The time-killing assay showed that the AMPs completely killed A. baumannii and S. epidermidis isolates after 1 and 5 h at five- and tenfold above their corresponding MICs, respectively. Anti-biofilm assay demonstrated that peptides were able to inhibit 50% of biofilm formation at sub-MIC of 1/8 MIC. Furthermore, moronecidin-like significantly inhibited biofilm formation more than moronecidin at 1/16 MIC. Collectively, our results revealed that antimicrobial and anti-biofilm activities of moronecidin-like are comparable to moronecidin. In addition, the hemolytic and cytotoxic activities of moronecidin-like were lower than those of moronecidin, suggesting it as a potential novel therapeutic agent, and a template to design new therapeutic AMPs.

Combating Drug-Resistant Fungi with Novel Imperfectly Amphipathic Palindromic Peptides

Antimicrobial peptides are an important weapon against invading pathogens and are potential candidates as novel antibacterial agents, but their antifungal activities are not fully developed. In this study, a set of imperfectly amphipathic peptides was developed based on the imperfectly amphipathic palindromic structure R _n(XRXXXRX)R _n ( n = 1, 2; X represents L, I, F, or W), and the engineered peptides exhibited high antimicrobial activities against all fungi and bacteria tested (including fluconazole-resistant Candida albicans), with geometric mean (GM) MICs ranging from 2.2 to 6.62 μM. Of such peptides, 13 (I6) (RRIRIIIRIRR-NH₂) that was Ile rich in its hydrophobic face had the highest antifungal activity (GM_fungi = 1.64 μM) while showing low toxicity and high salt and serum tolerance. It also had dramatic LPS-neutralizing propensity and a potent membrane-disruptive mechanism against microbial cells. In summary, these findings were useful for short AMPs design to combat the growing threat of drug-resistant fungal and bacterial infections.

Microbial Diseases of Bivalve Mollusks: Infections, Immunology and Antimicrobial Defense

A variety of bivalve mollusks (phylum Mollusca, class Bivalvia) constitute a prominent commodity in fisheries and aquacultures, but are also crucial in order to preserve our ecosystem’s complexity and function. Bivalve mollusks, such as clams, mussels, oysters and scallops, are relevant bred species, and their global farming maintains a high incremental annual growth rate, representing a considerable proportion of the overall fishery activities. Bivalve mollusks are filter feeders; therefore by filtering a great quantity of water, they may bioaccumulate in their tissues a high number of microorganisms that can be considered infectious for humans and higher vertebrates. Moreover, since some pathogens are also able to infect bivalve mollusks, they are a threat for the entire mollusk farming industry. In consideration of the leading role in aquaculture and the growing financial importance of bivalve farming, much interest has been recently devoted to investigate the pathogenesis of infectious diseases of these mollusks in order to be prepared for public health emergencies and to avoid dreadful income losses. Several bacterial and viral pathogens will be described herein. Despite the minor complexity of the organization of the immune system of bivalves, compared to mammalian immune systems, a precise description of the different mechanisms that induce its activation and functioning is still missing. In the present review, a substantial consideration will be devoted in outlining the immune responses of bivalves and their repertoire of immune cells. Finally, we will focus on the description of antimicrobial peptides that have been identified and characterized in bivalve mollusks. Their structural and antimicrobial features are also of great interest for the biotechnology sector as antimicrobial templates to combat the increasing antibiotic-resistance of different pathogenic bacteria that plague the human population all over the world.

Immunity in Protochordates: The Tunicate Perspective

Tunicates are the closest relatives of vertebrates, and their peculiar phylogenetic position explains the increasing interest toward tunicate immunobiology. They are filter-feeding organisms, and this greatly influences their defense strategies. The majority of the studies on tunicate immunity were carried out in ascidians. The tunic acts as a first barrier against pathogens and parasites. In addition, the oral siphon and the pharynx represent two major, highly vascularized, immune organs, where circulating hemocytes can sense non-self material and trigger immune responses that, usually, lead to inflammation and phagocytosis. Inflammation involves the recruitment of circulating cytotoxic, phenoloxidase (PO)-containing cells in the infected area, where they degranulate as a consequence of non-self recognition and release cytokines, complement factors, and the enzyme PO. The latter, acting on polyphenol substrata, produces cytotoxic quinones, which polymerize to melanin, and reactive oxygen species, which induce oxidative stress. Both the alternative and the lectin pathways of complement activation converge to activate C3: C3a and C3b are involved in the recruitment of hemocytes and in the opsonization of foreign materials, respectively. The interaction of circulating professional phagocytes with potentially pathogenic foreign material can be direct or mediated by opsonins, either complement dependent or complement independent. Together with cytotoxic cells, phagocytes are active in the encapsulation of large materials. Cells involved in immune responses, collectively called immunocytes, represent a large fraction of hemocytes, and the presence of a cross talk between cytotoxic cells and phagocytes, mediated by secreted humoral factors, was reported. Lectins play a pivotal role as pattern-recognition receptors and opsonizing agents. In addition, variable region-containing chitin-binding proteins, identified in the solitary ascidian Ciona intestinalis, control the settlement and colonization of bacteria in the gut.

pH Dependent Antimicrobial Peptides and Proteins, Their Mechanisms of Action and Potential as Therapeutic Agents

Antimicrobial peptides (AMPs) are potent antibiotics of the innate immune system that have been extensively investigated as a potential solution to the global problem of infectious diseases caused by pathogenic microbes. A group of AMPs that are increasingly being reported are those that utilise pH dependent antimicrobial mechanisms, and here we review research into this area. This review shows that these antimicrobial molecules are produced by a diverse spectrum of creatures, including vertebrates and invertebrates, and are primarily cationic, although a number of anionic examples are known. Some of these molecules exhibit high pH optima for their antimicrobial activity but in most cases, these AMPs show activity against microbes that present low pH optima, which reflects the acidic pH generally found at their sites of action, particularly the skin. The modes of action used by these molecules are based on a number of major structure/function relationships, which include metal ion binding, changes to net charge and conformational plasticity, and primarily involve the protonation of histidine, aspartic acid and glutamic acid residues at low pH. The pH dependent activity of pore forming antimicrobial proteins involves mechanisms that generally differ fundamentally to those used by pH dependent AMPs, which can be described by the carpet, toroidal pore and barrel-stave pore models of membrane interaction. A number of pH dependent AMPs and antimicrobial proteins have been developed for medical purposes and have successfully completed clinical trials, including kappacins, LL-37, histatins and lactoferrin, along with a number of their derivatives. Major examples of the therapeutic application of these antimicrobial molecules include wound healing as well as the treatment of multiple cancers and infections due to viruses, bacteria and fungi. In general, these applications involve topical administration, such as the use of mouth washes, cream formulations and hydrogel delivery systems. Nonetheless, many pH dependent AMPs and antimicrobial proteins have yet to be fully characterized and these molecules, as a whole, represent an untapped source of novel biologically active agents that could aid fulfillment of the urgent need for alternatives to conventional antibiotics, helping to avert a return to the pre-antibiotic era.

Membrane-active peptides from marine organisms--antimicrobials, cell-penetrating peptides and peptide toxins: applications and prospects

Marine organisms are known to be a rich and unique source of bioactive compounds as they are exposed to extreme conditions in the oceans. The present study is an attempt to briefly describe some of the important membrane-active peptides (MAPs) such as antimicrobial peptides (AMPs), cell-penetrating peptides (CPPs) and peptide toxins from marine organisms. Since both AMPs and CPPs play a role in membrane perturbation and exhibit interchangeable role, they can speculatively fall under the broad umbrella of MAPs. The study focuses on the structural and functional characteristics of different classes of marine MAPs. Further, AMPs are considered as a potential remedy to antibiotic resistance acquired by several pathogens. Peptides from marine organisms show novel post-translational modifications such as cysteine knots, halogenation and histidino-alanine bridge that enable these peptides to withstand harsh marine environmental conditions. These unusual modifications of AMPs from marine organisms are expected to increase their half-life in living systems, contributing to their increased bioavailability and stability when administered as drug in in vivo systems. Apart from AMPs, marine toxins with membrane-perturbing properties could be essentially investigated for their cytotoxic effect on various pathogens and their cell-penetrating activity across various mammalian cells. The current review will help in identifying the MAPs from marine organisms with crucial post-translational modifications that can be used as template for designing novel therapeutic agents and drug-delivery vehicles for treatment of human diseases.

Marine Antimicrobial Peptides: Nature Provides Templates for the Design of Novel Compounds against Pathogenic Bacteria

The discovery of antibiotics for the treatment of bacterial infections brought the idea that bacteria would no longer endanger human health. However, bacterial diseases still represent a worldwide treat. The ability of microorganisms to develop resistance, together with the indiscriminate use of antibiotics, is mainly responsible for this situation; thus, resistance has compelled the scientific community to search for novel therapeutics. In this scenario, antimicrobial peptides (AMPs) provide a promising strategy against a wide array of pathogenic microorganisms, being able to act directly as antimicrobial agents but also being important regulators of the innate immune system. This review is an attempt to explore marine AMPs as a rich source of molecules with antimicrobial activity. In fact, the sea is poorly explored in terms of AMPs, but it represents a resource with plentiful antibacterial agents performing their role in a harsh environment. For the application of AMPs in the medical field limitations correlated to their peptide nature, their inactivation by environmental pH, presence of salts, proteases, or other components have to be solved. Thus, these peptides may act as templates for the design of more potent and less toxic compounds.

Legionella pneumophila: The Paradox of a Highly Sensitive Opportunistic Waterborne Pathogen Able to Persist in the Environment

Legionella pneumophila, the major causative agent of Legionnaires’ disease, is found in freshwater environments in close association with free-living amoebae and multispecies biofilms, leading to persistence, spread, biocide resistance, and elevated virulence of the bacterium. Indeed, legionellosis outbreaks are mainly due to the ability of this bacterium to colonize and persist in water facilities, despite harsh physical and chemical treatments. However, these treatments are not totally efficient and, after a lag period, L. pneumophila may be able to quickly re-colonize these systems. Several natural compounds (biosurfactants, antimicrobial peptides…) with anti-Legionella properties have recently been described in the literature, highlighting their specific activities against this pathogen. In this review, we first consider this hallmark of Legionella to resist killing, in regard to its biofilm or host-associated life style. Then, we focus more accurately on natural anti-Legionella molecules described so far, which could provide new eco-friendly and alternative ways to struggle against this important pathogen in plumbing.

Using adjuvants and environmental factors to modulate the activity of antimicrobial peptides

The increase in antibiotic resistant and multi-drug resistant bacterial infections has serious implications for the future of health care. The difficulty in finding both new microbial targets and new drugs against existing targets adds to the concern. The use of combination and adjuvant therapies are potential strategies to counter this threat. Antimicrobial peptides (AMPs) are a promising class of antibiotics (ABs), particularly for topical and surface applications. Efforts have been directed toward a number of strategies, including the use of conventional ABs combined with AMPs, and the use of potentiating agents to increase the performance of AMPs. This review focuses on combination strategies such as adjuvants and the manipulation of environmental variables to improve the efficacy of AMPs as potential therapeutic agents. This article is part of a Special Issue entitled: Antimicrobial peptides edited by Karl Lohner and Kai Hilpert.

Potent antimicrobial peptides against Legionella pneumophila and its environmental host, Acanthamoeba castellanii

Legionella pneumophila, the major causative agent of Legionnaires' disease, is most often found in the environment in close association with free-living amoebae, leading to persistence, spread, biocide resistance, and elevated virulence of the bacterium. In the present study, we evaluated the anti-Legionella and anti-Acanthamoeba activities of three alpha-helical antimicrobial peptides (AMPs), namely, NK-2, Ci-MAM-A24, and Ci-PAP-A22, already known for the extraordinary efficacy against other microbes. Our data represent the first demonstration of the activity of a particular AMP against both the human facultative intracellular pathogen L. pneumophila and its pathogenic host, Acanthamoeba castellanii. Interestingly, the most effective peptide, Ci-MAM-A24, was also found to reduce the Legionella cell number within amoebae. Accordingly, this peptide was immobilized on gold surfaces to assess its antimicrobial activity. Surfaces were characterized, and activity studies revealed that the potent bactericidal activity of the peptide was conserved after its immobilization. In the frame of elaborating anti-Legionella surfaces, Ci-MAM-A24 represents, by its direct and indirect activity against Legionella, a potent peptide template for biological control of the bacterium in plumbings.

A small heat shock protein (sHSP) from Sinonovacula constricta against heavy metals stresses

Small heat shock proteins (sHSPs) are ATP-independent molecular chaperones and involved into many physiological and stress processes. In the present study, the full-length cDNA of sHSP was cloned from razor clam Sinonovacula constricta (denoted as ScsHSP) through cDNA library and PCR approaches. Some feature motifs like the typical α-crystalline domain with six beta strands, three susceptible phosphorylation serines (S(15), S(78), and S(82)) were conserved in the deduce amino acid of ScsHSP. Tissue distribution analysis of the ScsHSP revealed that the mRNA transcripts of ScsHSP were constitutively expressed in all examined tissues with the highest expressions in the haemocytes. The temporal expression of ScsHSP in gill and haemocytes after PbCl2 and CdCl2 exposure were recorded by qPCR. The suppressed expression patterns were detected in CdCl2 stress at both tissues, and the minimum expression were detected at 36 h with 0.58-fold decrease in haemocytes and 0.30-fold in gill compared to each control group. During the PbCl2 exposure experiment, the expression level of ScsHSP increased significantly with larger amplitude in haemocytes. As time progressed, the mRNA transcripts of ScsHSP recovered almost to the original level at 36 h. All our results indicated that ScsHSP was involved into mediating environmental pollutants exposure and considered to be a promising candidate bio-mark for heavy metals monitoring.

pH Dependence of microbe sterilization by cationic antimicrobial peptides

We recently described a family of cationic antimicrobial peptides (CAMPs) selected from a combinatorial library that exhibited potent, broad-spectrum activity at neutral pH and low ionic strength. To further delimit the utility and activity profiles of these peptides, we investigated the effects of solution conditions, such as pH and ionic strength, on the efficacy of the peptide antimicrobials against a panel of microorganisms. Peptide minimum sterilizing concentrations (MSCs) varied linearly with pH for each subtype within our family of CAMPs for all organisms tested. The peptides were much less effective against Gram-negative bacteria at high pH, consistent with a decrease in net positive charge on the peptides. A similar trend was observed for the fungus Candida albicans. Surprisingly, the opposite pH trend was observed with the Gram-positive Staphylococcus aureus. In addition, an additive ionic strength effect was observed with increasing buffer strengths at identical pH values. The extreme difference in the observed pH behavior between Gram-negative and Gram-positive organisms is attributed to the presence of native charged molecules in the much thicker peptidoglycan layer of the Gram-positive organism. The novel species-specific effects of pH observed here have important implications for applications using CAMPs and for the design of novel CAMPs.

Immune competence of the Ciona intestinalis pharynx: complement system-mediated activity

In the tunicate Ciona intestinalis, the ciliated pharynx, which connects the external environment to a highly developed and compartmentalized gastrointestinal system, represents the natural portal of entry for a vast and diverse, potentially pathogenic microbial community. To address the role of the pharynx in immune surveillance in Ciona, we asked whether C3, the key component of the complement system, was expressed in this organ and whether the encoded protein was functionally active. We found by real-time PCR that C3, constitutively expressed in the pharynx, is up-regulated by LPS injection. Using two specific anti-CiC3 and anti-CiC3a polyclonal antibodies in immunohistochemical staining of pharynx sections, we found that the gene product was localized to hemocytes of the pharyngeal bars (identified as granular amoebocytes) and in stigmata ciliated cells. Use of the same antibodies in Western blot analysis indicated that CiC3 and its activation products CiC3b and CiC3a are present in pharynx homogenates. Our observation that the amount of the bioactive fragment CiC3a increased in the pharynx of LPS-treated animals provides the first molecular and functional evidence for complement-mediated immunological activity in the tunicate pharynx.

Identification of an antimicrobial peptide from human methionine sulfoxide reductase B3

Human methionine sulfoxide reductase B3A (hMsrB3A) is an endoplasmic reticulum (ER) reductase that catalyzes the stereospecific reduction of methionine-R-sulfoxide to methionine in proteins. In this work, we identified an antimicrobial peptide from hMsrB3A protein. The N-terminal ER-targeting signal peptide (amino acids 1-31) conferred an antimicrobial effect in Escherichia coli cells. Sequence and structural analyses showed that the overall positively charged ER signal peptide had an Argand Pro-rich region and a potential hydrophobic α-helical segment that contains 4 cysteine residues. The potential α-helical region was essential for the antimicrobial activity within E. coli cells. A synthetic peptide, comprised of 2-26 amino acids of the signal peptide, was effective at killing Gram-negative E. coli, Klebsiella pneumoniae, and Salmonella paratyphi, but had no bactericidal activity against Gram-positive Staphylococcus aureus.

Toll-like receptors of deuterostome invertebrates

Defensive systems against pathogens are responsible not only for survival or lifetime of an individual but also for the evolution of a species. Innate immunity is expected to be more important for invertebrates than mammals, given that adaptive immunity has not been acquired in the former. Toll-like receptors (TLRs) have been shown to play a crucial role in host defense of pathogenic microbes in innate immunity of mammals. Recent genome-wide analyses have suggested that TLR or their related genes are conserved in invertebrates. In particular, numerous TLR-related gene candidates were detected in deuterostome invertebrates, including a sea urchin (222 TLR-related gene candidates) and amphioxus (72 TLR-related gene candidates). Molecular phylogenetic analysis verified that most of sea urchin or amphioxus TLR candidates are paralogous, suggesting that these organisms expanded TLR-related genes in a species-specific manner. In contrast, another deuterostome invertebrate, the ascidian Ciona intestinalis, was found to possess only two TLR genes. Moreover, Ciona TLRs, Ci-TLR1 and Ci-TLR2, were shown to possess “hybrid” functionality of mammalian TLRs. Such functionality of Ci-TLRs could not be predicted by sequence comparison with vertebrate TLRs, indicating confounding evolutionary lineages of deuterostome invertebrate TLRs or their candidates. In this review article, we present recent advances in studies of TLRs or their candidates among deuterostome invertebrates, and provide insight into an evolutionary process of TLRs.

Localization of antimicrobial peptides in the tunic of Ciona intestinalis (Ascidiacea, Tunicata) and their involvement in local inflammatory-like reactions

Tunicates comprising a wide variety of different species synthesize antimicrobial peptides as important effector molecules of the innate immune system. Recently, two putative gene families coding for antimicrobial peptides were identified in the expressed sequence tag database of the tunicate Ciona intestinalis. Two synthetic peptides representing the cationic core region of one member of each of the families displayed potent antibacterial and antifungal activities. Moreover, the natural peptides were demonstrated to be synthesized and stored in distinct hemocyte types. Here, we investigated the presence of these natural peptides, namely Ci-MAM-A and Ci-PAP-A, in the tunic of C. intestinalis considering that the ascidian tunic is a body surface barrier exposed to constant microbial assault. Furthermore, as the tunic may represent a major route of entry for pathogen invasion after its damage we monitored the location of these peptides upon a local inflammatory-like reaction induced by injection of foreign cells. Using immunocytochemistry and electron microscopy both peptides were localized to the tunic and were massively present in granulocytes of inflamed tissue. Conclusively, antimicrobial peptides may constitute a chemical barrier within the tunic of urochordates.