Antibiotic activities of host defense peptides: more to it than lipid bilayer perturbation

Partial peptides from rice defensin OsAFP1 exhibited antifungal activity against the rice blast pathogen Pyricularia oryzae

Rice blast caused by Pyricularia oryzae is one of the most devastating diseases worldwide. This study aimed to investigate the antifungal activity of rice defensin OsAFP1 and its partial peptides against P. oryzae. The partial peptides near the N- and C-terminal regions of OsAFP1 exhibited approximately the same antifungal activity as the entire protein against P. oryzae. These partial peptides have the potential to be used as fungicides.

A Cationic Polymer That Shows High Antifungal Activity against Diverse Human Pathogens

Invasive fungal diseases are generally difficult to treat and often fatal. The therapeutic agents available to treat fungi are limited, and there is a critical need for new agents to combat these deadly infections. Antifungal compound development has been hindered by the challenge of creating agents that are highly active against fungal pathogens but not toxic to the host. Host defense peptides (HDPs) are produced by eukaryotes as a component of the innate immune response to pathogens and have served as inspiration for the development of many new antibacterial compounds. HDP mimics, however, have largely failed to exhibit potent and selective antifungal activity. Here, we present an HDP-like nylon-3 copolymer that is effective against diverse fungi while displaying only mild to moderate toxicity toward mammalian cells. This polymer is active on its own and in synergy with existing antifungal drugs against multiple species of Candida and Cryptococcus, reaching levels of efficacy comparable to those of the clinical agents amphotericin B and fluconazole in some cases. In addition, the polymer acts synergistically with azoles against different species of Aspergillus, including some azole-resistant strains. These findings indicate that nylon-3 polymers are a promising lead for development of new antifungal therapeutic strategies.

Diversity, evolution and medical applications of insect antimicrobial peptides

Antimicrobial peptides (AMPs) are short proteins with antimicrobial activity. A large portion of known AMPs originate from insects, and the number and diversity of these molecules in different species varies considerably. Insect AMPs represent a potential source of alternative antibiotics to address the limitation of current antibiotics, which has been caused by the emergence and spread of multidrug-resistant pathogens. To get more insight into AMPs, we investigated the diversity and evolution of insect AMPs by mapping their phylogenetic distribution, allowing us to predict the evolutionary origins of selected AMP families and to identify evolutionarily conserved and taxon-specific families. Furthermore, we highlight the use of the nematode Caenorhabditis elegans as a whole-animal model in high-throughput screening methods to identify AMPs with efficacy against human pathogens, including Acinetobacter baumanii and methicillin-resistant Staphylococcus aureus. We also discuss the potential medical applications of AMPs, including their use as alternatives for conventional antibiotics in ectopic therapies, their combined use with antibiotics to restore the susceptibility of multidrug-resistant pathogens, and their use as templates for the rational design of peptidomimetic drugs that overcome the disadvantages of therapeutic peptides.

The article is part of the themed issue ‘Evolutionary ecology of arthropod antimicrobial peptides’.

In-cell production of a genetically-encoded library based on the θ-defensin RTD-1 using a bacterial expression system

We report the high-yield heterologous expression of bioactive θ-defensin RTD-1 inside Escherichia coli cells by making use of intracellular protein trans-splicing in combination with a high efficient split-intein. RTD-1 is a small backbone-cyclized polypeptide with three disulfide bridges and a natural inhibitor of anthrax lethal factor protease. Recombinant RTD-1 was natively folded and able to inhibit anthrax lethal factor protease. In-cell expression of RTD-1 was very efficient and yielded ≈0.7mg of folded RTD-1 per gram of wet E. coli cells. This approach was used to generate of a genetically-encoded RTD-1-based peptide library in live E. coli cells. These results clearly demonstrate the possibility of using genetically-encoded RTD-1-based peptide libraries in live E. coli cells, which is a critical first step for developing in-cell screening and directed evolution technologies using the cyclic peptide RTD-1asa molecular scaffold.

Newly identified invertebrate-type lysozyme (Splys-i) in mud crab (Scylla paramamosain) exhibiting muramidase-deficient antimicrobial activity

Lysozymes are widely distributed immune effectors exerting muramidase activity against the peptidoglycan of the bacterial cell wall to trigger cell lysis. However, some invertebrate-type (i-type) lysozymes deficient of muramidase activity still exhibit antimicrobial activity. To date, the mechanism underlying the antimicrobial effect of muramidase-deficient i-type lysozymes remains unclear. Accordingly, this study characterized a novel i-type lysozyme, Splys-i, in the mud crab Scylla paramamosain. Splys-i shared the highest identity with the Litopenaeus vannamei i-type lysozyme (Lvlys-i2, 54% identity) at the amino acid level. Alignment analysis and 3D structure comparison show that Splys-i may be a muramidase-deficient i-type lysozyme because it lacks the two conserved catalytic residues (Glu and Asp) that are necessary for muramidase activity. Splys-i is mainly distributed in the intestine, stomach, gills, hepatopancreas, and hemocytes, and it is upregulated by Vibrio harveyi or Staphylococcus aureus challenge. Recombinant Splys-i protein (rSplys-i) can inhibit the growth of Gram-negative bacteria (V. harveyi, Vibrio alginolyticus, Vibrio parahemolyticus, and Escherichia coli), Gram-positive bacteria (S. aureus, Bacillus subtilis, and Bacillus megaterium), and the fungus Candida albicans to varying degrees. In this study, two binding assays and a bacterial agglutination assay were conducted to elucidate the potential antimicrobial mechanisms of Splys-i. Results demonstrated that rSplys-i could bind to all nine aforementioned microorganisms. It also exhibited a strong binding activity to lipopolysaccharide from E. coli and lipoteichoic acid and peptidoglycan (PGN) from S. aureus but a weak binding activity to PGN from B. subtilis and β-glucan from fungi. Moreover, rSplys-i could agglutinate these nine types of microorganisms in the presence of Ca²⁺ at different protein concentrations. These results suggest that the binding activity and its triggered agglutinating activity might be two major mechanisms of action to realize the muramidase-deficient antibacterial activity. In addition, rSplys-i can hydrolyze the peptidoglycan of some Gram-positive bacteria because it exhibits weak isopeptidase activities in salt and protein concentration-dependent manner. This result indicates that such an isopeptidase activity may contribute to the muramidase-deficient antimicrobial activity to a certain degree. In conclusion, Splys-i is upregulated by pathogenic bacteria, and it inhibits bacterial growth by binding and agglutination activities as well as isopeptidase activity, suggesting that Splys-i is involved in immune defense against bacteria through several different mechanisms of action.

Targeting a cell wall biosynthesis hot spot

Covering: up to 2017History points to the bacterial cell wall biosynthetic network as a very effective target for antibiotic intervention, and numerous natural product inhibitors have been discovered. In addition to the inhibition of enzymes involved in the multistep synthesis of the macromolecular layer, in particular, interference with membrane-bound substrates and intermediates essential for the biosynthetic reactions has proven a valuable antibacterial strategy. A prominent target within the peptidoglycan biosynthetic pathway is lipid II, which represents a particular "Achilles' heel" for antibiotic attack, as it is readily accessible on the outside of the cytoplasmic membrane. Lipid II is a unique non-protein target that is one of the structurally most conserved molecules in bacterial cells. Notably, lipid II is more than just a target molecule, since sequestration of the cell wall precursor may be combined with additional antibiotic activities, such as the disruption of membrane integrity or disintegration of membrane-bound multi-enzyme machineries. Within the membrane bilayer lipid II is likely organized in specific anionic phospholipid patches that form a particular "landing platform" for antibiotics. Nature has invented a variety of different "lipid II binders" of at least 5 chemical classes, and their antibiotic activities can vary substantially depending on the compounds' physicochemical properties, such as amphiphilicity and charge, and thus trigger diverse cellular effects that are decisive for antibiotic activity.

Structure-Activity Relationships of Insect Defensins

Insects make up the largest and most diverse group of organisms on earth with several million species to exist in total. Considering the sheer number of insect species and the vast variety of ways they interact with their environment through chemistry, it is clear that they have significant potential as a source of new lead molecules. They have adapted to a range of ecological habitats and exhibit a symbiotic lifestyle with various microbes such as bacteria and fungi. Accordingly, numerous antimicrobial compounds have been identified including for example defensin peptides. Insect defensins were found to have broad-spectrum activity against various gram-positive/negative bacteria as well as fungi. They exhibit a unique structural topology involving the complex arrangement of three disulfide bonds as well as an alpha helix and beta sheets, which is known as cysteine-stabilized αβ motif. Their stability and amenability to peptide engineering make them promising candidates for the development of novel antibiotics lead molecules. This review highlights the current knowledge regarding the structure-activity relationships of insect defensin peptides and provides basis for future studies focusing on the rational design of novel cysteine-rich antimicrobial peptides.

A novel anti-lipopolysaccharide factor SpALF6 in mud crab Scylla paramamosain exhibiting different antimicrobial activity from its single amino acid mutant

In crustaceans, anti-lipopolysaccharide factors (ALFs) are important immune effectors that have sequence diversity and exhibit broad antimicrobial activities. In this study, we characterized a novel ALF homolog SpALF6 from mud crab Scylla paramamosain and its variant SpALF6-V, which was generated by mutations of two amino acids (H46 to R and A110 to P) due to the presence of two single nucleotide polymorphisms (SNPs). SpALF6 was an anionic peptide with isoelectric point (pI) 6.79, whereas SpALF6-V was a cationic protein with pI 7.98. These two proteins shared a common lipopolysaccharide (LPS)-binding domain (LBD) with pI 6.05. SpALF6 was expressed mainly in hemocytes and up-regulated by Vibrio parahaemolyticus or Staphylococcus aureus challenge, indicating that SpALF6 may participate in the antibacterial immune responses. To investigate the likely functional differences between SpALF6 and SpALF6-V and elucidate the underlying mechanisms, a single amino acid mutant SpALF6-M (from H46 to R, outside but very close to LBD), which had the same pI as SpALF6-V, was harvested by a fusion PCR. Then, both SpALF6 and SpALF6-M were overexpressed and purified to test antimicrobial activity and binding activity to microbial cells or polysaccharides. SpALF6-M exhibited more potent antimicrobial and cell-binding activity on Gram-positive bacteria and fungi than SpALF6. Furthermore, SpALF6-M possessed stronger lipoteichoic acid (LTA)-binding activity than SpALF6, demonstrating that this particular positively charged amino acid outside but close to LBD contributed to the increase in SpALF6-M antibacterial activity. In addition, SpALF6 LBD peptide and its biotin-labeled form were synthesized in this study. Results showed that this anionic LBD peptide itself did not exhibit any significant antimicrobial activity against 10 kinds of microorganisms but it possessed strong binding activity to LPS, LTA, and peptidoglycan. These findings suggested that this anionic LBD was still an important active center and required collaboration with some particular positively charged amino acids outside LBD to exhibit antibacterial activity. Thus, SpALF6-M antimicrobial activity was increased by the mutation of H46 to R instead of A110 to P, which did not change the protein charge, suggesting that SpALF6-V may have more potent antimicrobial activity than SpALF6 and play more important roles in antibacterial immunity. This study provided a new insight into the mechanisms of how ALF amino acid sequence diversity resulted in their functional divergence.

Oxidative stress induced in E. coli by the human antimicrobial peptide LL-37

Antimicrobial peptides (AMPs) are thought to kill bacterial cells by permeabilizing their membranes. However, some antimicrobial peptides inhibit E. coli growth more efficiently in aerobic than in anaerobic conditions. In the attack of the human cathelicidin LL-37 on E. coli, real-time, single-cell fluorescence imaging reveals the timing of membrane permeabilization and the onset of oxidative stress. For cells growing aerobically, a CellROX Green assay indicates that LL-37 induces rapid formation of oxidative species after entry into the periplasm, but before permeabilization of the cytoplasmic membrane (CM). A cytoplasmic Amplex Red assay signals a subsequent burst of oxidative species, most likely hydrogen peroxide, shortly after permeabilization of the CM. These signals are much stronger in the presence of oxygen, a functional electron transport chain, and a large proton motive force (PMF). They are much weaker in cells growing anaerobically, by either fermentation or anaerobic respiration. In aerobic growth, the oxidative signals are attenuated in a cytochrome oxidase–bd deletion mutant, but not in a –bo₃ deletion mutant, suggesting a specific effect of LL-37 on the electron transport chain. The AMPs melittin and LL-37 induce strong oxidative signals and exhibit O₂-sensitive MICs, while the AMPs indolicidin and cecropin A do not. These results suggest that AMP activity in different tissues may be tuned according to the local oxygen level. This may be significant for control of opportunistic pathogens while enabling growth of commensal bacteria.

Antimicrobial peptides play a significant role in the innate immune response of plants and animals, including humans. While it is well known that AMPs can permeabilize bacterial cell membranes, a growing body of evidence indicates that they cause a variety of additional deleterious effects. Here we use single-cell imaging methods to study the induction of oxidative stress in live E. coli by several natural cationic AMPs, including the human cathelicidin LL-37. Strong fluorescence signals indicative of oxidative stress correlate with smaller minimum inhibitory concentrations (MICs) in aerobic vs anaerobic growth conditions. A detailed mechanistic study suggests that LL-37 disrupts the proper flow of electrons through the electron transport chain, releasing oxidative species into the periplasm. Based on these results, we suggest that the degree of aeration in different tissue types may be used by the host to modulate AMP efficacy.

Potent in vitro and in vivo antifungal activity of a small molecule host defense peptide mimic through a membrane-active mechanism

Lethal systemic fungal infections of Candida species are increasingly common, especially in immune compromised patients. By in vitro screening of small molecule mimics of naturally occurring host defense peptides (HDP), we have identified several active antifungal molecules, which also exhibited potent activity in two mouse models of oral candidiasis. Here we show that one such compound, C4, exhibits a mechanism of action that is similar to the parent HDP upon which it was designed. Specifically, its initial interaction with the anionic microbial membrane is electrostatic, as its fungicidal activity is inhibited by cations. We observed rapid membrane permeabilization to propidium iodide and ATP efflux in response to C4. Unlike the antifungal peptide histatin 5, it did not require energy-dependent transport across the membrane. Rapid membrane disruption was observed by both fluorescence and electron microscopy. The compound was highly active in vitro against numerous fluconazole-resistant clinical isolates of C. albicans and non-albicans species, and it exhibited potent, dose-dependent activity in a mouse model of invasive candidiasis, reducing kidney burden by three logs after 24 hours, and preventing mortality for up to 17 days. Together the results support the development of this class of antifungal drug to treat invasive candidiasis.

Antifungal Activity against Filamentous Fungi of Ts1, a Multifunctional Toxin from Tityus serrulatus Scorpion Venom

Antimicrobial peptides (AMPs) are ubiquitous and multipotent components of the innate immune defense arsenal used by both prokaryotic and eukaryotic organisms. The search for new AMPs has increased in recent years, due to the growing development of microbial resistance to therapeutical drugs. In this work, we evaluate the effects of Tityus serrulatus venom (Tsv), its fractions and its major toxin Ts1, a beta-neurotoxin, on fungi growth. The fractions were obtained by ion-exchange chromatography of Tsv. The growth inhibition of 11 pathogenic and non-pathogenic filamentous fungi (Aspergillus fumigatus, A. nidulans, A. niger, A. terreus, Neurospora crassa, Penicillium corylophilum, P. ochrochloron, P. verrucosum, P. viridicatum, P. waksmanii, and Talaromyces flavus) was evaluated by quantitative microplate reader assay. Tsv (100 and 500 μg/well, which correspond to 1 and 5 mg/mL, respectively, of total soluble protein) was active in inhibiting growth of A. nidulans, A. terreus, P. corylophilum, and P. verrucosum, especially in the higher concentration used and at the first 30 h. After this period, fungi might have used Tsv components as alternative sources of nutrients, and therefore, increased their growth tax. Only fractions IX, X, XI, XIIA, XIIB (3 and 7.5 μg/well, which correspond to 30 and 75 μg/mL, respectively, of total soluble protein) and Ts1 (1.5, 3, and 6 μg/well, which correspond to 2.18, 4.36, and 8.72 μM, respectively) showed antifungal activity. Ts1 showed to be a non-morphogenic toxin with dose-dependent activity against A. nidulans, inhibiting 100% of fungal growth from 3 μg/well (4.36 μM). The inhibitory effect of Ts1 against A. nidulans growth was accompanied by fungistatic effects and was not amended by 1 mM CaCl₂ or tetrodotoxin (46.98 and 93.96 μM). The structural differences between Ts1 and drosomycin, a potent cysteine-rich antifungal peptide, are discussed here. Our results highlight the antifungal potential of the first cysteine-containing scorpion toxin. Since Ts1 is a multifunctional toxin, we suggest that it could be used as a template in the design of engineered scorpion AMPs and in the search for new mechanisms of action of antifungal drugs.

Investigations into the killing activity of an antimicrobial peptide active against extensively antibiotic-resistant K. pneumon iae and P. aeruginosa

SET-M33 is a multimeric antimicrobial peptide active against Gram-negative bacteria in vitro and in vivo. Insights into its killing mechanism could elucidate correlations with selectivity. SET-M33 showed concentration-dependent bactericidal activity against colistin-susceptible and resistant isolates of P. aeruginosa and K. pneumoniae. Scanning and transmission microscopy studies showed that SET-M33 generated cell blisters, blebs, membrane stacks and deep craters in K. pneumoniae and P. aeruginosa cells. NMR analysis and CD spectra in the presence of sodium dodecyl sulfate micelles showed a transition from an unstructured state to a stable α-helix, driving the peptide to arrange itself on the surface of micelles. SET-M33 kills Gram-negative bacteria after an initial interaction with bacterial LPS. The molecule becomes then embedded in the outer membrane surface, thereby impairing cell function. This activity of SET-M33, in contrast to other similar antimicrobial peptides such as colistin, does not generate resistant mutants after 24h of exposure, non-specific interactions or toxicity against eukaryotic cell membranes, suggesting that SET-M33 is a promising new option for the treatment of Gram-negative antibiotic-resistant infections.

Plant-microbial interactions in agriculture and the use of farming systems to improve diversity and productivity

A thorough understanding of the services provided by microorganisms to the agricultural ecosystem is integral to understanding how management systems can improve or deteriorate soil health and production over the long term. Yet it is hampered by the difficulty in measuring the intersection of plant, microbe, and environment, in no small part because of the situational specificity to some plant-microbial interactions, related to soil moisture, nutrient content, climate, and local diversity. Despite this, perspective on soil microbiota in agricultural settings can inform management practices to improve the sustainability of agricultural production.

Peptides, Peptidomimetics, and Polypeptides from Marine Sources: A Wealth of Natural Sources for Pharmaceutical Applications

Nature provides a variety of peptides that are expressed in most living species. Evolutionary pressure and natural selection have created and optimized these peptides to bind to receptors with high affinity. Hence, natural resources provide an abundant chemical space to be explored in peptide-based drug discovery. Marine peptides can be extracted by simple solvent extraction techniques. The advancement of analytical techniques has made it possible to obtain pure peptides from natural resources. Extracted peptides have been evaluated as possible therapeutic agents for a wide range of diseases, including antibacterial, antifungal, antidiabetic and anticancer activity as well as cardiovascular and neurotoxin activity. Although marine resources provide thousands of possible peptides, only a few peptides derived from marine sources have reached the pharmaceutical market. This review focuses on some of the peptides derived from marine sources in the past ten years and gives a brief review of those that are currently in clinical trials or on the market.

Antifungal plant defensins: increased insight in their mode of action as a basis for their use to combat fungal infections

Plant defensins are small, cationic peptides with a highly conserved 3D structure. They have been studied extensively in the past decades. Various biological activities have been attributed to plant defensins, such as anti-insect and antimicrobial activities, but they are also known to affect ion channels and display antitumor activity. This review focuses on the structure, biological activity and antifungal mode of action of some well-characterized plant defensins, with particular attention to their fungal membrane target(s), their induced cell death mechanisms as well as their antibiofilm activity. As plant defensins are, in general, not toxic to human cells, show in vivo efficacy and have low frequencies of resistance occurrence, they are of particular interest in the fight against fungal infections.

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.

New Found Hope for Antibiotic Discovery: Lipid II Inhibitors

Research into antibacterial agents has recently gathered pace in light of the disturbing crisis of antimicrobial resistance. The development of modern tools offers the opportunity of reviving the fallen era of antibacterial discovery through uncovering novel lead compounds that target vital bacterial cell components, such as lipid II. This paper provides a summary of the role of lipid II as well as an overview and insight into the structural features of macrocyclic peptides that inhibit this bacterial cell wall component. The recent discovery of teixobactin, a new class of lipid II inhibitor has generated substantial research interests. As such, the significant progress that has been achieved towards its development as a promising antibacterial agent is discussed.

Low pH Enhances the Action of Maximin H5 against Staphylococcus aureus and Helps Mediate Lysylated Phosphatidylglycerol-Induced Resistance

Maximin H5 (MH5) is an amphibian antimicrobial peptide specifically targeting Staphylococcus aureus. At pH 6, the peptide showed an improved ability to penetrate (ΔΠ = 6.2 mN m(-1)) and lyse (lysis = 48%) Staphylococcus aureus membrane mimics, which incorporated physiological levels of lysylated phosphatidylglycerol (Lys-PG, 60%), compared to that at pH 7 (ΔΠ = 5.6 mN m(-1) and lysis = 40% at pH 7) where levels of Lys-PG are lower (40%). The peptide therefore appears to have optimal function at pH levels known to be optimal for the organism's growth. MH5 killed S. aureus (minimum inhibitory concentration of 90 μM) via membranolytic mechanisms that involved the stabilization of α-helical structure (approximately 45-50%) and showed similarities to the "Carpet" mechanism based on its ability to increase the rigidity (Cs(-1) = 109.94 mN m(-1)) and thermodynamic stability (ΔGmix = -3.0) of physiologically relevant S. aureus membrane mimics at pH 6. On the basis of theoretical analysis, this mechanism might involve the use of a tilted peptide structure, and efficacy was noted to vary inversely with the Lys-PG content of S. aureus membrane mimics for each pH studied (R(2) ∼ 0.97), which led to the suggestion that under biologically relevant conditions, low pH helps mediate Lys-PG-induced resistance in S. aureus to MH5 antibacterial action. The peptide showed a lack of hemolytic activity (<2% hemolysis) and merits further investigation as a potential template for development as an antistaphylococcal agent in medically and biotechnically relevant areas.

Phenylbutyrate induces LL-37-dependent autophagy and intracellular killing of Mycobacterium tuberculosis in human macrophages

LL-37 is a human antimicrobial peptide (AMP) of the cathelicidin family with multiple activities including a mediator of vitamin D-induced autophagy in human macrophages, resulting in intracellular killing of Mycobacterium tuberculosis (Mtb). In a previous trial in healthy volunteers, we have shown that LL-37 expression and subsequent Mtb-killing can be further enhanced by 4-phenylbutyrate (PBA), also an inducer of LL-37 expression. Here, we explore a potential mechanism(s) behind PBA and LL-37-induced autophagy and intracellular killing of Mtb. Mtb infection of macrophages downregulated the expression of both the CAMP transcript and LL-37 peptide as well as certain autophagy-related genes (BECN1 and ATG5) at both the mRNA and protein levels. In addition, activation of LC3-II in primary macrophages and THP-1 cells was not detected. PBA and the active form of vitamin D3 (1,25[OH]2D3), separately or particularly in combination, were able to overcome Mtb-induced suppression of LL-37 expression. Notably, reactivation of autophagy occurred by stimulation of macrophages with PBA and promoted colocalization of LL-37 and LC3-II in autophagosomes. Importantly, PBA treatment failed to induce autophagy in Mtb-infected THP-1 cells, when the expression of LL-37 was silenced. However, PBA-induced autophagy was restored when the LL-37 knockdown cells were supplemented with synthetic LL-37. Interestingly, we have found that LL-37-induced autophagy was mediated via P2RX7 receptor followed by enhanced cytosolic free Ca(2+), and activation of AMPK and PtdIns3K pathways. Altogether, these results suggest a novel activity for PBA as an inducer of autophagy, which is LL-37-dependent and promotes intracellular killing of Mtb in human macrophages.

Lipopolysaccharide Biosynthesis Genes of Yersinia pseudotuberculosis Promote Resistance to Antimicrobial Chemokines

Antimicrobial chemokines (AMCs) are a recently described family of host defense peptides that play an important role in protecting a wide variety of organisms from bacterial infection. Very little is known about the bacterial targets of AMCs or factors that influence bacterial susceptibility to AMCs. In an effort to understand how bacterial pathogens resist killing by AMCs, we screened Yersinia pseudotuberculosis transposon mutants for those with increased binding to the AMCs CCL28 and CCL25. Mutants exhibiting increased binding to AMCs were subjected to AMC killing assays, which revealed their increased sensitivity to chemokine-mediated cell death. The majority of the mutants exhibiting increased binding to AMCs contained transposon insertions in genes related to lipopolysaccharide biosynthesis. A particularly strong effect on susceptibility to AMC mediated killing was observed by disruption of the hldD/waaF/waaC operon, necessary for ADP-L-glycero-D-manno-heptose synthesis and a complete lipopolysaccharide core oligosaccharide. Periodate oxidation of surface carbohydrates also enhanced AMC binding, whereas enzymatic removal of surface proteins significantly reduced binding. These results suggest that the structure of Y. pseudotuberculosis LPS greatly affects the antimicrobial activity of AMCs by shielding a protein ligand on the bacterial cell surface.

Psd1 binding affinity toward fungal membrane components as assessed by SPR: The role of glucosylceramide in fungal recognition and entry

Psd1 is a plant defensin that has antifungal activity against several pathogenic and nonpathogenic fungi. Previous analysis of Psd1 chemical shift perturbations by nuclear magnetic resonance (NMR) spectroscopy demonstrated that this defensin interacts with phospholipids and the sphingolipid glucosylceramide isolated from Fusarium solani (GlcCer(Fusarium solani)). In this study, these interactions were evaluated by real-time surface plasmon resonance (SPR) analysis. The data obtained demonstrated that Psd1 could bind more strongly to small unilamellar vesicles (SUV)-containing GlcCer(Fusarium solani) than to SUV that was composed of phosphatidylcholine (PC) alone or was enriched with GlcCer that had been isolated from soybeans. An increase in the SPR response after cholesterol or ergosterol incorporation in PC-SUV was detected; however, SUV composed of PC:Erg (7:3; molar:molar) became unstable in the presence of Psd1, suggesting membrane destabilization. We also observed a lack of Psd1 internalization in Candida albicans strains that were deficient in the glucosyl ceramide synthase gene. Together, these data indicate that GlcCer is essential for Psd1 anchoring in the fungal plasma membrane as well as internalization.

Pseudomonas aeruginosa: targeting cell-wall metabolism for new antibacterial discovery and development

Pseudomonas aeruginosa is a leading cause of hospital-acquired infections and is resistant to most antibiotics. With therapeutic options against P. aeruginosa dwindling, and the lack of new antibiotics in advanced developmental stages, strategies for preserving the effectiveness of current antibiotics are urgently required. β-Lactam antibiotics are important agents for treating P. aeruginosa infections, thus, adjuvants that potentiate the activity of these compounds are desirable for extending their lifespan while new antibiotics - or antibiotic classes - are discovered and developed. In this review, we discuss recent research that has identified exploitable targets of cell-wall metabolism for the design and development of compounds that hinder resistance and potentiate the activity of antipseudomonal β-lactams.

Reduction Impairs the Antibacterial Activity but Benefits the LPS Neutralization Ability of Human Enteric Defensin 5

Oxidized human defensin 5 (HD5_OX), a Paneth cell-secreted antibacterial peptide with three characteristic disulfide bonds, protects the host from invasion by morbigenous microbes in the small intestine. HD5_OX can be reduced by thioredoxin (Trx) in vitro, while the biochemical properties of the reduced linear peptide, HD5_RED, remain unclear. Here, we first confirm that HD5_RED does exist in vivo. Furthermore, we reveal that the recruitment of HD5_RED to the outer membrane of Gram-negative bacteria and to the anionic lipid A is lower than that of HD5_OX, and HD5_RED is less efficient in penetrating bacterial outer and inner membranes and inducing membrane depolarization, which confers an attenuated antibacterial activity to HD5_RED. However, due to its higher structural flexibility, the binding of HD5_RED to bacterial lipopolysaccharide (LPS) is markedly stronger than that of HD5_OX. Consequently, HD5_RED is more effective in suppressing the production of the pro-inflammatory cytokine TNF-α in LPS-stimulated macrophages by blocking the interaction between LPS and LPS-binding protein, thus suggesting that HD5_RED might act as a scavenger to neutralize LPS in the gut. This study provides insights into the antibacterial and immunoregulatory effects of HD5_RED and expands the known repertoire of the enteric defensins.

Expression of antimicrobial peptides under control of a camalexin-biosynthetic promoter confers enhanced resistance against Pseudomonas syringae

In Arabidopsis thaliana phytoalexin biosynthesis is tightly regulated. The camalexin biosynthetic gene CYP71B15/PAD3 is highly expressed in response to pathogens and specific abiotic triggers, while constitutive expression is very low. Based on this property we expressed artificial antimicrobial peptides under control of the CYP71B15 promoter avoiding potential toxic effects to the plant related to constitutive expression. Significant and substantial growth inhibition of Pseudomonas syringae was observed, demonstrating that expression of these peptides under control of a phytoalexin promoter is an effective approach for enhancement of resistance against bacterial pathogens.

The radish defensins RsAFP1 and RsAFP2 act synergistically with caspofungin against Candida albicans biofilms

The radish defensin RsAFP2 was previously characterized as a peptide with potent antifungal activity against several plant pathogenic fungi and human pathogens, including Candida albicans. RsAFP2 induces apoptosis and impairs the yeast-to-hypha transition in C. albicans. As the yeast-to-hypha transition is considered important for progression to mature biofilms, we analyzed the potential antibiofilm activity of recombinant (r)RsAFP2, heterologously expressed in Pichia pastoris, against C. albicans biofilms. We found that rRsAFP2 prevents C. albicans biofilm formation with a BIC-2 (i.e., the minimal rRsAFP2 concentration that inhibits biofilm formation by 50% as compared to control treatment) of 1.65 ± 0.40 mg/mL. Moreover, biofilm-specific synergistic effects were observed between rRsAFP2 doses as low as 2.5 μg/mL to 10 μg/mL and the antimycotics caspofungin and amphotericin B, pointing to the potential of RsAFP2 as a novel antibiofilm compound. In addition, we characterized the solution structure of rRsAFP2 and compared it to that of RsAFP1, another defensin present in radish seeds. These peptides have similar amino acid sequences, except for two amino acids, but rRsAFP2 is more potent than RsAFP1 against planktonic and biofilm cultures. Interestingly, as in case of rRsAFP2, also RsAFP1 acts synergistically with caspofungin against C. albicans biofilms in a comparable low dose range as rRsAFP2. A structural comparison of both defensins via NMR analysis revealed that also rRsAFP2 adopts the typical cysteine-stabilized αβ-motif of plant defensins, however, no structural differences were found between these peptides that might result in their differential antifungal/antibiofilm potency. This further suggests that the conserved structure of RsAFP1 and rRsAFP2 bears the potential to synergize with antimycotics against C. albicans biofilms.

A hemocyanin-derived antimicrobial peptide from the penaeid shrimp adopts an alpha-helical structure that specifically permeabilizes fungal membranes

BACKGROUND

Hemocyanins are respiratory proteins with multiple functions. In diverse crustaceans hemocyanins can release histidine-rich antimicrobial peptides in response to microbial challenge. In penaeid shrimp, strictly antifungal peptides are released from the C-terminus of hemocyanins.

METHODS

The three-dimensional structure of the antifungal peptide PvHCt from Litopenaeus vannamei was determined by NMR. Its mechanism of action against the shrimp pathogen Fusarium oxysporum was investigated using immunochemistry, fluorescence and transmission electron microscopy.

RESULTS

PvHCt folded into an amphipathic α-helix in membrane-mimicking media and displayed a random conformation in aqueous environment. In contact with F. oxysporum, PvHCt bound massively to the surface of fungal hyphae without being imported into the cytoplasm. At minimal inhibitory concentrations, PvHCt made the fungal membrane permeable to SYTOX-green and fluorescent dextran beads of 4 kDa. Higher size beads could not enter the cytoplasm. Therefore, PvHCt likely creates local damages to the fungal membrane. While the fungal cell wall appeared preserved, gradual degeneration of the cytoplasm most often resulting in cell lysis was observed in fungal spores and hyphae. In the remaining fungal cells, PvHCt induced a protective response by the formation of daughter hyphae.

CONCLUSION

The massive accumulation of PvHCt at the surface of fungal hyphae and subsequent insertion into the plasma membrane disrupt its integrity as a permeability barrier, leading to disruption of internal homeostasis and fungal death.

GENERAL SIGNIFICANCE

The histidine-rich antimicrobial peptide PvHCt derived from shrimp hemocyanin is a strictly antifungal peptide, which adopts an amphipathic α-helical structure, and selectively binds to and permeabilizes fungal cells.

Lipid Requirements for the Enzymatic Activity of MraY Translocases and in Vitro Reconstitution of the Lipid II Synthesis Pathway

Screening of new compounds directed against key protein targets must continually keep pace with emerging antibiotic resistances. Although periplasmic enzymes of bacterial cell wall biosynthesis have been among the first drug targets, compounds directed against the membrane-integrated catalysts are hardly available. A promising future target is the integral membrane protein MraY catalyzing the first membrane associated step within the cytoplasmic pathway of bacterial peptidoglycan biosynthesis. However, the expression of most MraY homologues in cellular expression systems is challenging and limits biochemical analysis. We report the efficient production of MraY homologues from various human pathogens by synthetic cell-free expression approaches and their subsequent characterization. MraY homologues originating from Bordetella pertussis, Helicobacter pylori, Chlamydia pneumoniae, Borrelia burgdorferi, and Escherichia coli as well as Bacillus subtilis were co-translationally solubilized using either detergent micelles or preformed nanodiscs assembled with defined membranes. All MraY enzymes originating from Gram-negative bacteria were sensitive to detergents and required nanodiscs containing negatively charged lipids for obtaining a stable and functionally folded conformation. In contrast, the Gram-positive B. subtilis MraY not only tolerates detergent but is also less specific for its lipid environment. The MraY·nanodisc complexes were able to reconstitute a complete in vitro lipid I and lipid II forming pipeline in combination with the cell-free expressed soluble enzymes MurA-F and with the membrane-associated protein MurG. As a proof of principle for future screening platforms, we demonstrate the inhibition of the in vitro lipid II biosynthesis with the specific inhibitors fosfomycin, feglymycin, and tunicamycin.

An intimate link between antimicrobial peptide sequence diversity and binding to essential components of bacterial membranes

Antimicrobial peptides and proteins (AMPs) are widespread in the living kingdom. They are key effectors of defense reactions and mediators of competitions between organisms. They are often cationic and amphiphilic, which favors their interactions with the anionic membranes of microorganisms. Several AMP families do not directly alter membrane integrity but rather target conserved components of the bacterial membranes in a process that provides them with potent and specific antimicrobial activities. Thus, lipopolysaccharides (LPS), lipoteichoic acids (LTA) and the peptidoglycan precursor Lipid II are targeted by a broad series of AMPs. Studying the functional diversity of immune effectors tells us about the essential residues involved in AMP mechanism of action. Marine invertebrates have been found to produce a remarkable diversity of AMPs. Molluscan defensins and crustacean anti-LPS factors (ALF) are diverse in terms of amino acid sequence and show contrasted phenotypes in terms of antimicrobial activity. Their activity is directed essentially against Gram-positive or Gram-negative bacteria due to their specific interactions with Lipid II or Lipid A, respectively. Through those interesting examples, we discuss here how sequence diversity generated throughout evolution informs us on residues required for essential molecular interaction at the bacterial membranes and subsequent antibacterial activity. Through the analysis of molecular variants having lost antibacterial activity or shaped novel functions, we also discuss the molecular bases of functional divergence in AMPs. This article is part of a Special Issue entitled: Antimicrobial peptides edited by Karl Lohner and Kai Hilpert.

Antimicrobial Peptides in Human Sepsis

Nearly 100 years ago, antimicrobial peptides (AMPs) were identified as an important part of innate immunity. They exist in species from bacteria to mammals and can be isolated in body fluids and on surfaces constitutively or induced by inflammation. Defensins have anti-bacterial effects against Gram-positive and Gram-negative bacteria as well as anti-viral and anti-yeast effects. Human neutrophil peptides (HNP) 1-3 and human beta-defensins (HBDs) 1-3 are some of the most important defensins in humans. Recent studies have demonstrated higher levels of HNP 1-3 and HBD-2 in sepsis. The bactericidal/permeability-increasing protein (BPI) attenuates local inflammatory response and decreases systemic toxicity of endotoxins. Moreover, BPI might reflect the severity of organ dysfunction in sepsis. Elevated plasma lactoferrin is detected in patients with organ failure. HNP 1-3, lactoferrin, BPI, and heparin-binding protein are increased in sepsis. Human lactoferrin peptide 1-11 (hLF 1-11) possesses antimicrobial activity and modulates inflammation. The recombinant form of lactoferrin [talactoferrin alpha (TLF)] has been shown to decrease mortality in critically ill patients. A phase II/III study with TLF in sepsis did not confirm this result. The growing number of multiresistant bacteria is an ongoing problem in sepsis therapy. Furthermore, antibiotics are known to promote the liberation of pro-inflammatory cell components and thus augment the severity of sepsis. Compared to antibiotics, AMPs kill bacteria but also neutralize pathogenic factors such as lipopolysaccharide. The obstacle to applying naturally occurring AMPs is their high nephro- and neurotoxicity. Therefore, the challenge is to develop peptides to treat septic patients effectively without causing harm. This overview focuses on natural and synthetic AMPs in human and experimental sepsis and their potential to provide significant improvements in the treatment of critically ill with severe infections.

Synergistic Activity of the Plant Defensin HsAFP1 and Caspofungin against Candida albicans Biofilms and Planktonic Cultures

Plant defensins are small, cysteine-rich peptides with antifungal activity against a broad range of yeast and fungi. In this study we investigated the antibiofilm activity of a plant defensin from coral bells (Heuchera sanguinea), i.e. HsAFP1. To this end, HsAFP1 was heterologously produced using Pichia pastoris as a host. The recombinant peptide rHsAFP1 showed a similar antifungal activity against the plant pathogen Fusarium culmorum as native HsAFP1 purified from seeds. NMR analysis revealed that rHsAFP1 consists of an α-helix and a triple-stranded antiparallel β-sheet stabilised by four intramolecular disulfide bonds. We found that rHsAFP1 can inhibit growth of the human pathogen Candida albicans as well as prevent C. albicans biofilm formation with a BIC50 (i.e. the minimum rHsAFP1 concentration required to inhibit biofilm formation by 50% as compared to control treatment) of 11.00 ± 1.70 μM. As such, this is the first report of a plant defensin exhibiting inhibitory activity against fungal biofilms. We further analysed the potential of rHsAFP1 to increase the activity of the conventional antimycotics caspofungin and amphotericin B towards C. albicans. Synergistic effects were observed between rHsAFP1 and these compounds against both planktonic C. albicans cells and biofilms. Most notably, concentrations of rHsAFP1 as low as 0.53 μM resulted in a synergistic activity with caspofungin against pre-grown C. albicans biofilms. rHsAFP1 was found non-toxic towards human HepG2 cells up to 40 μM, thereby supporting the lack of a general cytotoxic activity as previously reported for HsAFP1. A structure-function study with 24-mer synthetic peptides spanning the entire HsAFP1 sequence revealed the importance of the γ-core and its adjacent regions for HsAFP1 antibiofilm activity. These findings point towards broad applications of rHsAFP1 and its derivatives in the field of antifungal and antibiofilm drug development.

Liquid storage of boar semen: Current and future perspectives on the use of cationic antimicrobial peptides to replace antibiotics in semen extenders

Antibiotics are of great importance in boar semen extenders to ensure long shelf life of spermatozoa and to reduce transmission of pathogens into the female tract. However, the use of antibiotics carries a risk of developing resistant bacterial strains in artificial insemination laboratories and their spread via artificial insemination. Development of multiresistant bacteria is a major concern if mixtures of antibiotics are used in semen extenders. Minimal contamination prevention techniques and surveillance of critical hygiene control points proved to be efficient in reducing bacterial load and preventing development of antibiotic resistance. Nevertheless, novel antimicrobial concepts are necessary for efficient bacterial control in extended boar semen with a minimum risk of evoking antibiotic resistance. Enhanced efforts have been made in recent years in the design and use of antimicrobial peptides (AMPs) as alternatives to conventional antibiotics. The male genital tract harbors a series of endogenic substances with antimicrobial activity and additional functions relevant to the fertilization process. However, exogenic AMPs often exert dose- and time-dependent toxic effects on mammalian spermatozoa. Therefore, it is important that potential newly designed AMPs have only minor impacts on eukaryotic cells. Recently, synthetic magainin derivatives and cyclic hexapeptides were tested for their application in boar semen preservation. Bacterial selectivity, proteolytic stability, thermodynamic resistance, and potential synergistic interaction with conventional antibiotics propel predominantly cyclic hexapeptides into highly promising, leading candidates for further development in semen preservation. The time scale for the development of resistant pathogens cannot be predicted at this moment.

Insect Antimicrobial Peptide Complexes Prevent Resistance Development in Bacteria

In recent decades much attention has been paid to antimicrobial peptides (AMPs) as natural antibiotics, which are presumably protected from resistance development in bacteria. However, experimental evolution studies have revealed prompt resistance increase in bacteria to any individual AMP tested. Here we demonstrate that naturally occurring compounds containing insect AMP complexes have clear advantage over individual peptide and small molecule antibiotics in respect of drug resistance development. As a model we have used the compounds isolated from bacteria challenged maggots of Calliphoridae flies. The compound isolated from blow fly Calliphora vicina was found to contain three distinct families of cell membrane disrupting/permeabilizing peptides (defensins, cecropins and diptericins), one family of proline rich peptides and several unknown antimicrobial substances. Resistance changes under long term selective pressure of the compound and reference antibiotics cefotaxime, meropenem and polymyxin B were tested using Escherichia coli, Klebsiella pneumonia and Acinetobacter baumannii clinical strains. All the strains readily developed resistance to the reference antibiotics, while no signs of resistance growth to the compound were registered. Similar results were obtained with the compounds isolated from 3 other fly species. The experiments revealed that natural compounds containing insect AMP complexes, in contrast to individual AMP and small molecule antibiotics, are well protected from resistance development in bacteria. Further progress in the research of natural AMP complexes may provide novel solutions to the drug resistance problem.

Potential effect of cationic liposomes on interactions with oral bacterial cells and biofilms

CONTEXT

Although oral infectious diseases have been attributed to bacteria, drug treatments remain ineffective because bacteria and their products exist as biofilms. Cationic liposomes have been suggested to electrostatically interact with the negative charge on the bacterial surface, thereby improving the effects of conventional drug therapies. However, the electrostatic interaction between oral bacteria and cationic liposomes has not yet been examined in detail.

OBJECTIVE

The aim of the present study was to examine the behavior of cationic liposomes and Streptococcus mutans in planktonic cells and biofilms.

MATERIALS AND METHODS

Liposomes with or without cationic lipid were prepared using a reverse-phase evaporation method. The zeta potentials of conventional liposomes (without cationic lipid) and cationic liposomes were -13 and 8 mV, respectively, and both had a mean particle size of approximately 180 nm. We first assessed the interaction between liposomes and planktonic bacterial cells with a flow cytometer. We then used a surface plasmon resonance method to examine the binding of liposomes to biofilms. We confirmed the binding behavior of liposomes with biofilms using confocal laser scanning microscopy.

RESULTS

The interactions between cationic liposomes and S. mutans cells and biofilms were stronger than those of conventional liposomes. Microscopic observations revealed that many cationic liposomes interacted with the bacterial mass and penetrated the deep layers of biofilms.

DISCUSSION AND CONCLUSION

In this study, we demonstrated that cationic liposomes had higher affinity not only to oral bacterial cells, but also biofilms than conventional liposomes. This electrostatic interaction may be useful as a potential drug delivery system to biofilms.

The increasing role of phosphatidylethanolamine as a lipid receptor in the action of host defence peptides

Host defence peptides (HDPs) are antimicrobial agents produced by organisms across the prokaryotic and eukaryotic kingdoms. Many prokaryotes produce HDPs, which utilise lipid and protein receptors in the membranes of bacterial competitors to facilitate their antibacterial action and thereby survive in their niche environment. As a major example, it is well established that cinnamycin and duramycins from Streptomyces have a high affinity for phosphatidylethanolamine (PE) and exhibit activity against other Gram-positive organisms, such as Bacillus. In contrast, although eukaryotic HDPs utilise membrane interactive mechanisms to facilitate their antimicrobial activity, the prevailing view has long been that these mechanisms do not involve membrane receptors. However, this view has been recently challenged by reports that a number of eukaryotic HDPs such as plant cyclotides also use PE as a receptor to promote their antimicrobial activities. Here, we review current understanding of the mechanisms that underpin the use of PE as a receptor in the antimicrobial and other biological actions of HDPs and describe medical and biotechnical uses of these peptides, which range from tumour imaging and detection to inclusion in topical microbicidal gels to prevent the sexual transmission of HIV.

Structural variations of the cell wall precursor lipid II in Gram-positive bacteria - Impact on binding and efficacy of antimicrobial peptides

Antimicrobial peptides (AMPs) are natural antibiotics produced by virtually all living organisms. Typically, AMPs are cationic and amphiphilic and first contacts with target microbes involve interactions with negatively charged components of the cell envelope such as lipopolysaccharide (LPS), and wall- or lipoteichoic acids (WTA, LTA). The importance of charge-mediated interactions of AMPs with the cell envelope is reflected by effective microbial resistance mechanisms which are based on reduction of the overall charge of these polymers. The anionic polymers are linked in various ways to the stress-bearing polymer of the cell envelope, the peptidoglycan, which is made of a highly conserved building block, a disaccharide-pentapeptide moiety that also contains charged residues. This structural element, in spite of its conservation throughout the bacterial world, can undergo genus- and species-specific modifications that also impact significantly on the overall charge of the cell envelope and on the binding affinity of AMPs. The modification reactions involved largely occur on the membrane-bound peptidoglycan building block, the so-called lipid II, which is a most prominent target for AMPs. In this review, we focus on modifications of lipid II and peptidoglycan and discuss their consequences for the interactions with various classes of AMPs, such as defensins, lantibiotics and glyco-(lipo)-peptide antibiotics. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.

Evidence for a novel mechanism of antimicrobial action of a cyclic R-,W-rich hexapeptide

The development of antimicrobial peptides as new class of antibiotic agents requires structural characterisation and understanding of their diverse mechanisms of action. As the cyclic hexapeptide cWFW (cyclo(RRRWFW)) does not exert its rapid cell killing activity by membrane permeabilisation, in this study we investigated alternative mechanisms of action, such as peptide translocation into the cytoplasm and peptide interaction with components of the phospholipid matrix of the bacterial membrane. Using fluorescence microscopy and an HPLC-based strategy to analyse peptide uptake into the cells we could confirm the cytoplasmic membrane as the major peptide target. However, unexpectedly we observed accumulation of cWFW at distinct sites of the membrane. Further characterisation of peptide-membrane interaction involved live cell imaging to visualise the distribution of the lipid cardiolipin (CL) and isothermal titration calorimetry to determine the binding affinity to model membranes with different bacterial lipid compositions. Our results demonstrate a distribution of the cyclic peptide similar to that of cardiolipin within the membrane and highly preferred affinity of cWFW for CL-rich phosphatidylethanolamine (POPE) matrices. These observations point to a novel mechanism of antimicrobial killing for the cyclic hexapeptide cWFW which is neither based on membrane permeabilisation nor translocation into the cytoplasm but rather on preferred partitioning into particular lipid domains. As the phospholipids POPE/CL play a key role in the dynamic organisation of bacterial membranes we discuss the consequences of this peptide-lipid-interaction and outline the impact on antimicrobial peptide research.

Antibacterial products of marine organisms

Marine organisms comprising microbes, plants, invertebrates, and vertebrates elaborate an impressive array of structurally diverse antimicrobial products ranging from small cyclic compounds to macromolecules such as proteins. Some of these biomolecules originate directly from marine animals while others arise from microbes associated with the animals. It is noteworthy that some of the biomolecules referred to above are structurally unique while others belong to known classes of compounds, peptides, and proteins. Some of the antibacterial agents are more active against Gram-positive bacteria while others have higher effectiveness on Gram-negative bacteria. Some are efficacious against both Gram-positive and Gram-negative bacteria and against drug-resistant strains as well. The mechanism of antibacterial action of a large number of the chemically identified antibacterial agents, possible synergism with currently used antibiotics, and the issue of possible toxicity on mammalian cells and tissues await elucidation. The structural characteristics pivotal to antibacterial activity have been ascertained in only a few studies. Demonstration of efficacy of the antibacterial agents in animal models of bacterial infection is highly desirable. Structural characterization of the active principles present in aqueous and organic extracts of marine organisms with reportedly antibacterial activity would be desirable.

Concatemerization increases the inhibitory activity of short, cell-penetrating, cationic and tryptophan-rich antifungal peptides

There are short cationic and tryptophan-rich antifungal peptides such as the hexapeptide PAF26 (RKKWFW) that have selective toxicity and cell penetration properties against fungal cells. This study demonstrates that concatemeric peptides with tandem repeats of the heptapeptide PAF54 (which is an elongated PAF26 sequence) show increased fungistatic and bacteriostatic activities while maintaining the absence of hemolytic activity of the monomer. The increase in antimicrobial activity of the double-repeated PAF sequences (diPAFs), compared to the nonrepeated PAF, was higher (4-8-fold) than that seen for the triple-repeated sequences (triPAFs) versus the diPAFs (2-fold). However, concatemerization diminished the fungicidal activity against quiescent spores of the filamentous fungus Penicillium digitatum. Peptide solubility and sensitivity to proteolytic degradation were affected by the design of the concatemers: incorporation of the AGPA sequence hinge to separate PAF54 repeats increased solubility while the C-terminal addition of the KDEL sequence decreased in vitro stability. These results led to the design of the triPAF sequence PAF102 of 30 amino acid residues, with increased antimicrobial activity and minimal inhibitory concentration (MIC) value of 1-5 μM depending on the fungus. Further characterization of the mode-of-action of PAF102 demonstrated that it colocalizes first with the fungal cell wall, it is thereafter internalized in an energy dependent manner into hyphal cells of the filamentous fungus Fusarium proliferatum, and finally kills hyphal cells intracellularly. Therefore, PAF102 showed mechanistic properties against fungi similar to the parental PAF26. These observations are of high interest in the future development of PAF-based antimicrobial molecules optimized for their production in biofactories.

ARACINs, Brassicaceae-specific peptides exhibiting antifungal activities against necrotrophic pathogens in Arabidopsis

Plants have developed a variety of mechanisms to cope with abiotic and biotic stresses. In a previous subcellular localization study of hydrogen peroxide-responsive proteins, two peptides with an unknown function (designated ARACIN1 and ARACIN2) have been identified. These peptides are structurally very similar but are transcriptionally differentially regulated during abiotic stresses during Botrytis cinerea infection or after benzothiadiazole and methyl jasmonate treatments. In Arabidopsis (Arabidopsis thaliana), these paralogous genes are positioned in tandem within a cluster of pathogen defense-related genes. Both ARACINs are small, cationic, and hydrophobic peptides, known characteristics for antimicrobial peptides. Their genes are expressed in peripheral cell layers prone to pathogen entry and are lineage specific to the Brassicaceae family. In vitro bioassays demonstrated that both ARACIN peptides have a direct antifungal effect against the agronomically and economically important necrotrophic fungi B. cinerea, Alternaria brassicicola, Fusarium graminearum, and Sclerotinia sclerotiorum and yeast (Saccharomyces cerevisiae). In addition, transgenic Arabidopsis plants that ectopically express ARACIN1 are protected better against infections with both B. cinerea and A. brassicicola. Therefore, we can conclude that both ARACINs act as antimicrobial peptides.

Multitasking antimicrobial peptides in plant development and host defense against biotic/abiotic stress

Crop losses due to pathogens are a major threat to global food security. Plants employ a multilayer defense against a pathogen including the use of physical barriers (cell wall), induction of hypersensitive defense response (HR), resistance (R) proteins, and synthesis of antimicrobial peptides (AMPs). Unlike a complex R gene-mediated immunity, AMPs directly target diverse microbial pathogens. Many a times, R-mediated immunity breaks down and plant defense is compromised. Although R-gene dependent pathogen resistance has been well studied, comparatively little is known about the interactions of AMPs with host defense and physiology. AMPs are ubiquitous, low molecular weight peptides that display broad spectrum resistance against bacteria, fungi and viruses. In plants, AMPs are mainly classified into cyclotides, defensins, thionins, lipid transfer proteins, snakins, and hevein-like vicilin-like and knottins. Genetic distance lineages suggest their conservation with minimal effect of speciation events during evolution. AMPs provide durable resistance in plants through a combination of membrane lysis and cellular toxicity of the pathogen. Plant hormones - gibberellins, ethylene, jasmonates, and salicylic acid, are among the physiological regulators that regulate the expression of AMPs. Transgenically produced AMP-plants have become a means showing that AMPs are able to mitigate host defense responses while providing durable resistance against pathogens.

Resistance to Antimicrobial Peptides in Vibrios

Vibrios are associated with a broad diversity of hosts that produce antimicrobial peptides (AMPs) as part of their defense against microbial infections. In particular, vibrios colonize epithelia, which function as protective barriers and express AMPs as a first line of chemical defense against pathogens. Recent studies have shown they can also colonize phagocytes, key components of the animal immune system. Phagocytes infiltrate infected tissues and use AMPs to kill the phagocytosed microorganisms intracellularly, or deliver their antimicrobial content extracellularly to circumvent tissue infection. We review here the mechanisms by which vibrios have evolved the capacity to evade or resist the potent antimicrobial defenses of the immune cells or tissues they colonize. Among their strategies to resist killing by AMPs, primarily vibrios use membrane remodeling mechanisms. In particular, some highly resistant strains substitute hexaacylated Lipid A with a diglycine residue to reduce their negative surface charge, thereby lowering their electrostatic interactions with cationic AMPs. As a response to envelope stress, which can be induced by membrane-active agents including AMPs, vibrios also release outer membrane vesicles to create a protective membranous shield that traps extracellular AMPs and prevents interaction of the peptides with their own membranes. Finally, once AMPs have breached the bacterial membrane barriers, vibrios use RND efflux pumps, similar to those of other species, to transport AMPs out of their cytoplasmic space.

In vitro synergistic activity of clofazimine and other antituberculous drugs against multidrug-resistant Mycobacterium tuberculosis isolates

Clofazimine (CLO) is a promising candidate drug for use in the management of multidrug-resistant tuberculosis (MDR-TB) patients. In this study, the minimum inhibitory concentration (MIC) method was used to investigate drug susceptibility to CLO as well as potential synergies between CLO and other antituberculous drugs, including ethambutol (EMB), levofloxacin (LEV), moxifloxacin (MOX), amikacin (AMK) and capreomycin (CAP), among MDR-TB isolates from China. A total of 195 MDR-TB isolates were collected from the national drug resistance survey conducted in China. Of the 195 MDR-TB isolates, 54 (27.7%) were classified as CLO-resistant, whilst 141 (72.3%) were CLO-susceptible with MICs of ≤ 1 μg/mL. In addition, the prevalence of CLO-resistant isolates among the extensively drug-resistant (XDR)-TB group was 61.5% (8/13), which was significantly higher than that of the MDR-TB group (23.0%) (P = 0.006). When fractional inhibitory concentration indexes (FICIs) were calculated for 24 isolates, synergy was found in 11 isolates (45.8%) against the CLO/EMB combination, 6 (25.0%) against the CLO/LEV combination, 8 (33.3%) against the CLO/MOX combination, 4 (16.7%) against the CLO/AMK combination and 5 (20.8%) against the CLO/CAP combination. In addition, <15% of MDR-TB isolates showed antagonistic effects against these five combinations. In conclusion, these findings demonstrate that the combination of CLO and EMB shows better synergism than the other combinations containing CLO. The CLO/MOX combination is more likely to show synergy against MDR-TB isolates than the CLO/LEV combination. Taken together, we suggest that CLO, in combination with EMB or MOX, may be a promising drug regimen for the treatment of MDR-TB.