Antifungal membranolytic activity of the tyrocidines against filamentous plant fungi

Bdelloid rotifers deploy horizontally acquired biosynthetic genes against a fungal pathogen

Coevolutionary antagonism generates relentless selection that can favour genetic exchange, including transfer of antibiotic synthesis and resistance genes among bacteria, and sexual recombination of disease resistance alleles in eukaryotes. We report an unusual link between biological conflict and DNA transfer in bdelloid rotifers, microscopic animals whose genomes show elevated levels of horizontal gene transfer from non-metazoan taxa. When rotifers were challenged with a fungal pathogen, horizontally acquired genes were over twice as likely to be upregulated as other genes - a stronger enrichment than observed for abiotic stressors. Among hundreds of upregulated genes, the most markedly overrepresented were clusters resembling bacterial polyketide and nonribosomal peptide synthetases that produce antibiotics. Upregulation of these clusters in a pathogen-resistant rotifer species was nearly ten times stronger than in a susceptible species. By acquiring, domesticating, and expressing non-metazoan biosynthetic pathways, bdelloids may have evolved to resist natural enemies using antimicrobial mechanisms absent from other animals.

Update on fungal lipid biosynthesis inhibitors as antifungal agents

Fungal diseases today represent a world-wide problem. Poor hygiene and decreased immunity are the main reasons behind the manifestation of this disease. After COVID-19, an increase in the rate of fungal infection has been observed in different countries. Different classes of antifungal agents, such as polyenes, azoles, echinocandins, and anti-metabolites, as well as their combinations, are currently employed to treat fungal diseases; these drugs are effective but can cause some side effects and toxicities. Therefore, the identification and development of newer antifungal agents is a current need. The fungal cell comprises many lipids, such as ergosterol, phospholipids, and sphingolipids. Ergosterol is a sterol lipid that is only found in fungal cells. Various pathways synthesize all these lipids, and the activities of multiple enzymes govern these pathways. Inhibiting these enzymes will ultimately impede the lipid synthesis pathway, and this phenomenon could be a potential antifungal therapy. This review will discuss various lipid synthesis pathways and multiple antifungal agents identified as having fungal lipid synthesis inhibition activity. This review will identify novel compounds that can inhibit fungal lipid synthesis, permitting researchers to direct further deep pharmacological investigation and help develop drug delivery systems for such compounds.

Cyclic Peptides with Antifungal Properties Derived from Bacteria, Fungi, Plants, and Synthetic Sources

Fungal infections remain a significant concern for human health. The emergence of microbial resistance, the improper use of antimicrobial drugs, and the need for fewer toxic antifungal treatments in immunocompromised patients have sparked substantial interest in antifungal research. Cyclic peptides, classified as antifungal peptides, have been in development as potential antifungal agents since 1948. In recent years, there has been growing attention from the scientific community to explore cyclic peptides as a promising strategy for combating antifungal infections caused by pathogenic fungi. The identification of antifungal cyclic peptides from various sources has been possible due to the widespread interest in peptide research in recent decades. It is increasingly important to evaluate narrow- to broad-spectrum antifungal activity and the mode of action of synthetic and natural cyclic peptides for both synthesized and extracted peptides. This short review aims to highlight some of the antifungal cyclic peptides isolated from bacteria, fungi, and plants. This brief review is not intended to present an exhaustive catalog of all known antifungal cyclic peptides but rather seeks to showcase selected cyclic peptides with antifungal properties that have been isolated from bacteria, fungi, plants, and synthetic sources. The addition of commercially available cyclic antifungal peptides serves to corroborate the notion that cyclic peptides can serve as a valuable source for the development of antifungal drugs. Additionally, this review discusses the potential future of utilizing combinations of antifungal peptides from different sources. The review underscores the need for the further exploration of the novel antifungal therapeutic applications of these abundant and diverse cyclic peptides.

Potent Antifungal Functions of a Living Modified Organism Protein, CP4-EPSPS, against Pathogenic Fungal Cells

Various proteins introduced into living modified organism (LMO) crops function in plant defense mechanisms against target insect pests or herbicides. This study analyzed the antifungal effects of an introduced LMO protein, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) from Agrobacterium sp. strain CP4 (CP4-EPSPS). Pure recombinant CP4-EPSPS protein, expressed in Escherichia coli, inhibited the growth of human and plant fungal pathogens (Candida albicans, C. tropicalis, C. krusei, Colletotrichum gloeosporioides, Fusarium solani, F. graminearum, and Trichoderma virens), at minimum inhibitory concentrations (MICs) that ranged from 62.5 to 250 µg/mL. It inhibited fungal spore germination as well as cell proliferation on C. gloeosporioides. Rhodamine-labeled CP4-EPSPS accumulated on the fungal cell wall and within intracellular cytosol. In addition, the protein induced uptake of SYTOX Green into cells, but not into intracellular mitochondrial reactive oxygen species (ROS), indicating that its antifungal action was due to inducing the permeability of the fungal cell wall. Its antifungal action showed cell surface damage, as observed from fungal cell morphology. This study provided information on the effects of the LMO protein, EPSPS, on fungal growth.

Antimicrobial nano-assemblies of tryptocidine C, a tryptophan-rich cyclic decapeptide, from ethanolic solutions

Tryptocidine C (TpcC), a Trp-rich cyclodecapeptide is a minor constituent in the antibiotic tyrothricin complex from Brevibacillus parabrevis. TpcC possesses a high tendency to oligomerise in aqueous solutions and dried TpcC forms distinct self-assembled nanoparticles. High-resolution scanning electron microscopy revealed the influence of different ethanol:water solvent systems on TpcC self-assembly, with the TpcC, dried from a high concentration in 15% ethanol, primarily assembling into small nanospheres with 24.3 nm diameter and 0.05 polydispersity. TpcC at 16 μM, near its CMC, formed a variety of structures such as small nanospheres, large dense nanospheroids and facetted 3-D-crystals, as well as sheets and coarse carpet-like structures which depended on ethanol concentration. Drying 16 μM TpcC from 75% ethanol resulted in highly facetted 3-D crystals, as well as small nanospheres, while those in 10% ethanol preparation had less defined facets. Drying from 20 to 50% ethanol led to polymorphic architectures with a few defined nanospheroids and various small nanoparticles, imbedded in carpet- and sheet-like structures. These polymorphic surface morphologies correlated with maintenance of fluorescence properties and the surface-derived antibacterial activity against Staphylococcus aureus over time, while there was a significant change in fluorescence and loss in activity in the 10% and 75% preparations where 3-D crystals were observed. This indicated that TpcC oligomerisation in solutions with 20-50% ethanol leads to metastable structures with a high propensity for release of antimicrobial moieties, while those leading to crystallisation limit active moieties release. TpcC nano-assemblies can find application in antimicrobial coatings, surface disinfectants, food packaging and wound healing materials.

Creating Robust Antimicrobial Materials with Sticky Tyrocidines

Modified antimicrobial and antifouling materials and surfaces can be used to limit the propagation of microorganisms on various surfaces and minimise the occurrence of infection, transfer, and spoilage. Increased demand for ‘green’ solutions for material treatment has pushed the focus towards to naturally produced antimicrobials. Tyrocidines, cyclo-decapeptides naturally produced by a soil bacterium Brevibacillus parabrevis, have a broad spectrum of activity against Gram-positive and Gram-negative bacteria, filamentous fungi, and yeasts. Continual losses in tyrocidine production highlighted the possible association of peptides to surfaces. It was found in this study that tyrocidines readily associates with many materials, with a selectivity towards polysaccharide-type materials, such as cellulose. Peptide-treated cellulose was found to remain active after exposure to a broad pH range, various temperatures, salt solutions, water washes, and organic solvents, with the sterilising activity only affected by 1% SDS and 70% acetonitrile. Furthermore, a comparison to other antimicrobial peptides showed the association between tyrocidines and cellulose to be unique in terms of antimicrobial activity. The robust association between the tyrocidines and various materials holds great promise in applications focused on preventing surface contamination and creating self-sterilising materials.

Purification and erythrocyte-membrane perturbing activity of a ketose-specific lectin from Moringa oleifera seeds

This study purified a hemagglutinating protein (MoL) from Moringa oleifera seed, and investigated its hemolytic activity. Molecular weight and stability of MoL were also determined. Modification of some amino acid residues was carried out and the effect on MoL hemagglutinating activity determined. Other investigated parameters are the effects of temperature, concentration, incubation period, pH, and sugars on the protein's hemagglutinating and hemolytic activities. The native and subunit molecular weights were estimated as 30 and 27.5 kDa respectively. Hemagglutinating activity of MoL was slightly inhibited by fructose and sucrose, stable at temperature up to 90°C and within pH range of 2-4. Modification of tryptophan and arginine residues resulted in partial loss of hemagglutinating activity. The hemolytic activity of MoL was concentration, temperature, pH, and time-dependent. The study concluded that MoL showed hemolytic (membrane-perturbing) activity in moderate acidic conditions. This suggests its potential exploitation in improved intracellular delivery of bioactive compounds.

The Influence of Cellulose-Type Formulants on Anti-Candida Activity of the Tyrocidines

Candida species are highly adaptable to environmental changes with their phenotypic flexibility allowing for the evasion of most host defence mechanisms. Moreover, increasing resistance of human pathogenic Candida strains has been reported against all four classes of available antifungal drugs, which highlights the need for combinational therapies. Tyrocidines are cyclic antimicrobial peptides that have shown synergistic activity with antifungal drugs such as caspofungin and amphotericin B. However, these cyclodecapeptides have haemolytic activity and cytotoxicity, but they have been used for decades in the clinic for topical applications. The tyrocidines tend to form higher-order structures in aqueous solutions and excessive aggregation can result in variable or diminished activity. Previous studies have shown that the tyrocidines prefer ordered association to celluloses. Therefore, a formulation with soluble cellulose was used to control the oligomer stability and size, thereby increasing the activity against Candida spp. Of the formulants tested, it was found that commercial hydroxy-propyl-methyl cellulose, E10M, yielded the best results with increased stability, increased anti-Candida activity, and improved selectivity. This formulation holds promise in topical applications against Candida spp. infections.

Oligomerisation of tryptocidine C, a Trp-rich cyclodecapeptide from the antimicrobial tyrothricin complex

Tryptocidine C (TpcC, cyclo[D-Phe¹-Pro²-Trp³-D-Trp⁴-Asn⁵-Gln⁶-Trp⁷-Val⁸-Orn⁹-Leu¹⁰]) is a broad-spectrum antimicrobial peptide in the tyrothricin complex produced by a soil bacterium, Brevibacillus parabrevis. Electrospray mass spectrometric studies reveal the oligomerisation of TpcC into dimers and higher oligomers, analogous to tyrocidine C (TrcC, Trp⁷ replaced by Tyr⁷). Ion mobility mass spectrometry (IMMS) further confirms the formation of stable peptide dimers and tetramers with diameters of 2.7 nm and 3.3 nm, respectively, calculated from collisional cross section (CCS). Molecular dynamic simulations and docking studies support the formation of amphipathic dimers, with a diameter of 2.5 ± 0.07 nm calculated from low energy model CCS. Circular dichroism and IMMS studies point towards dynamic hydrogen-bonded conformational changes up to 28-33 μM after which the structures become more static (or in equilibrium). Fluorescence studies indicate aromatic stacking of Trp residues with a CMC of 18 μM in aqueous solutions. The concentration and time dependent interaction of Trp in oligomers indicate cooperativity in the TpcC oligomerisation that leads to the formation of higher order microscopic structures. Scanning electron microscopy studies unequivocally shows that TpcC forms nanospheres with a mean diameter of 25 nm. Repeated smaller oligomeric units, possibly dimers and tetramers, self-assemble to form these nanospheres.

Revealing the Mechanisms of Synergistic Action of Two Magainin Antimicrobial Peptides

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

d-Amino acids in antimicrobial peptides: a potential approach to treat and combat antimicrobial resistance

Antimicrobial resistance is one of the leading challenges in the human healthcare segment. Advances in antimicrobial resistance have triggered exploration of natural alternatives to stabilize its seriousness. Antimicrobial peptides are small, positively charged oligopeptides that are as potent as commercially available antibiotics against a wide spectrum of organisms, such as Gram-positive bacteria, Gram-negative bacteria, viruses, and fungal strains. In addition to their antibiotic capabilities, these peptides possess anticancer activity, activate the immune response, and regulate inflammation. Peptides have distinct modes of action and fall into various categories due to their amino acid composition. Although antimicrobial peptides specifically target the bacterial cytoplasmic membrane, they can also target the cell nucleus and protein synthesis. Owing to the increasing demand for novel treatments against the threat of antimicrobial resistance, naturally synthesized peptides are a beneficial development concept. Antimicrobial peptides are pervasive and can easily be modified using de-novo synthesis technology. Antimicrobial peptides can be isolated from natural resources such as humans, plants, bacteria, and fungi. This review gives a brief overview of antimicrobial peptides and their diastereomeric composition. Other current trends, the future scope of antimicrobial peptides, and the role of d-amino acids are also discussed, with a specific emphasis on the design and development of new drugs.

Little Antimicrobial Peptides with Big Therapeutic Roles

Antimicrobial Peptides (AMPs) are short amphipathic biological molecules generally with less than 100 amino acids. AMPs not only present high bioactivities against bacteria, fungi or protists-induced infections, but also play important roles in anticancer activity, immune response and inflammation regulation. AMPs are classified as ribosomally synthesized, non-ribosomally synthesized and post-translationally modified, non-ribosomally synthesized ones and several synthetic or semisynthetic peptides according to their synthesis with or without the involvement of ribosomes. The molecular characterization and bioactivity action mechanisms are summarized for several ribosomally synthesized AMPs and main non-ribosomally synthesized members (cyclopeptides, lipopeptides, glycopeptides, lipoglycopeptides). We also analyze challenges and new strategies to overcome drug resistance and application limitations for AMP discovery. In conclusion, the growing novel small molecular AMPs have huge therapeutic potentials of antibacterial, antiviral, anticancer and immunoregulatory bioactivities through new techniquesdriven drug discovery strategy including bioinformatics prediction, de novo rational design and biosynthesis.

Tyrocidine A interactions with saccharides investigated by CD and NMR spectroscopies

Tyrocidines are a family of cyclic decapeptides produced by the soil bacterium, Brevibacillus parabrevis. These antibiotic peptides can be used to prevent infections in agriculture and food industry but also to prepare antimicrobial lozenges, creams, and dressings for medical applications. It has been observed that the tyrocidines interact with saccharides such as cellulose from their soil environment, as well as sugars in culture media and glycans in fungal cell walls. Here, we investigated the interactions of tyrocidines with glucose, sucrose, and cellotetraose (as cellulose model) in a quantitative fashion utilising CD and NMR spectroscopy. The CD and NMR spectra of tyrocidine A (TrcA) were analysed as a function of solvent composition, and the spectral properties agree with the formation of oligomeric structures that are governed by β-sheet secondary structures once the acetonitrile content of the solvent is increased. Saccharides seem to also induce TrcA spectral changes reverting those induced by organic solvents. The CD spectral changes of TrcA in the presence of glucose agree with new ordered H-bonding, possibly β-sheet structures. The amides involved in intramolecular H-bonding remained largely unaffected by the environmental changes. In contrast, amides exposed to the exterior and/or involved in TrcA intermolecular association show the largest ¹ H chemical shift changes. CD and NMR spectroscopic investigations correlated well with TrcA-glucose interactions characterized by a dissociation constant around 200 μM. Interestingly, the association of cellotetraose corresponds closely to the additive effect from four glucose moieties, while a much higher dissociation constant was observed for sucrose. Similar trends to TrcA for binding to the three saccharides were observed for the analogous tyrocidines, tyrocidine B, and tyrocidine C. These results therefore indicate that the tyrocidine interactions with the glucose monosaccharide unit are fairly specific and reversible.

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

Even 30 years after the discovery of magainins, biophysical and structural investigations on how these peptides interact with membranes can still bear surprises and add new interesting detail to how these peptides exert their antimicrobial action. Early on, using oriented solid-state NMR spectroscopy, it was found that the amphipathic helices formed by magainins are active when being oriented parallel to the membrane surface. More recent investigations indicate that this in-planar alignment is also found when PGLa and magainin in combination exert synergistic pore-forming activities, where studies on the mechanism of synergistic interaction are ongoing. In a related manner, the investigation of dimeric antimicrobial peptide sequences has become an interesting topic of research which bears promise to refine our views how antimicrobial action occurs. The molecular shape concept has been introduced to explain the effects of lipids and peptides on membrane morphology, locally and globally, and in particular of cationic amphipathic helices that partition into the membrane interface. This concept has been extended in this review to include more recent ideas on soft membranes that can adapt to external stimuli including membrane-disruptive molecules. In this manner, the lipids can change their shape in the presence of low peptide concentrations, thereby maintaining the bilayer properties. At higher peptide concentrations, phase transitions occur which lead to the formation of pores and membrane lytic processes. In the context of the molecular shape concept, the properties of lipopeptides, including surfactins, are shortly presented, and comparisons with the hydrophobic alamethicin sequence are made.

The Multifaceted Antibacterial Mechanisms of the Pioneering Peptide Antibiotics Tyrocidine and Gramicidin S

Cyclic β-sheet decapeptides, such as tyrocidines and gramicidin S, were among the first antibiotics in clinical application. Although they have been used for such a long time, there is virtually no resistance to them, which has led to a renewed interest in this peptide class. Both tyrocidines and gramicidin S are thought to disrupt the bacterial membrane. However, this knowledge is mainly derived from in vitro studies, and there is surprisingly little knowledge about how these long-established antibiotics kill bacteria. Our results shed new light on the antibacterial mechanism of β-sheet peptide antibiotics and explain why they are still so effective and why there is so little resistance to them.

Cyclic β-sheet decapeptides from the tyrocidine group and the homologous gramicidin S were the first commercially used antibiotics, yet it remains unclear exactly how they kill bacteria. We investigated their mode of action using a bacterial cytological profiling approach. Tyrocidines form defined ion-conducting pores, induce lipid phase separation, and strongly reduce membrane fluidity, resulting in delocalization of a broad range of peripheral and integral membrane proteins. Interestingly, they also cause DNA damage and interfere with DNA-binding proteins. Despite sharing 50% sequence identity with tyrocidines, gramicidin S causes only mild lipid demixing with minor effects on membrane fluidity and permeability. Gramicidin S delocalizes peripheral membrane proteins involved in cell division and cell envelope synthesis but does not affect integral membrane proteins or DNA. Our results shed a new light on the multifaceted antibacterial mechanisms of these antibiotics and explain why resistance to them is virtually nonexistent.

Molecular mechanism of Arabidopsis thaliana profilins as antifungal proteins

BACKGROUND

It remains an open question whether plant phloem sap proteins are functionally involved in plant defense mechanisms.

METHODS

The antifungal effects of two profilin proteins from Arabidopsis thaliana, AtPFN1 and AtPFN2, were tested against 11 molds and 4 yeast fungal strains. Fluorescence profiling, biophysical, and biochemical analyses were employed to investigate their antifungal mechanism.

RESULTS

Recombinant AtPFN1 and AtPFN2 proteins, expressed in Escherichia coli, inhibited the cell growth of various pathogenic fungal strains at concentrations ranging from 10 to 160 μg/mL. The proteins showed significant intracellular accumulation and cell-binding affinity for fungal cells. Interestingly, the AtPFN proteins could penetrate the fungal cell wall and membrane and act as inhibitors of fungal growth via generation of cellular reactive oxygen species and mitochondrial superoxide. This triggered the AtPFN variant-induced cell apoptosis, resulting in morphological changes in the cells.

CONCLUSION

PFNs may play a critical role as antifungal proteins in the Arabidopsis defense system against fungal pathogen attacks.

GENERAL SIGNIFICANCE

The present study indicates that two profilin proteins, AtPFN1 and AtPFN2, can act as natural antimicrobial agents in the plant defense system.

Biophysical Investigations Elucidating the Mechanisms of Action of Antimicrobial Peptides and Their Synergism

Biophysical and structural investigations are presented with a focus on the membrane lipid interactions of cationic linear antibiotic peptides such as magainin, PGLa, LL37, and melittin. Observations made with these peptides are distinct as seen from data obtained with the hydrophobic peptide alamethicin. The cationic amphipathic peptides predominantly adopt membrane alignments parallel to the bilayer surface; thus the distribution of polar and non-polar side chains of the amphipathic helices mirror the environmental changes at the membrane interface. Such a membrane partitioning of an amphipathic helix has been shown to cause considerable disruptions in the lipid packing arrangements, transient openings at low peptide concentration, and membrane disintegration at higher peptide-to-lipid ratios. The manifold supramolecular arrangements adopted by lipids and peptides are represented by the 'soft membranes adapt and respond, also transiently' (SMART) model. Whereas molecular dynamics simulations provide atomistic views on lipid membranes in the presence of antimicrobial peptides, the biophysical investigations reveal interesting details on a molecular and supramolecular level, and recent microscopic imaging experiments delineate interesting sequences of events when bacterial cells are exposed to such peptides. Finally, biophysical studies that aim to reveal the mechanisms of synergistic interactions of magainin 2 and PGLa are presented, including unpublished isothermal titration calorimetry (ITC), circular dichroism (CD) and dynamic light scattering (DLS) measurements that suggest that the peptides are involved in liposome agglutination by mediating intermembrane interactions. A number of structural events are presented in schematic models that relate to the antimicrobial and synergistic mechanism of amphipathic peptides when they are aligned parallel to the membrane surface.

Antifungal peptides: To be or not to be membrane active

Most antifungal peptides (AFPs), if not all, have membrane activity, while some also have alternative targets. Fungal membranes share many characteristics with mammalian membranes with only a few differences, such as differences in sphingolipids, phosphatidylinositol (PI) content and the main sterol is ergosterol. Fungal membranes are also more negative and a better target for cationic AFPs. Targeting just the fungal membrane lipids such as phosphatidylinositol and/or ergosterol by AFPs often translates into mammalian cell toxicity. Conversely, a specific AFP target in the fungal pathogen, such as glucosylceramide, mannosyldiinositol phosphorylceramide or a fungal protein target translates into high pathogen selectivity. However, a lower target concentration, absence or change in the specific fungal target can naturally lead to resistance, although such resistance in turn could result in reduced pathogen virulence. The question is then to be or not to be membrane active - what is the best choice for a successful AFP? In this review we deliberate on this question by focusing on the recent advances in our knowledge on how natural AFPs target fungi.