Antimicrobial Action of Prototypic Amphipathic Cationic Decapeptides and Their Branched Dimers

Apoptin NLS2 homodimerization strategy for improved antibacterial activity and bio-stability

The emergence of antibiotic resistance prompts exploration of viable antimicrobial peptides (AMPs) designs. The present study explores the antimicrobial prospects of Apoptin nuclear localization sequence (NLS2)-derived peptide ANLP (PRPRTAKRRIRL). Further, we examined the utility of the NLS dimerization strategy for improvement in antimicrobial activity and sustained bio-stability of AMPs. Initially, the antimicrobial potential of ANLP using antimicrobial peptide databases was analyzed. Then, ANLP along with its two homodimer variants namely ANLP-K1 and ANLP-K2 were synthesized and evaluated for antimicrobial activity against Escherichia coli and Salmonella. Among three AMPs, ANLP-K2 showed efficient antibacterial activity with 12 µM minimum inhibitory concentration (MIC). Slow degradation of ANLP-K1 (26.48%) and ANLP-K2 (13.21%) compared with linear ANLP (52.33%) at 480 min in serum stability assay indicates improved bio-stability of dimeric peptides. The AMPs presented no cytotoxicity in Vero cells. Dye penetration assays confirmed the membrane interacting nature of AMPs. The zeta potential analysis reveals effective charge neutralization of both lipopolysaccharide (LPS) and bacterial cells by dimeric AMPs. The dimeric AMPs on scanning electron microscopy studies showed multiple pore formations on the bacterial surface. Collectively, proposed Lysine scaffold dimerization of Apoptin NLS2 strategy resulted in enhancing antibacterial activity, bio-stability, and could be effective in neutralizing the off-target effect of LPS. In conclusion, these results suggest that nuclear localization sequence with a modified dimeric approach could represent a rich source of template for designing future antimicrobial peptides.

Oligomer Dynamics of LL-37 Truncated Fragments Probed by α-Hemolysin Pore and Molecular Simulations

Oligomerization of antimicrobial peptides (AMPs) is critical in their effects on pathogens. LL-37 and its truncated fragments are widely investigated regarding their structures, antimicrobial activities, and application, such as developing new antibiotics. Due to the small size and weak intermolecular interactions of LL-37 fragments, it is still elusive to establish the relationship between oligomeric states and antimicrobial activities. Here, an α-hemolysin nanopore, mass spectrometry (MS), and molecular dynamic (MD) simulations are used to characterize the oligomeric states of two LL-37 fragments. Nanopore studies provide evidence of trapping events related to the oligomer formation and provide further details on their stabilities, which are confirmed by MS and MD simulations. Furthermore, simulation results reveal the molecular basis of oligomer dynamics and states of LL-37 fragments. This work provides unique insights into the relationship between the oligomer dynamics of AMPs and their antimicrobial activities at the single-molecule level. The study demonstrates how integrating methods allows deciphering single molecule level understanding from nanopore sensing approaches.

Synthesis, biological evaluation and mechanistic studies of 4-(1,3-thiazol-2-yl)morpholine-benzimidazole hybrids as a new structural class of antimicrobials

In spite of several attempts to develop newer pharmacophores as potential antimicrobial agents, the benzimidazole scaffold is still considered as one of the most sought after structural component towards the design of compounds that act against a wide spectrum of microbes. Herein, we report the design and synthesis of a new structural class of 4-(1,3-thiazol-2-yl)morpholine-benzimidazole hybrids as antimicrobial agents. The most potent analog, 6g shows IC₅₀ of 1.3 µM, 2.7 µM, 10.8 µM, 5.4 µM and 10.8 µM against Cryptococcus neoformans, Candida albicans, Candida parapsilosis, Escherichia coli and Staphylococcus aureus, respectively. Interestingly 6g exhibits selectivity towards the cryptococcal cells with fungicidal behavior. Propidium iodide uptake study shows permeabilization of pathogenic cells in the presence of 6g. Flow cytometric analysis confirms that cell death is predominantly due to apoptosis. Moreover, electron microscopic analysis specifies that it shrinks, disrupts and initiate pore(s) formation in the cell membrane leading to cell lysis.

Unnatural amino acids: promising implications for the development of new antimicrobial peptides

The increasing incidence and rapid spread of bacterial resistance to conventional antibiotics are a serious global threat to public health, highlighting the need to develop new antimicrobial alternatives. Antimicrobial peptides (AMPs) represent a class of promising natural antibiotic candidates due to their broad-spectrum activity and low tendency to induce resistance. However, the development of AMPs for medical use is hampered by several obstacles, such as moderate activity, lability to proteolytic degradation, and low bioavailability. To date, many researchers have focussed on the optimization or design of novel artificial AMPs with desired properties. Unnatural amino acids (UAAs) are valuable building blocks in the manufacture of a variety of pharmaceuticals, and have been used to develop artificial AMPs with specific structural and physicochemical properties. Rational incorporation of UAAs has become a very promising approach to endow AMPs with strong and long-lasting activity but no toxicity. This review aims to summarize key approaches that have been used to incorporate UAAs to develop novel AMPs with improved properties and better performance. It is anticipated that this review will guide future design considerations for UAA-based antimicrobial applications.

Synthetic amino acids-based short amphipathic peptides exhibit antifungal activity by targeting cell membrane disruption

Availability of a limited number of antifungal drugs created a necessity to develop new antifungals with distinct mode of action. Investigation on a new series of peptides led us to identify Boc-His-Trp-His[1-(4-tert-butylphenyl)] (10g) as the most promising inhibitor exhibiting IC₅₀ value of 4.4 µg/mL against Cryptococcus neoformans. Analog 10g exhibit high selectivity to fungal cells and was nonhemolytic and noncytotoxic at its minimum inhibitory concentration. 10g produced fungicidal effect on growing cryptococcal cells and displayed synergistic effect with amphotericin B. Overall cationic character of 10g resulted in interaction with negatively charged fungal membrane while hydrophobicity enhanced penetration inside the cryptococcal cells causing hole(s) formation and disruption to the membrane as evident by the scanning electron microscopy, transmission electron microscopy, and confocal laser scanning microscopy analyses. Flow cytometric investigation revealed rapid death of fungal cells by apopotic pathway.

Anticryptococcal activity and mechanistic studies of short amphipathic peptides

Cryptococcus neoformans, an opportunistic fungal pathogen, causes cryptococcosis in immunocompromised persons. A series of modified L-histidines-containing peptides are synthesized that exhibit promising activity against C. neoformans. Analog 11d [L-His(2-adamantyl)-L-Trp-L-His(2-phenyl)-OMe] produced potency with an IC₅₀ of 3.02 µg/ml (MIC = 5.49 µg/ml). This peptide is noncytotoxic and nonhaemolytic at the MIC and displays synergistic effects with amphotericin B at subinhibitory concentration. Mechanistic investigation of 11d using microscopic tools indicates cell wall and membrane disruption of C. neoformans, while flow cytometric analysis confirms cell death by apoptosis. This study indicates that 11d exhibits antifungal potential and acts via the rapid onset of action.

Synergy by Perturbing the Gram-Negative Outer Membrane: Opening the Door for Gram-Positive Specific Antibiotics

New approaches to target antibacterial agents toward Gram-negative bacteria are key, given the rise of antibiotic resistance. Since the discovery of polymyxin B nonapeptide as a potent Gram-negative outer membrane (OM)-permeabilizing synergist in the early 1980s, a vast amount of literature on such synergists has been published. This Review addresses a range of peptide-based and small organic compounds that disrupt the OM to elicit a synergistic effect with antibiotics that are otherwise inactive toward Gram-negative bacteria, with synergy defined as a fractional inhibitory concentration index (FICI) of <0.5. Another requirement for the inclusion of the synergists here covered is their potentiation of a specific set of clinically used antibiotics: erythromycin, rifampicin, novobiocin, or vancomycin. In addition, we have focused on those synergists with reported activity against Gram-negative members of the ESKAPE family of pathogens namely, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and/or Acinetobacter baumannii. In cases where the FICI values were not directly reported in the primary literature but could be calculated from the published data, we have done so, allowing for more direct comparison of potency with other synergists. We also address the hemolytic activity of the various OM-disrupting synergists reported in the literature, an effect that is often downplayed but is of key importance in assessing the selectivity of such compounds for Gram-negative bacteria.

Chemically modified and conjugated antimicrobial peptides against superbugs

Antimicrobial resistance (AMR) is one of the greatest threats to human health that, by 2050, will lead to more deaths from bacterial infections than cancer. New antimicrobial agents, both broad-spectrum and selective, that do not induce AMR are urgently required. Antimicrobial peptides (AMPs) are a novel class of alternatives that possess potent activity against a wide range of Gram-negative and positive bacteria with little or no capacity to induce AMR. This has stimulated substantial chemical development of novel peptide-based antibiotics possessing improved therapeutic index. This review summarises recent synthetic efforts and their impact on analogue design as well as their various applications in AMP development. It includes modifications that have been reported to enhance antimicrobial activity including lipidation, glycosylation and multimerization through to the broad application of novel bio-orthogonal chemistry, as well as perspectives on the direction of future research. The subject area is primarily the development of next-generation antimicrobial agents through selective, rational chemical modification of AMPs. The review further serves as a guide toward the most promising directions in this field to stimulate broad scientific attention, and will lead to new, effective and selective solutions for the several biomedical challenges to which antimicrobial peptidomimetics are being applied.

Rational Design of Dipicolylamine-Containing Carbazole Amphiphiles Combined with Zn2+ as Potent Broad-Spectrum Antibacterial Agents with a Membrane-Disruptive Mechanism

Antibiotic resistance has become one of the most urgently important problems facing healthcare providers. A novel series of dipicolylamine-containing carbazole amphiphiles with strong Zn²⁺ chelating ability were synthesized, biomimicking cationic antimicrobial peptides. Effective broad-spectrum 16 combined with 12.5 μg/mL Zn²⁺ was identified as the most promising antimicrobial candidate. 16 combined with 12.5 μg/mL Zn²⁺ exhibited excellent antimicrobial activity against both Gram-positive and Gram-negative bacteria (MICs = 0.78-3.125 μg/mL), weak hemolytic activity, and low cytotoxicity. Time-kill kinetics and mechanism studies revealed 16 combined with 12.5 μg/mL Zn²⁺ had rapid bacterial killing properties, as evidenced by disruption of the integrity of bacterial cell membranes, effectively preventing bacterial resistance development. Importantly, 16 combined with 12.5 μg/mL Zn²⁺ showed excellent in vivo efficacy in a murine keratitis model caused by Staphylococcus aureus ATCC29213 or Pseudomonas aeruginosa ATCC9027. Therefore, 16 combined with 12.5 μg/mL Zn²⁺ could be a promising candidate for treating bacterial infections.

Fusogenic porous silicon nanoparticles as a broad-spectrum immunotherapy against bacterial infections

Bacterial infections are re-emerging as substantial threats to global health due to the limited selection of antibiotics that are capable of overcoming antibiotic-resistant strains. By deterring such mutations whilst minimizing the need to develop new pathogen-specific antibiotics, immunotherapy offers a broad-spectrum therapeutic solution against bacterial infections. In particular, pathology resulting from excessive immune response (i.e. fibrosis, necrosis, exudation, breath impediment) contributes significantly to negative disease outcome. Herein, we present a nanoparticle that is targeted to activated macrophages and loaded with siRNA against the Irf5 gene. This formulation is able to induce >80% gene silencing in activated macrophages in vivo, and it inhibits the excessive inflammatory response, generating a significantly improved therapeutic outcome in mouse models of bacterial infection. The versatility of the approach is demonstrated using mice with antibiotic-resistant Gram-positive (methicillin-resistant Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa) muscle and lung infections, respectively. Effective depletion of the Irf5 gene in macrophages is found to significantly improve the therapeutic outcome of infected mice, regardless of the bacteria strain and type.

Vogel H, Park Y. Antifungal and Antibiofilm Activities and the Mechanism of Action of Repeating Lysine-Tryptophan Peptides against Candida albicans

The rapid increase in the emergence of antifungal-resistant Candida albicans strains is becoming a serious health concern. Because antimicrobial peptides (AMPs) may provide a potential alternative to conventional antifungal agents, we have synthesized a series of peptides with a varying number of lysine and tryptophan repeats (KWn-NH2). The antifungal activity of these peptides increased with peptide length, but only the longest KW5 peptide displayed cytotoxicity towards a human keratinocyte cell line. The KW4 and KW5 peptides exhibited strong antifungal activity against C. albicans, even under conditions of high-salt and acidic pH, or the addition of fungal cell wall components. Moreover, KW4 inhibited biofilm formation by a fluconazole-resistant C. albicans strain. Circular dichroism and fluorescence spectroscopy indicated that fungal liposomes could interact with the longer peptides but that they did not release the fluorescent dye calcein. Subsequently, fluorescence assays with different dyes revealed that KW4 did not disrupt the membrane integrity of intact fungal cells. Scanning electron microscopy showed no changes in fungal morphology, while laser-scanning confocal microscopy indicated that KW4 can localize into the cytosol of C. albicans. Gel retardation assays revealed that KW4 can bind to fungal RNA as a potential intracellular target. Taken together, our data indicate that KW4 can inhibit cellular functions by binding to RNA and DNA after it has been translocated into the cell, resulting in the eradication of C. albicans.

Short to ultrashort peptide-based hydrogels as a platform for biomedical applications

Short peptides have attracted significant attention from researchers in the past few years due to their easy design, synthesis and characterization, diverse functionalisation possibilities, low cost, possibility to make a large range of hierarchical nanostructures and most importantly their high biocompatibility and biodegradability. Generally, short peptides are also relatively more stable than their longer variants, non-immunogenic in nature and many of them self-assemble to provide an exciting range of nanostructures, including hydrogels. Thus, the development of short peptide-based hydrogels has become an area of intense investigation. Although these hydrogels have a water content of greater than 90%, they are surprisingly highly stable structures, and thus have been used for various biomedical applications, including cell therapeutics, drug delivery, tissue engineering and regeneration, contact lenses, biosensors, and wound healing, by different researchers. Herein, we review the progress of research in the rapidly expanding field of short to ultrashort peptide-based hydrogels and their possible applications. Special attention is paid to address and review this field with regard to the stability of peptide-based hydrogels, particularly to enzymatic degradation.

Dimerization of Antimicrobial Peptides: A Promising Strategy to Enhance Antimicrobial Peptide Activity

Antimicrobial resistance is a global health problem with strong social and economic impacts. The development of new antimicrobial agents is considered an urgent challenge. In this regard, Antimicrobial Peptides (AMPs) appear to be novel candidates to overcome this problem. The mechanism of action of AMPs involves intracellular targets and membrane disruption. Although the exact mechanism of action of AMPs remains controversial, most AMPs act through membrane disruption of the target cell. Several strategies have been used to improve AMP activity, such as peptide dimerization. In this review, we focus on AMP dimerization, showing many examples of dimerized peptides and their effects on biological activity. Although more studies are necessary to elucidate the relationship between peptide properties and the dimerization effect on antimicrobial activity, dimerization constitutes a promising strategy to improve the effectiveness of AMPs.

Geometry of a TLR2-Agonist-Based Adjuvant Can Affect the Resulting Antigen-Specific Immune Response

Targeted delivery of otherwise nonimmunogenic antigens to Toll-like receptors (TLRs) expressed on dendritic cells (DCs) has proven to be an effective means of improving immunogenicity. For this purpose, we have used a branched cationic lipopeptide, R₄Pam₂Cys, which is an agonist for TLR2 and enables electrostatic association with antigen for this purpose. Here, we compare the immunological properties of ovalbumin formulated with different geometrical configurations of R₄Pam₂Cys. Our results demonstrate that notwithstanding the presence of the same adjuvant, branched forms of R₄Pam₂Cys are more effective at inducing immune responses than are linear geometries. CD8⁺ T-cell-mediated responses are particularly improved, resulting in significantly higher levels of antigen-specific cytokine secretion and cytolysis of antigen-bearing target cells in vivo. The results correlate with the ability of branched R₄Pam₂Cys conformations to encourage higher levels of DC maturation and facilitate superior antigen uptake, leading to increased production of proinflammatory cytokines. These differences are not attributable to particle size because both branched and linear lipopeptides associate with antigen-forming complexes of similar size, but rather the ability of branched lipopeptides to induce more efficient TLR2-mediated cell signaling. Branched lipopeptides are also more resistant to trypsin-mediated proteolysis, suggesting greater stability than their linear counterparts. The branched lipopeptide facilitates presentation of antigen more efficiently to CD8⁺ T cells, resulting in rapid cell division and upregulation of early cell surface activation markers. These results as well as cognate recognition of Pam₂Cys by TLR2 indicate that the adjuvant's efficiency is also dependent on its geometry.

Fabrication of cationic nanostructures from short self-assembling amphiphilic mixed α/β-pentapeptide: Potential candidates for drug delivery, gene delivery, and antimicrobial applications

The present article describes designing and fabrication of nanostructures from a mixed α/β-pentapeptide, Lys-βAla-βAla-Lys-βAla, which majorly contains non-natural β-alanine residues in the backbone with two α-lysine residues at 1- and 4-positions. The amphiphilic pentapeptide showed the ability to self-assemble into cationic nanovesicles in an aqueous solution. The average size of peptide nanostructures was found to be ~270 nm with a very high cationic charge of ~+40 mV. TEM micrographs revealed the average size of the same nanostructures ~80 nm bearing vesicular morphology. CD and FTIR spectroscopic studies on self-assembled pentapeptide hinted at random coil conformation which was also correlated with conformational search program using Hyper Chem 8.0. The pentapeptide nanostructures were then tested for encapsulation of hydrophobic model drug moieties, L-Dopa, and curcumin. Transfection efficiency of the generated cationic nanostructures was evaluated on HEK293 cells and compared the results with those obtained in the presence of chloroquine. The cytotoxicity assay performed using MTT depicted ~75-80% cell viability. The obtained nanostructures also gave positive results against both Gram-negative and Gram-positive bacterial strains. Altogether the results advocate the promising potential of the pentapeptide foldamer, H-Lys-βAla-βAla-Lys-βAla-OEt, for drug and gene delivery applications along with the antimicrobial activity.

The singular behavior of a β-type semi-synthetic two branched polypeptide: three-dimensional structure and mode of action

Dendrimeric peptides make a versatile group of bioactive peptidomimetics and a potential new class of antimicrobial agents to tackle the pressing threat of multi-drug resistant pathogens. These are branched supramolecular assemblies where multiple copies of the bioactive unit are linked to a central core. Beyond their antimicrobial activity, dendrimeric peptides could also be designed to functionalize the surface of nanoparticles or materials for other medical uses. Despite these properties, however, little is known about the structure-function relationship of such compounds, which is key to unveil the fundamental physico-chemical parameters and design analogues with desired attributes. To close this gap, we focused on a semi-synthetic, two-branched peptide, SB056, endowed with remarkable activity against both Gram-positive and Gram-negative bacteria and limited cytotoxicity. SB056 can be considered the smallest prototypical dendrimeric peptide, with the core restricted to a single lysine residue and only two copies of the same highly cationic 10-mer polypeptide; an octanamide tail is present at the C-terminus. Combining NMR and Molecular Dynamics simulations, we have determined the 3D structure of two analogues. Fluorescence spectroscopy was applied to investigate the water-bilayer partition in the presence of vesicles of variable charge. Vesicle leakage assays were also performed and the experimental data were analyzed by applying an iterative Monte Carlo scheme to estimate the minimum number of bound peptides needed to achieve the release. We unveiled a singular beta hairpin-type structure determined by the peptide chains only, with the octanamide tail available for further functionalization to add new potential properties without affecting the structure.

Antimicrobial activity of Bulbothrix setschwanensis (Zahlbr.) Hale lichen by cell wall disruption of Staphylococcus aureus and Cryptococcus neoformans

In the present study, antimicrobial activity of a common Himalayan lichen viz. Bulbothrix setschwanensis (Zahlbr.) Hale extract in three common solvents (acetone, chloroform and methanol) was evaluated against six bacterial and seven fungal clinical strains. The acetone extract showed promising antimicrobial activity against S. aureus (1.56 mg/mL) and C. neoformans (6.25 mg/mL). Further, GC-MS analysis revealed 2,3-bis(2-methylpentanoyloxy)propyl 2-methylpentanoate and Ethyl 2-[(2R,3R,4aR,8aS)-3-hydroxy-2,3,4,4a,6,7,8,8a-octahydropyrano [3,2-b]pyran-2-yl]acetate as the predominant compounds. The combination of acetone extract with antibacterial drugs [kanamycin (KAN), rifampicin (RIF)] and antifungal drugs [amphotericin B (Amp B) and fluconazole (FLC)] showed lysis of S. aureus and C. neoformans at non-inhibitory concentration (FICI values were 0.31 for KAN, 0.18 for RIF, 0.37 for Amp B and 0.30 for FLC, respectively). Notably, the acetone extract confirmed cell wall damage of both S. aureus and C. neoformans cells and was clearly visualized under scanning electron microscopy (SEM), flow cytometry and confocal microscopy. Besides this, the three extracts also have less significant cytotoxic activity at MIC concentrations against mammalian cells (HEK-293 and HeLa). This study for the first time suggests that the chemical compounds present in the acetone extract of B. setschwanensis could be used against S. aureus and C. neoformans infections.

Branched Peptide, B2088, Disrupts the Supramolecular Organization of Lipopolysaccharides and Sensitizes the Gram-negative Bacteria

Dissecting the complexities of branched peptide-lipopolysaccharides (LPS) interactions provide rationale for the development of non-cytotoxic antibiotic adjuvants. Using various biophysical methods, we show that the branched peptide, B2088, binds to lipid A and disrupts the supramolecular organization of LPS. The disruption of outer membrane in an intact bacterium was demonstrated by fluorescence spectroscopy and checkerboard assays, the latter confirming strong to moderate synergism between B2088 and various classes of antibiotics. The potency of synergistic combinations of B2088 and antibiotics was further established by time-kill kinetics, mammalian cell culture infections model and in vivo model of bacterial keratitis. Importantly, B2088 did not show any cytotoxicity to corneal epithelial cells for at least 96 h continuous exposure or hemolytic activity even at 20 mg/ml. Peptide congeners containing norvaline, phenylalanine and tyrosine (instead of valine in B2088) displayed better synergism compared to other substitutions. We propose that high affinity and subsequent disruption of the supramolecular assembly of LPS by the branched peptides are vital for the development of non-cytotoxic antibiotic adjuvants that can enhance the accessibility of conventional antibiotics to the intracellular targets, decrease the antibiotic consumption and holds promise in averting antibiotic resistance.

Heteroaryl chalcone allied triazole conjugated organosilatranes: synthesis, spectral analysis, antimicrobial screening, photophysical and theoretical investigations

A series of heteroaryl tethered triazole conjoined organosilatranes were synthesized and studied for their solvatochromism experimentally and theoretically followed by antimicrobial screening.

Effects and mechanisms of the secondary structure on the antimicrobial activity and specificity of antimicrobial peptides

A 15-mer cationic α-helical antimicrobial peptide HPRP-A1 was used as the parent peptide to study the effects of peptide secondary structure on the biophysical properties and biological activities. Without changing the amino acid composition of HPRP-A1, we designed two α-helical peptides with either higher or lower helicity compared with the parent peptide, a β-sheet peptide and a random coiled peptide using de novo design approach. The secondary structures were confirmed by circular dichroism spectroscopy. The three α-helical peptides exhibited comparable antibacterial activities, but their hemolytic activity varied from extreme hemolysis to no hemolysis, which is correlated with their helicity. The β-sheet peptide shows poor antibacterial and strong hemolytic activities. More interestingly, the random coil peptide shows no antibacterial activity against Gram-negative bacteria, weak antibacterial activity against Gram-positive bacteria, and extremely weak hemolytic activity. Bacterial membrane permeabilization was also testified on peptides with different secondary structures. Tryptophan fluorescence experiment revealed that the peptide binding preference to the lipid vesicles for mimicking the prokaryotic or eukaryotic membranes was consistent with their biological activities. With the de novo design approach, we proved that it is important to maintain certain contents of amphipathic secondary structure for a desirable biological activity. We believe that the de novo design approach of relocation of the amino acids within a template sequence could be an effective approach in optimizing the specificity of an antimicrobial peptide.

Bactericidal activity of curcumin I is associated with damaging of bacterial membrane

Curcumin, an important constituent of turmeric, is known for various biological activities, primarily due to its antioxidant mechanism. The present study focused on the antibacterial activity of curcumin I, a significant component of commercial curcumin, against four genera of bacteria, including those that are Gram-positive (Staphylococcus aureus and Enterococcus faecalis) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa). These represent prominent human pathogens, particularly in hospital settings. Our study shows the strong antibacterial potential of curcumin I against all the tested bacteria from Gram-positive as well as Gram-negative groups. The integrity of the bacterial membrane was checked using two differential permeabilization indicating fluorescent probes, namely, propidium iodide and calcein. Both the membrane permeabilization assays confirmed membrane leakage in Gram-negative and Gram-positive bacteria on exposure to curcumin I. In addition, scanning electron microscopy and fluorescence microscopy were employed to confirm the membrane damages in bacterial cells on exposure to curcumin I. The present study confirms the broad-spectrum antibacterial nature of curcumin I, and its membrane damaging property. Findings from this study could provide impetus for further research on curcumin I regarding its antibiotic potential against rapidly emerging bacterial pathogens.

Antimicrobial peptides in echinoderm host defense

Antimicrobial peptides (AMPs) are important effector molecules in innate immunity. Here we briefly summarize characteristic traits of AMPs and their mechanisms of antimicrobial activity. Echinoderms live in a microbe-rich marine environment and are known to express a wide range of AMPs. We address two novel AMP families from coelomocytes of sea urchins: cysteine-rich AMPs (strongylocins) and heterodimeric AMPs (centrocins). These peptide families have conserved preprosequences, are present in both adults and pluteus stage larvae, have potent antimicrobial properties, and therefore appear to be important innate immune effectors. Strongylocins have a unique cysteine pattern compared to other cysteine-rich peptides, which suggests a novel AMP folding pattern. Centrocins and SdStrongylocin 2 contain brominated tryptophan residues in their native form. This review also includes AMPs isolated from other echinoderms, such as holothuroidins, fragments of beta-thymosin, and fragments of lectin (CEL-III). Echinoderm AMPs are crucial molecules for the understanding of echinoderm immunity, and their potent antimicrobial activity makes them potential precursors of novel drug leads.

Effects of single amino acid substitution on the biophysical properties and biological activities of an amphipathic α-helical antibacterial peptide against Gram-negative bacteria

An antimicrobial peptide, known as V13K, was utilized as the framework to study the effects of charge, hydrophobicity and helicity on the biophysical properties and biological activities of α-helical peptides. Six amino acids (Lys, Glu, Gly, Ser, Ala, and Leu) were individually used to substitute the original hydrophobic valine at the selected sixteenth location on the non-polar face of V13K. The results showed that the single amino acid substitutions changed the hydrophobicity of peptide analogs as monitored by RP-HPLC, but did not cause significant changes on peptide secondary structures both in a benign buffer and in a hydrophobic environment. The biological activities of the analogs exhibited a hydrophobicity-dependent behavior. The mechanism of peptide interaction with the outer membrane and cytoplasmic membrane of Gram-negative bacteria was investigated. We demonstrated that this single amino acid substitution method has valuable potential for the rational design of antimicrobial peptides with enhanced activities.

Rationally designed transmembrane peptide mimics of the multidrug transporter protein Cdr1 act as antagonists to selectively block drug efflux and chemosensitize azole-resistant clinical isolates of Candida albicans

Drug-resistant pathogenic fungi use several families of membrane-embedded transporters to efflux antifungal drugs from the cells. The efflux pump Cdr1 (Candida drug resistance 1) belongs to the ATP-binding cassette (ABC) superfamily of transporters. Cdr1 is one of the most predominant mechanisms of multidrug resistance in azole-resistant (AR) clinical isolates of Candida albicans. Blocking drug efflux represents an attractive approach to combat the multidrug resistance of this opportunistic human pathogen. In this study, we rationally designed and synthesized transmembrane peptide mimics (TMPMs) of Cdr1 protein (Cdr1p) that correspond to each of the 12 transmembrane helices (TMHs) of the two transmembrane domains of the protein to target the primary structure of the Cdr1p. Several FITC-tagged TMPMs specifically bound to Cdr1p and blocked the efflux of entrapped fluorescent dyes from the AR (Gu5) isolate. These TMPMs did not affect the efflux of entrapped fluorescent dye from cells expressing the Cdr1p homologue Cdr2p or from cells expressing a non-ABC transporter Mdr1p. Notably, the time correlation of single photon counting fluorescence measurements confirmed the specific interaction of FITC-tagged TMPMs with their respective TMH. By using mutant variants of Cdr1p, we show that these TMPM antagonists contain the structural information necessary to target their respective TMHs of Cdr1p and specific binding sites that mediate the interactions between the mimics and its respective helix. Additionally, TMPMs that were devoid of any demonstrable hemolytic, cytotoxic, and antifungal activities chemosensitize AR clinical isolates and demonstrate synergy with drugs that further improved the therapeutic potential of fluconazole in vivo.

Anti-plasmodial action of de novo-designed, cationic, lysine-branched, amphipathic, helical peptides

BACKGROUND

A lack of vaccine and rampant drug resistance demands new anti-malarials.

METHODS

In vitro blood stage anti-plasmodial properties of several de novo-designed, chemically synthesized, cationic, amphipathic, helical, antibiotic peptides were examined against Plasmodium falciparum using SYBR Green assay. Mechanistic details of anti-plasmodial action were examined by optical/fluorescence microscopy and FACS analysis.

RESULTS

Unlike the monomeric decapeptides {(Ac-GXRKXHKXWA-NH2) (X = F,ΔF) (Fm, ΔFm IC50 >100 μM)}, the lysine-branched,dimeric versions showed far greater potency {IC50 (μM) Fd 1.5 , ΔFd 1.39}. The more helical and proteolytically stable ΔFd was studied for mechanistic details. ΔFq, a K-K2 dendrimer of ΔFm and (ΔFm)2 a linear dimer of ΔFm showed IC50 (μM) of 0.25 and 2.4 respectively. The healthy/infected red cell selectivity indices were >35 (ΔFd), >20 (ΔFm)2 and 10 (ΔFq). FITC-ΔFd showed rapid and selective accumulation in parasitized red cells. Overlaying DAPI and FITC florescence suggested that ΔFd binds DNA. Trophozoites and schizonts incubated with ΔFd (2.5 μM) egressed anomalously and Band-3 immunostaining revealed them not to be associated with RBC membrane. Prematurely egressed merozoites from peptide-treated cultures were found to be invasion incompetent.

CONCLUSION

Good selectivity (>35), good resistance index (1.1) and low cytotoxicity indicate the promise of ΔFd against malaria.

Progressive structuring of a branched antimicrobial peptide on the path to the inner membrane target

In recent years, interest has grown in the antimicrobial properties of certain natural and non-natural peptides. The strategy of inserting a covalent branch point in a peptide can improve its antimicrobial properties while retaining host biocompatibility. However, little is known regarding possible structural transitions as the peptide moves on the access path to the presumed target, the inner membrane. Establishing the nature of the interactions with the complex bacterial outer and inner membranes is important for effective peptide design. Structure-activity relationships of an amphiphilic, branched antimicrobial peptide (B2088) are examined using environment-sensitive fluorescent probes, electron microscopy, molecular dynamics simulations, and high resolution NMR in solution and in condensed states. The peptide is reconstituted in bacterial outer membrane lipopolysaccharide extract as well as in a variety of lipid media mimicking the inner membrane of Gram-negative pathogens. Progressive structure accretion is observed for the peptide in water, LPS, and lipid environments. Despite inducing rapid aggregation of bacteria-derived lipopolysaccharides, the peptide remains highly mobile in the aggregated lattice. At the inner membranes, the peptide undergoes further structural compaction mediated by interactions with negatively charged lipids, probably causing redistribution of membrane lipids, which in turn results in increased membrane permeability and bacterial lysis. These findings suggest that peptides possessing both enhanced mobility in the bacterial outer membrane and spatial structure facilitating its interactions with the membrane-water interface may provide excellent structural motifs to develop new antimicrobials that can overcome antibiotic-resistant Gram-negative pathogens.

Fabrication of nanostructures through molecular self-assembly of small amphiphilic glyco-dehydropeptides

Self-assembled peptide-based nanostructures have been the focus of research in the past decade because of their potential applications in various biological systems. Normally, small self-assembled peptide nanostructures contain hydrophobic moieties, therefore, their solubility in aqueous systems poses the important challenge in the field of molecular self-assembly in order to make effective use of these in a wide variety of applications. To improve their aqueous solubility, the self-assembled amphiphilic α,β-dehydrophenylalanine containing small glyco-dehydropeptides, Boc-Phe-ΔPhe-εAhx-GA (I) and H-Phe-ΔPhe-εAhx-GA (II) with glucosamine (GA) attached at the C-terminal through a 6-aminocaproic acid linker, were synthesized, demonstrating the formation of nanostructures in aqueous media, which were characterized by DLS, AFM and TEM. Further, nanostructure II reduced auric chloride to gold nanoparticles and formed a peptide-gold conjugate (VII). The feasibility of using the nanostructures I and II as nanovectors for drug delivery was demonstrated by loading hydrophobic molecules, eosin and N-fluoresceinyl-2-aminoethanol (FAE) dyes. Besides, these peptides displayed antimicrobial activity against Micrococcus flavus, Bacillus subtilis and Pseudomonas aeruginosa. All these results advocate the potential of these nanostructures as efficient vectors for drug delivery applications.

Structural and biophysical characterization of an antimicrobial peptide chimera comprised of lactoferricin and lactoferrampin

Lactoferricin and lactoferrampin are two antimicrobial peptides found in the N-terminal lobe of bovine lactoferrin with broad spectrum antimicrobial activity against a range of Gram-positive and Gram-negative bacteria as well as Candida albicans. A heterodimer comprised of lactoferrampin joined to a fragment of lactoferricin was recently reported in which these two peptides were joined at their C-termini through the two amino groups of a single Lys residue (Bolscher et al., 2009, Biochimie 91(1):123-132). This hybrid peptide, termed LFchimera, has significantly higher antimicrobial activity compared to the individual peptides or an equimolar mixture of the two. In this work, the underlying mechanism behind the increased antibacterial activity of LFchimera was investigated. Differential scanning calorimetry studies demonstrated that all the peptides influenced the thermotropic phase behaviour of anionic phospholipid suspensions. Calcein leakage and vesicle fusion experiments with anionic liposomes revealed that LFchimera had enhanced membrane perturbing properties compared to the individual peptides. Peptide structures were evaluated using circular dichroism and NMR spectroscopy to gain insight into the structural features of LFchimera that contribute to the increased antimicrobial activity. The NMR solution structure, determined in a miscible co-solvent mixture of chloroform, methanol and water, revealed that the Lys linkage increased the helical content in LFchimera compared to the individual peptides, but it did not fix the relative orientations of lactoferricin and lactoferrampin with respect to each other. The structure of LFchimera provides insight into the conformation of this peptide in a membranous environment and improves our understanding of its antimicrobial mechanism of action.

Inhibitory effects and mechanisms of physiological conditions on the activity of enantiomeric forms of an α-helical antibacterial peptide against bacteria

Enantiomeric amphipathic α-helical antibacterial peptides were synthesized and their biophysical and biological properties under different physiological conditions were studied. In the absence of physiological factors, the L- and D-peptides exhibited similar antimicrobial activities against a broad spectrum of bacteria, even against clinical isolates with resistance to traditional antibiotics. However, in the presence of NaCl, CaCl₂ or human serum albumin (HSA) at physiological concentrations, the enantiomers revealed bacterium-species dependent attenuations in antibacterial activity. In the presence of salts the electrostatic interaction between the peptides and the biomembrane was inhibited. Salts, especially CaCl₂ weakened the ability of the peptides to permeabilize the outer membrane of Gram-negative bacteria, as determined by a 1-N-phenylnaphthylamine uptake assay. HSA exhibited variable inhibitory effects on the activity of the peptides when incubated with different bacterial strains. The peptides showed different binding association abilities to HSA at different molar ratios, regardless of their chirality, resulting in reduced peptide biological activity. The D-peptide performed better than its L-enantiomer in all conditions tested because of its resistance to proteolysis, and may therefore represent a promising candidate for development as a therapeutic agent.

De novo design of α,β-didehydrophenylalanine containing peptides: from models to applications

The de novo design of peptides and proteins has emerged as an approach for investigating protein structure and function. The success relies heavily on the ability to design relatively short peptides that can adopt stable secondary structures. To this end, substitution with α,β-dehydroamino acids, especially α,β-didehydrophenylalanine (ΔPhe or ΔF) has blossomed in manifold directions, providing a rich diversity of well-defined structural motifs. Introduction of α,β-didehydrophenylalanine induces β-bends in small and 3(10)-helices in longer peptide sequences. Most favorable conformation of ΔF residues are (φ,ψ) ∼(60°, 30°), (-60°, -30°), (-60°, 150°), and (60°, -150°). These features have been exploited in designing helix-turn-helix, helical bundle arrangements, and glycine zipper type super secondary structural motifs. The unusual capability of α,β-didehydrophenylalanine ring to form a variety of multicentered interactions (N-H…O, C-H…O, C-H…π, and N-H…π) suggests its possible exploitation for future de novo design of supramolecular structures. This work has now been extended to the de novo design of peptides with antibiotic, antifibrillization activity, etc. More recently, self-assembling properties of small dehydropeptides have been explored. This review focuses primarily on the structural and functional behavior of α,β-didehydrophenylalanine containing peptides.

Rationale-based, de novo design of dehydrophenylalanine-containing antibiotic peptides and systematic modification in sequence for enhanced potency

Increased microbial drug resistance has generated a global requirement for new anti-infective agents. As part of an effort to develop new, low-molecular-mass peptide antibiotics, we used a rationale-based minimalist approach to design short, nonhemolytic, potent, and broad-spectrum antibiotic peptides with increased serum stability. These peptides were designed to attain an amphipathic structure in helical conformations. VS1 was used as the lead compound, and its properties were compared with three series of derivates obtained by (i) N-terminal amino acid addition, (ii) systematic Trp substitution, and (iii) peptide dendrimerization. The Trp substitution approach underlined the optimized sequence of VS2 in terms of potency, faster membrane permeation, and cost-effectiveness. VS2 (a variant of VS1 with two Trp substitutions) was found to exhibit good antimicrobial activity against both the Gram-negative Escherichia coli and the Gram-positive bacterium Staphylococcus aureus. It was also found to have noncytolytic activity and the ability to permeate and depolarize the bacterial membrane. Lysis of the bacterial cell wall and inner membrane by the peptide was confirmed by transmission electron microscopy. A combination of small size, the presence of unnatural amino acids, high antimicrobial activity, insignificant hemolysis, and proteolytic resistance provides fundamental information for the de novo design of an antimicrobial peptide useful for the management of infectious disease.

C-terminal amino acids of alpha-melanocyte-stimulating hormone are requisite for its antibacterial activity against Staphylococcus aureus

Alpha-melanocyte-stimulating hormone (α-MSH) is an endogenous neuropeptide that is known for its anti-inflammatory and antipyretic activities. We recently demonstrated that α-MSH possesses staphylocidal activity and causes bacterial membrane damage. To understand the role of its amino acid sequences in the staphylocidal mechanism, in the present study we investigated the antimicrobial activities of different fragments of α-MSH, i.e., α-MSH(6-13), α-MSH(11-13), and α-MSH(1-5), and compared them with that of the entire peptide. Our results showed that peptides containing the C-terminal region of α-MSH, namely, α-MSH(6-13) and α-MSH(11-13), efficiently killed >90% of both methicillin-sensitive and -resistant Staphylococcus aureus cells in the micromolar range and ∼50% of these cells in the nanomolar range; their efficiency was comparable to that of the entire α-MSH, whereas the peptide containing the N-terminal region, α-MSH(1-5), was found to be ineffective against S. aureus. The antimicrobial activity of α-MSH and its C-terminal fragments was not affected by the presence of NaCl or even divalent cations such as Ca2+ and Mg2+. Similar to the case for the parent peptide, α-MSH(6-13) and α-MSH(11-13) also depolarized and permeabilized Staphylococcus cells (∼70 to 80% of the cells were depolarized and lysed after 2 h of peptide exposure at micromolar concentrations). Furthermore, scanning and transmission electron microscopy showed remarkable morphological and ultrastructural changes on S. aureus cell surface due to exposure to α-MSH-based peptides. Thus, our observations indicate that C-terminal fragments of α-MSH retain the antimicrobial activity of entire peptide and that their mechanism of action is similar to that of full-length peptide. These observations are important and are critical in the rational design of α-MSH-based therapeutics with optimal efficacy.

Studies on mechanism of action of anticancer peptides by modulation of hydrophobicity within a defined structural framework

In the present study, the hydrophobicity of a 26-residue α-helical peptide (peptide P) was altered to study the effects of peptide hydrophobicity on the mechanism of action of cationic anticancer peptides. Hydrophobicity of the nonpolar face of the peptides was shown to correlate with peptide helicity. The self-association ability of peptides in aqueous environment, determined by the reversed-phase high performance liquid chromatography temperature profiling, showed strong influence on anticancer activity. The peptide analogues with greater hydrophobicity showed stronger anticancer activity determined by IC(50) values with a necrotic-like membrane disruption mechanism. Peptide analogues exhibited high specificity against cancer cells and much higher anticancer activity than widely-used anticancer chemical drugs. The mechanism of action of anticancer peptides was also investigated. The hydrophobicity of peptides plays a crucial role in the mechanism of action against cancer cells, which could present a way, using a de novo design approach, to create anticancer peptides as potential therapeutics in clinical practices.

Structural features governing the activity of lactoferricin-derived peptides that act in synergy with antibiotics against Pseudomonas aeruginosa in vitro and in vivo

Pseudomonas aeruginosa is naturally resistant to many antibiotics, and infections caused by this organism are a serious threat, especially to hospitalized patients. The intrinsic low permeability of P. aeruginosa to antibiotics results from the coordinated action of several mechanisms, such as the presence of restrictive porins and the expression of multidrug efflux pump systems. Our goal was to develop antimicrobial peptides with an improved bacterial membrane-permeabilizing ability, so that they enhance the antibacterial activity of antibiotics. We carried out a structure activity relationship analysis to investigate the parameters that govern the permeabilizing activity of short (8- to 12-amino-acid) lactoferricin-derived peptides. We used a new class of constitutional and sequence-dependent descriptors called PEDES (peptide descriptors from sequence) that allowed us to predict (Spearman's ρ = 0.74; P < 0.001) the permeabilizing activity of a new peptide generation. To study if peptide-mediated permeabilization could neutralize antibiotic resistance mechanisms, the most potent peptides were combined with antibiotics, and the antimicrobial activities of the combinations were determined on P. aeruginosa strains whose mechanisms of resistance to those antibiotics had been previously characterized. A subinhibitory concentration of compound P2-15 or P2-27 sensitized P. aeruginosa to most classes of antibiotics tested and counteracted several mechanisms of antibiotic resistance, including loss of the OprD porin and overexpression of several multidrug efflux pump systems. Using a mouse model of lethal infection, we demonstrated that whereas P2-15 and erythromycin were unable to protect mice when administered separately, concomitant administration of the compounds afforded long-lasting protection to one-third of the animals.

Multivalent Antimicrobial Peptides as Therapeutics: Design Principles and Structural Diversities

This review highlights the design principles, progress and advantages attributed to the structural diversity associated with both natural and synthetic multivalent antimicrobial peptides (AMPs). Natural homo- or hetero-dimers of AMPs linked by intermolecular disulfide bonds existed in the animal kingdom, but the multivalency strategy has been adopted to create synthetic branched or polymeric AMPs that do not exist in nature. The multivalent strategy for the design of multivalent AMPs provides advantages to overcome the challenges faced in clinical applications of AMPs, such as: stability, efficiency, toxicity, maintenance of activity in high salt concentrations and under physiological conditions, and importantly overcoming bacterial resistance which is currently a leading health problem in the world. The multivalency strategy is valuable for moving multivalent AMPs toward clinical applications.

Isolation, structure elucidation, and synergistic antibacterial activity of a novel two-component lantibiotic lichenicidin from Bacillus licheniformis VK21

A novel synergetic lantibiotic pair, Lchalpha (3249.51 Da) and Lchbeta (3019.36 Da), termed lichenicidin VK21, was isolated from the producer strain Bacillus licheniformis VK21. Chemical and spatial structures of Lchalpha and Lchbeta were determined. Each peptide contains 31 amino acid residues linked by 4 intramolecular thioether bridges and the N-terminal 2-oxobutyryl group. Spatial structures of Lchalpha and Lchbeta were studied by NMR spectroscopy in methanol solution. The Lchalpha peptide displays structural homology with mersacidin-like lantibiotics and involves relatively well-structured N- and C-terminal domains connected by a flexible loop stabilized by a thioether bridge Ala11-S-Ala21. In contrast, the Lchbeta peptide represents a prolonged hydrophobic alpha-helix flanked with more flexible N- and C-terminal domains. A lantibiotic cluster of the Bacillus licheniformis VK21 genome which comprises the structural genes, lchA1 and lchA2, encoding the lantibiotics precursors, as well as the gene of a modifying enzyme lchM1, was amplified and sequenced. The mature peptides, Lchalpha and Lchbeta, interact synergistically to possess antibiotic activity against Gram-positive bacteria within a nanomolar concentration range, though the individual peptides were shown to be active at micromolar concentrations. Our results afford molecular insight into the mechanism of lichenicidin VK21 action.

Synergy with rifampin and kanamycin enhances potency, kill kinetics, and selectivity of de novo-designed antimicrobial peptides

By choosing membranes as targets of action, antibacterial peptides offer the promise of providing antibiotics to which bacteria would not become resistant. However, there is a need to increase their potency against bacteria along with achieving a reduction in toxicity to host cells. Here, we report that three de novo-designed antibacterial peptides (DeltaFm, DeltaFmscr, and Ud) with poor to moderate antibacterial potencies and kill kinetics improved significantly in all of these aspects when synergized with rifampin and kanamycin against Escherichia coli. (DeltaFm and DeltaFmscr [a scrambled-sequence version of DeltaFm] are isomeric, monomeric decapeptides containing the nonproteinogenic amino acid alpha,beta-didehydrophenylalanine [DeltaF] in their sequences. Ud is a lysine-branched dimeric peptide containing the helicogenic amino acid alpha-aminoisobutyric acid [Aib].) In synergy with rifampin, the MIC of DeltaFmscr showed a 34-fold decrease (67.9 microg/ml alone, compared to 2 microg/ml in combination). A 20-fold improvement in the minimum bactericidal concentration of Ud was observed when the peptide was used in combination with rifampin (369.9 microg/ml alone, compared to 18.5 microg/ml in combination). Synergy with kanamycin resulted in an enhancement in kill kinetics for DeltaFmscr (no killing until 60 min for DeltaFmscr alone, versus 50% and 90% killing within 20 min and 60 min, respectively, in combination with kanamycin). Combination of the dendrimeric peptide DeltaFq (a K-K2 dendrimer for which the sequence of DeltaFm constitutes each of the four branches) (MIC, 21.3 microg/ml) with kanamycin (MIC, 2.1 microg/ml) not only lowered the MIC of each by 4-fold but also improved the therapeutic potential of this highly hemolytic (37% hemolysis alone, compared to 4% hemolysis in combination) and cytotoxic (70% toxicity at 10x MIC alone, versus 30% toxicity in combination) peptide. Thus, synergy between peptide and nonpeptide antibiotics has the potential to enhance the potency and target selectivity of antibacterial peptides, providing regimens which are more potent, faster acting, and safer for clinical use.