Phospholipid-driven differences determine the action of the synthetic antimicrobial peptide OP-145 on Gram-positive bacterial and mammalian membrane model systems

Unlocking roles of cationic and aromatic residues in peptide amphiphiles in treating drug-resistant gram-positive pathogens

Multidrug resistance (MDR) is a rising threat to global health because the number of essential antibiotics used for treating MDR infections is increasingly compromised. In this work we report a group of new amphiphilic peptides (AMPs) derived from the well-studied G3 (G(IIKK)3I-NH2) to fight infections from Gram-positive bacteria including susceptible Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MRSA), focusing on membrane interactions. Time-dependent killing experiments revealed that substitutions of II by WW (GWK), II by FF (GFK) and KK by RR (GIR) resulted in improved bactericidal efficiencies compared to G3 (GIK) on both S. aureus and MRSA, with the order of GWK > GIR > GFK > GIK. Electronic microscopy imaging revealed structural disruptions of AMP binding to bacterial cell walls. Fluorescence assays including AMP binding to anionic lipoteichoic acids (LTA) in cell-free and cell systems indicated concentration and time-dependent membrane destabilization associated with bacterial killing. Furthermore, AMP's binding to anionic plasma membrane via similar fluorescence assays revealed a different extent of membrane depolarization and leakage. These observations were supported by the penetration of AMPs into the LTA barrier and the subsequent structural compromise to the cytoplasmic membrane as revealed from SANS (small angle neutron scattering). Both experiments and molecular dynamics (MD) simulations revealed that GWK and GIR could make the membrane more rigid but less effective in diffusive efficiency than GIK and GFK through forming intramembrane peptide nanoaggregates associated with hydrophobic mismatch and formation of fluidic and rigid patches. The reported peptide-aggregate-induced phase-separation emerged as a crucial factor in accelerated membrane disintegration and fast bacterial killing. This work has demonstrated the importance of membrane interactions to the development of more effective AMPs and the relevance of the approaches as reported in assisting this area of research.

Chirality and pH Influence the Self-Assembly of Antimicrobial Lipopeptides with Diverse Nanostructures

Chirality plays a crucial role in the self-assembly of biomolecules in nature. Peptides show chirality-dependent conformation and self-assembly. Lipidation of peptides occurs in vivo and has recently been exploited in designed conjugates to drive self-assembly and enhance bioactivity. Here, a library of pH-responsive homochiral and heterochiral lipidated tripeptides has been designed. The designed lipopeptides comprise homochiral C16-YKK or C16-WKK (where all the amino acids are l-isomers), and two heterochiral conjugates C16-Ykk and C16-Wkk (where the two lysines are d-isomers). The self-assembly of all the synthesized lipopeptides in aqueous solution was examined using a combination of spectroscopic methods along with cryogenic-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS). Interestingly, it was observed that at acidic pH all the lipopeptides self-assemble into micelles, whereas at basic pH the homochiral lipopeptides self-assemble into nanofibers, whereas the heterochiral lipopeptides self-assemble into nanotapes and nanotubes. A pH switch was demonstrated using a thioflavin T fluorescence probe of β-sheet structure present in the extended structures at pH 8. We demonstrate that both chirality and pH in lipopeptides influence the self-assembly behavior of the model tripeptides, which also show promising bioactivity. Good cytocompatibility is observed in hemolytic assays and antimicrobial activity against both Gram-negative and Gram-positive bacteria is shown through the determination of minimum inhibition concentration (MIC) and minimum bactericidal concentration (MBC) values and live/dead bacteria staining assay.

Small-angle X-ray and neutron scattering applied to lipid-based nanoparticles: Recent advancements across different length scales

Lipid-based nanoparticles (LNPs), ranging from nanovesicles to non-lamellar assemblies, have gained significant attention in recent years, as versatile carriers for delivering drugs, vaccines, and nutrients. Small-angle scattering methods, employing X-rays (SAXS) or neutrons (SANS), represent unique tools to unveil structure, dynamics, and interactions of such particles on different length scales, spanning from the nano to the molecular scale. This review explores the state-of-the-art on scattering methods applied to unveil the structure of lipid-based nanoparticles and their interactions with drugs and bioactive molecules, to inform their rational design and formulation for medical applications. We will focus on complementary information accessible with X-rays or neutrons, ranging from insights on the structure and colloidal processes at a nanoscale level (SAXS) to details on the lipid organization and molecular interactions of LNPs (SANS). In addition, we will review new opportunities offered by Time-resolved (TR)-SAXS and -SANS for the investigation of dynamic processes involving LNPs. These span from real-time monitoring of LNPs structural evolution in response to endogenous or external stimuli (TR-SANS), to the investigation of the kinetics of lipid diffusion and exchange upon interaction with biomolecules (TR-SANS). Finally, we will spotlight novel combinations of SAXS and SANS with complementary on-line techniques, recently enabled at Large Scale Facilities for X-rays and neutrons. This emerging technology enables synchronized multi-method investigation, offering exciting opportunities for the simultaneous characterization of the structure and chemical or mechanical properties of LNPs.

Characterization of an antimicrobial peptide family from the venom gland of Heteropoda venatoria

Spider venom boasts extensive peptide diversity, constituting a natural biochemical arsenal for defense and predation. The new family HvAMPs, including 9 homologous members, were identified from the unnormalized cDNA library of Heteropoda venatoria venom gland by Sanger sequencing. The putative mature peptide is composed of 22 aliphatic amino acid residues. The mature peptides of HvAMP1 and HvAMP5, with 3 different amino acids, were synthesized and both were shown to adopt an amphipathic α-helical structure and amphipathicity in SDS buffer by CD spectroscopy. In comparison to HvAMP1, HvAMP5 exhibits higher antibacterial activity, particularly against Gram-positive bacteria, coupled with reduced hemolytic activity and cytotoxicity. Results from SYTO 9/PI staining indicate that HvAMP5 acts by disrupting bacterial cell membranes. Analysis of the relationships between structures and functions suggests that HvAMP5 enhances antibacterial activity and reduces mammalian cell toxicity by increasing positive charge and proline substitution. The three residues variation can augment the electrostatic attraction of antibacterial peptides to the bacterial phospholipid bilayer. The present study suggests that the HvAMPs may exert lytic action against cells of different origins to increase cellular and tissue barrier permeability to facilitate spider's defense or predation. Moreover, HvAMP5 holds promise as a novel antibacterial agent for treating Gram-positive bacterial infections. Simultaneously, the numerous diverse amino acid residue substitutions within the HvAMP family offer a template for future study.

Antiviral Action against SARS-CoV-2 of a Synthetic Peptide Based on a Novel Defensin Present in the Transcriptome of the Fire Salamander (Salamandra salamandra)

The potential emergence of zoonotic diseases has raised significant concerns, particularly in light of the recent pandemic, emphasizing the urgent need for scientific preparedness. The bioprospection and characterization of new molecules are strategically relevant to the research and development of innovative drugs for viral and bacterial treatment and disease management. Amphibian species possess a diverse array of compounds, including antimicrobial peptides. This study identified the first bioactive peptide from Salamandra salamandra in a transcriptome analysis. The synthetic peptide sequence, which belongs to the defensin family, was characterized through MALDI TOF/TOF mass spectrometry. Molecular docking assays hypothesized the interaction between the identified peptide and the active binding site of the spike WT RBD/hACE2 complex. Although additional studies are required, the preliminary evaluation of the antiviral potential of synthetic SS-I was conducted through an in vitro cell-based SARS-CoV-2 infection assay. Additionally, the cytotoxic and hemolytic effects of the synthesized peptide were assessed. These preliminary findings highlighted the potential of SS-I as a chemical scaffold for drug development against COVID-19, hindering viral infection. The peptide demonstrated hemolytic activity while not exhibiting cytotoxicity at the antiviral concentration.

Intracellular bacterial eradication using a novel peptide in vitro

AIM

To determine the effect of a novel antimicrobial peptide (AMP; OP145) and cell-penetrating peptide (Octa-arginine/R8) conjugate on the killing of intracellular Enterococcus faecalis, compared to OP145 and an antibiotic combination recommended for regenerative endodontic procedures.

METHODOLOGY

The biocompatible concentrations of OP145 and OP145-R8 were determined by assessing their cytotoxicity against human macrophages and red blood cells. Spatiotemporal internalization of the peptides into macrophages was investigated qualitatively and quantitatively by confocal laser scanning microscopy and flow cytometry respectively. Killing of extracellular and intracellular E. faecalis OG1RF by the peptides was determined by counting the colony-forming units (CFU). Intracellular antibacterial activity of the peptides was compared to a double antibiotic combination. Confocal microscopy was used to confirm the intracellular bacterial eradication. Significant differences between the different test groups were analysed using one-way analysis of variance. p < .05 was considered to be statistically significant.

RESULTS

Peptides at a concentration of 7.5 μmol/L were chosen for subsequent experiments based on the results of the alamarBlue™ cell viability assay and haemolytic assay. OP145-R8 selectively internalized into lysosomal compartments and the cytosol of macrophages. Conjugation with R8 improved the internalization of OP145 into macrophages in a temporal manner (70.53% at 1 h to 77.13% at 2 h), while no temporal increase was observed for OP145 alone (60.53% at 1 h with no increase at 2 h). OP145-R8 demonstrated significantly greater extracellular and intracellular antibacterial activity compared to OP145 at all investigated time-points and concentrations (p < .05). OP145-R8 at 7.5 μmol/L eradicated intracellular E. faecalis after 2 h (3.5 log reduction compared to the control; p < .05), while the antibiotics could not reduce more than 0.5 log CFU compared to the control (p > .05). Confocal microscopy showed complete absence of E. faecalis within the OP145-R8 treated macrophages.

CONCLUSIONS

The results of this study demonstrated that the conjugation of an AMP OP145 to a cell-penetrating peptide R8 eradicated extracellular and intracellular E. faecalis OG1RF without toxic effects on the host cells.

Are Antimicrobial Peptides a 21st-Century Solution for Atopic Dermatitis?

Atopic dermatitis (AD) is a chronic inflammatory skin disorder that is the result of various environmental, bacterial and genetic stimuli, which culminate in the disruption of the skin's barrier function. Characterized by highly pruritic skin lesions, xerosis and an array of comorbidities among which skin infections are the most common, this condition results in both a significant loss of quality of life and in the need for life-long treatments (e.g., corticosteroids, monoclonal antibodies and regular antibiotic intake), all of which may have harmful secondary effects. This, in conjunction with AD's rising prevalence, made the development of alternative treatment strategies the focus of both the scientific community and the pharmaceutical industry. Given their potential to both manage the skin microbiome, fight infections and even modulate the local immune response, the use of antimicrobial peptides (AMPs) from more diverse origins has become one of the most promising alternative solutions for AD management, with some being already used with some success towards this end. However, their production and use also exhibit some limitations. The current work seeks to compile the available information and provide a better understanding of the state of the art in the understanding of AMPs' true potential in addressing AD.

Bactericidal Activity to Escherichia coli: Different Modes of Action of Two 24-Mer Peptides SAAP-148 and OP-145, Both Derived from Human Cathelicidine LL-37

OP-145 and SAAP-148, two 24-mer antimicrobial peptides derived from human cathelicidin LL-37, exhibit killing efficacy against both Gram-positive and Gram-negative bacteria at comparable peptide concentrations. However, when it comes to the killing activity against Escherichia coli, the extent of membrane permeabilization does not align with the observed bactericidal activity. This is the case in living bacteria as well as in model membranes mimicking the E. coli cytoplasmic membrane (CM). In order to understand the killing activity of both peptides on a molecular basis, here we studied their mode of action, employing a combination of microbiological and biophysical techniques including differential scanning calorimetry (DSC), zeta potential measurements, and spectroscopic analyses. Various membrane dyes were utilized to monitor the impact of the peptides on bacterial and model membranes. Our findings unveiled distinct binding patterns of the peptides to the bacterial surface and differential permeabilization of the E. coli CM, depending on the smooth or rough/deep-rough lipopolysaccharide (LPS) phenotypes of E. coli strains. Interestingly, the antimicrobial activity and membrane depolarization were not significantly different in the different LPS phenotypes investigated, suggesting a general mechanism that is independent of LPS. Although the peptides exhibited limited permeabilization of E. coli membranes, DSC studies conducted on a mixture of synthetic phosphatidylglycerol/phosphatidylethanolamine/cardiolipin, which mimics the CM of Gram-negative bacteria, clearly demonstrated disruption of lipid chain packing. From these experiments, we conclude that depolarization of the CM and alterations in lipid packing plays a crucial role in the peptides' bactericidal activity.

The Membrane Activity of the Amphibian Temporin B Peptide Analog TB_KKG6K Sheds Light on the Mechanism That Kills Candida albicans

Temporin B (TB) is a 13-amino-acid-long, cationic peptide secreted by the granular glands of the European frog Rana temporaria. We recently showed that the modified TB peptide analog TB_KKG6K rapidly killed planktonic and sessile Candida albicans at low micromolar concentrations and was neither hemolytic nor cytotoxic to mammalian cells in vitro. The present study aimed to shed light into its mechanism of action, with a focus on its fungal cell membrane activity. We utilized different fluorescent dyes to prove that it rapidly induces membrane depolarization and permeabilization. Studies on model membrane systems revealed that the TB analog undergoes hydrophobic and electrostatic membrane interactions, showing a preference for anionic lipids, and identified phosphatidylinositol and cardiolipin as possible peptide targets. Fluorescence microscopy using fluorescein isothiocyanate-labeled TB_KKG6K in the presence of the lipophilic dye FM4-64 indicated that the peptide compromises membrane integrity and rapidly enters C. albicans cells in an energy-independent manner. Peptide-treated cells analyzed by cryo-based electron microscopy exhibited no signs of cell lysis; however, subcellular structures had disintegrated, suggesting that intracellular activity may form part of the killing mechanism of the peptide. Taken together, this study proved that TB_KKG6K compromises C. albicans membrane function, which explains the previously observed rapid, fungicidal mode of action and supports its great potential as a future anti-Candida therapeutic. IMPORTANCE Fungal infections with the opportunistic human pathogen C. albicans are associated with high mortality rates in immunocompromised patients. This is partly due to the yeast's ability to rapidly develop resistance toward currently available antifungals. Small, cationic, membrane-active peptides are promising compounds to fight against resistance development, as many of them effectuate rapid fungal cell death. This fast killing is believed to hamper the development of resistance, as the fungi do not have sufficient time to adapt to the antifungal compound. We previously reported that the synthetic variant of the amphibian TB peptide, TB_KKG6K, rapidly kills C. albicans. In the current study, the mechanism of action of the TB analog was investigated. We show that this TB analog is membrane-active and impairs cell membrane function, highlighting its potential to be developed as an attractive alternative anti-C. albicans therapeutic that may hinder the development of resistance.

Where Electrostatics Matter: Bacterial Surface Neutralization and Membrane Disruption by Antimicrobial Peptides SAAP-148 and OP-145

The need for alternative treatment of multi-drug-resistant bacteria led to the increased design of antimicrobial peptides (AMPs). AMPs exhibit a broad antimicrobial spectrum without a distinct preference for a specific species. Thus, their mechanism, disruption of fundamental barrier function by permeabilization of the bacterial cytoplasmic membrane is considered to be rather general and less likely related to antimicrobial resistance. Of all physico-chemical properties of AMPs, their positive charge seems to be crucial for their interaction with negatively charged bacterial membranes. Therefore, we elucidate the role of electrostatic interaction on bacterial surface neutralization and on membrane disruption potential of two potent antimicrobial peptides, namely, OP-145 and SAAP-148. Experiments were performed on Escherichia coli, a Gram-negative bacterium, and Enterococcus hirae, a Gram-positive bacterium, as well as on their model membranes. Zeta potential measurements demonstrated that both peptides neutralized the surface charge of E. coli immediately after their exposure, but not of E. hirae. Second, peptides neutralized all model membranes, but failed to efficiently disrupt model membranes mimicking Gram-negative bacteria. This was further confirmed by flow cytometry showing reduced membrane permeability for SAAP-148 and the lack of OP-145 to permeabilize the E. coli membrane. As neutralization of E. coli surface charges was achieved before the cells were killed, we conclude that electrostatic forces are more important for actions on the surface of Gram-negative bacteria than on their cytoplasmic membranes.

The antimicrobial effects of PLGA microspheres containing the antimicrobial peptide OP-145 on clinically isolated pathogens in bone infections

Infection after fracture is a significant problem for the healing of fractures. Antimicrobial peptides combined with PLGA (poly-lactic-co-glycolic acid) microspheres can open new horizons for treating bone infections. Twenty rats in the control group were treated with physiologic saline solution after surgery, and 20 rats in the treatment group were treated with OP-145 PLGA microspheres and vancomycin after surgery. The biofilms from treatment and control groups were analyzed by fluorescence microscopy. Blood samples were collected at 12, 24, 36, 48, and 72 h. OP-145 PLGA microspheres showed significant inhibitory effects on clinically isolated strains (P < 0.05) and there were significant differences in serum CRP (P < 0.05) levels compared with control group. In conclusion, OP-145 PLGA microspheres could slowly release antimicrobial peptides and significantly reduce biofilm formation and levels of inflammatory factors.

Antimicrobial Peptides and Cationic Nanoparticles: A Broad-Spectrum Weapon to Fight Multi-Drug Resistance Not Only in Bacteria

In the last few years, antibiotic resistance and, analogously, anticancer drug resistance have increased considerably, becoming one of the main public health problems. For this reason, it is crucial to find therapeutic strategies able to counteract the onset of multi-drug resistance (MDR). In this review, a critical overview of the innovative tools available today to fight MDR is reported. In this direction, the use of membrane-disruptive peptides/peptidomimetics (MDPs), such as antimicrobial peptides (AMPs), has received particular attention, due to their high selectivity and to their limited side effects. Moreover, similarities between bacteria and cancer cells are herein reported and the hypothesis of the possible use of AMPs also in anticancer therapies is discussed. However, it is important to take into account the limitations that could negatively impact clinical application and, in particular, the need for an efficient delivery system. In this regard, the use of nanoparticles (NPs) is proposed as a potential strategy to improve therapy; moreover, among polymeric NPs, cationic ones are emerging as promising tools able to fight the onset of MDR both in bacteria and in cancer cells.

Disruption of the Cytoplasmic Membrane Structure and Barrier Function Underlies the Potent Antiseptic Activity of Octenidine in Gram-Positive Bacteria

The antimicrobial killing mechanism of octenidine (OCT), a well-known antiseptic is poorly understood. We recently reported its interaction with Gram-negative bacteria by insertion of OCT into the outer and cytoplasmic membrane of Escherichia coli, resulting in a chaotic lipid rearrangement and rapid disruption of the cell envelope. Its action primarily disturbs the packing order of the hydrophobic moiety of a lipid, which consequently might result in a cascade of multiple effects at a cellular level. Here, we investigated OCT's impact on two different Gram-positive bacteria, Enterococcus hirae and Bacillus subtilis, and their respective model membranes. In accordance with our previous results, OCT induced membrane disorder in all investigated model systems. Electron and fluorescence microscopy clearly demonstrated changes in cellular structure and membrane integrity. These changes were accompanied by neutralization of the surface charge in both E. hirae and B. subtilis and membrane disturbances associated with permeabilization. Similar permeabilization and disordering of the lipid bilayer was also observed in model membranes. Furthermore, experiments performed on strongly versus partly anionic membranes showed that the lipid disordering effect induced by OCT is a result of maximized hydrophobic over electrostatic forces without distinct neutralization of the surface charge or discrimination between the lipid head groups. Indeed, mutants lacking specific lipid head groups were also susceptible to OCT to a similar extent as the wild type. The observed unspecific mode of action of OCT underlines its broad antimicrobial profile and renders the development of bacterial resistance to this molecule less likely. IMPORTANCE OCT is a well-established antiseptic molecule routinely used in a large field of clinical applications. Since the spread of antimicrobial resistance has restricted the use of antibiotics worldwide, topically applied antiseptics like OCT, with a broad spectrum of antimicrobial activity and high safety profile, gain increasing importance for effective infection prevention and therapy. To eliminate a wide spectrum of disease-causing microorganisms, a compound's antiseptic activity should be unspecific or multitarget. Our results demonstrate an unspecific mechanism of action for OCT, which remained largely unknown for years. OCT disturbs the barrier function of a bacterial cell, a function that is absolutely fundamental for survival. Because OCT does not distinguish between lipids, the building blocks of bacterial membranes, its mode of action might be attributed to all bacteria, including (multi)drug-resistant isolates. Our results underpin OCT's potent antiseptic activity for successful patient outcome.

Membrane Activity of LL-37 Derived Antimicrobial Peptides against Enterococcus hirae: Superiority of SAAP-148 over OP-145

The development of antimicrobial agents against multidrug-resistant bacteria is an important medical challenge. Antimicrobial peptides (AMPs), human cathelicidin LL-37 and its derivative OP-145, possess a potent antimicrobial activity and were under consideration for clinical trials. In order to overcome some of the challenges to their therapeutic potential, a very promising AMP, SAAP-148 was designed. Here, we studied the mode of action of highly cationic SAAP-148 in comparison with OP-145 on membranes of Enterococcus hirae at both cellular and molecular levels using model membranes composed of major constituents of enterococcal membranes, that is, anionic phosphatidylglycerol (PG) and cardiolipin (CL). In all assays used, SAAP-148 was consistently more efficient than OP-145, but both peptides displayed pronounced time and concentration dependences in killing bacteria and performing at the membrane. At cellular level, Nile Red-staining of enterococcal membranes showed abnormalities and cell shrinkage, which is also reflected in depolarization and permeabilization of E. hirae membranes. At the molecular level, both peptides abolished the thermotropic phase transition and induced disruption of PG/CL. Interestingly, the membrane was disrupted before the peptides neutralized the negative surface charge of PG/CL. Our results demonstrate that SAAP-148, which kills bacteria at a significantly lower concentration than OP-145, shows stronger effects on membranes at the cellular and molecular levels.

Structural Plasticity of LL-37 Indicates Elaborate Functional Adaptation Mechanisms to Bacterial Target Structures

The human cathelicidin LL-37 is a multifunctional peptide of the human innate immune system. Among the various functions of LL-37, its antimicrobial activity is important in controlling the microorganisms of the human body. The target molecules of LL-37 in bacteria include membrane lipids, lipopolysaccharides (LPS), lipoteichoic acid (LTA), proteins, DNA and RNA. In this mini-review, we summarize the entity of LL-37 structural data determined over the last 15 years and specifically discuss features implicated in the interactions with lipid-like molecules. For this purpose, we discuss partial and full-length structures of LL-37 determined in the presence of membrane-mimicking detergents. This constantly growing structural database is now composed of monomers, dimers, tetramers, and fiber-like structures. The diversity of these structures underlines an unexpected plasticity and highlights the conformational and oligomeric adaptability of LL-37 necessary to target different molecular scaffolds. The recent co-crystal structures of LL-37 in complex with detergents are particularly useful to understand how these molecules mimic lipids and LPS to induce oligomerization and fibrillation. Defined detergent binding sites provide deep insights into a new class of peptide scaffolds, widening our view on the fascinating world of the LL-37 structural factotum. Together, the new structures in their evolutionary context allow for the assignment of functionally conserved residues in oligomerization and target interactions. Conserved phenylalanine and arginine residues primarily mediate those interactions with lipids and LPS. The interactions with macromolecules such as proteins or DNA remain largely unexplored and open a field for future studies aimed at structures of LL-37 complexes. These complexes will then allow for the structure-based rational design of LL-37-derived peptides with improved antibiotic properties.

Octominin: An antibacterial and anti-biofilm peptide for controlling the multidrug resistance and pathogenic Streptococcus parauberis

Streptococcus parauberis is a pathogenic gram-positive bacterium that causes streptococcosis infection in fish. Since S. parauberis is becoming resistant to multiple antibiotics, the development of alternatives, such as antimicrobial peptides, has gained great attention. Octominin, derived from the defense protein of Octopus minor, showed a significant antimicrobial activity against multidrug resistance S. parauberis, with a minimum inhibitory concentration (MIC) and a minimum bactericidal concentration (MBC) of 50 and 100 μg/mL, respectively. Furthermore, time-kill kinetics, agar diffusion, and bacterial viability assays confirmed the concentration-dependent antibacterial activity of Octominin against S. parauberis. Field emission scanning electron microscopy analysis showed morphological and ultra-structural changes in S. parauberis upon Octominin treatment. Moreover, Octominin treatment demonstrated changes in membrane permeability, induced reactive oxygen species (ROS), and its binding ability to genomic DNA, suggesting its strong bactericidal activity with multiple modes of action. We confirmed the inhibition of biofilm formation and the eradication of existing biofilms in a concentration-dependent manner. Additionally, Octominin on S. parauberis at transcriptional level exhibited downregulation of membrane formation (pgsA and cds1), DNA repairing (recF), biofilm formation (pgaB and epsF) genes, while upregulation of ROS detoxification (sodA) and DNA protecting (ahpF) related genes. An in vivo study confirmed a significantly (P < 0.05) higher relative percentage survival in Octominin-treated larval zebrafish exposed to S. parauberis (93.3%) compared to the control group (20.0%). Collectively, our results confirm that Octominin could be a potential antibacterial and anti-biofilm agent against S. parauberis.

Bridging the Antimicrobial Activity of Two Lactoferricin Derivatives in E. coli and Lipid-Only Membranes

We coupled the antimicrobial activity of two well-studied lactoferricin derivatives, LF11-215 and LF11-324, in Escherichia coli and different lipid-only mimics of its cytoplasmic membrane using a common thermodynamic framework for peptide partitioning. In particular, we combined an improved analysis of microdilution assays with ζ-potential measurements, which allowed us to discriminate between the maximum number of surface-adsorbed peptides and peptides fully partitioned into the bacteria. At the same time, we measured the partitioning of the peptides into vesicles composed of phosphatidylethanolamine (PE), phosphatidylgylcerol (PG), and cardiolipin (CL) mixtures using tryptophan fluorescence and determined their membrane activity using a dye leakage assay and small-angle X-ray scattering. We found that the vast majority of LF11-215 and LF11-324 readily enter inner bacterial compartments, whereas only 1-5% remain surface bound. We observed comparable membrane binding of both peptides in membrane mimics containing PE and different molar ratios of PG and CL. The peptides' activity caused a concentration-dependent dye leakage in all studied membrane mimics; however, it also led to the formation of large aggregates, part of which contained collapsed multibilayers with sandwiched peptides in the interstitial space between membranes. This effect was least pronounced in pure PG vesicles, requiring also the highest peptide concentration to induce membrane permeabilization. In PE-containing systems, we additionally observed an effective shielding of the fluorescent dyes from leakage even at highest peptide concentrations, suggesting a coupling of the peptide activity to vesicle fusion, being mediated by the intrinsic lipid curvatures of PE and CL. Our results thus show that LF11-215 and LF11-324 effectively target inner bacterial components, while the stored elastic stress makes membranes more vulnerable to peptide translocation.

Review: Lessons Learned From Clinical Trials Using Antimicrobial Peptides (AMPs)

Antimicrobial peptides (AMPs) or host defense peptides protect the host against various pathogens such as yeast, fungi, viruses and bacteria. AMPs also display immunomodulatory properties ranging from the modulation of inflammatory responses to the promotion of wound healing. More interestingly, AMPs cause cell disruption through non-specific interactions with the membrane surface of pathogens. This is most likely responsible for the low or limited emergence of bacterial resistance against many AMPs. Despite the increasing number of antibiotic-resistant bacteria and the potency of novel AMPs to combat such pathogens, only a few AMPs are in clinical use. Therefore, the current review describes (i) the potential of AMPs as alternatives to antibiotics, (ii) the challenges toward clinical implementation of AMPs and (iii) strategies to improve the success rate of AMPs in clinical trials, emphasizing the lessons we could learn from these trials.

Interactions of an Anionic Antimicrobial Peptide with Zinc(II): Application to Bacterial Mimetic Membranes

While the majority of known antimicrobial peptides are cationic, a small number consist of short Asp-rich sequences that are anionic. These require metal ions to become biologically active. Here, we report the study of the zinc complexes of the peptide GADDDDD (GAD5), an antimicrobial peptide. Using a combination of dynamic light scattering (DLS), ζ-potential, infrared, Raman, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM), we find that adding zinc ions to GAD5 forces it into a compact structure. Higher amounts of zinc ions favor a larger structure, possibly a dimer. SEM images show that zinc ions reduce the size of the fibrillar structures of GAD5. TGA curves show that the addition of zinc ions increases the thermal stability of the structure of the peptide. TGA and DSC indicate that the association of GAD5 with a zwitterionic phospholipid in the presence of zinc ions is the most stable. The stability of that complex is due to the presence of a sharp endothermic peak in the 200-300 °C range, suggesting the presence of interlamellar water that is essential to the stabilization of the structure. These results indicate that the Zn-GAD5 complex prefers the bacteria-mimicking neutral (zwitterionic) membranes. In the presence of negatively charged phospholipids, the complex remains unordered and unstable. In terms of mechanism of action, the Zn-GAD5 complex promotes a possible endocytic uptake with respect to neutral (zwitterionic) membranes while promoting membrane disruption by forming pores with respect to negatively charged phospholipids.

Targeted and Intracellular Antibacterial Activity against S. agalactiae of the Chimeric Peptides Based on Pheromone and Cell-Penetrating Peptides

The significance of the complex bacterial ecosystem in the human body and the impediment of the mammalian membrane against many antibiotics together emphasize the necessity to develop antimicrobial agents with precise antimicrobial and cell-penetrating activities. A simple and feasible method for generating dual-function antimicrobial peptides inspired by highly hydrophobic peptide pheromone and cationic cell-penetrating peptides is presented. Furthermore, the extension of the peptide candidate library is achieved by modifying the charged domain. The bacteria-selective peptides L1, L2, L10, and L11 kill Streptococcus agalactiae by disrupting the membrane structure, and the targeted mechanism is suggested where the peptides offset the entrapment of S. agalactiae rather than of other bacteria. Moreover, L2 and L10 possess intracellular antibacterial activity and carrier property, which is mainly dependent on endocytosis. Given their suitable biocompatibility, high tolerance, no drug resistance, and effective antimicrobial capacity in a mouse mastitis model, L2 and L10 can be powerful weapons against S. agalactiae pathogen infection.

Strategies in Translating the Therapeutic Potentials of Host Defense Peptides

The golden era of antibiotics, heralded by the discovery of penicillin, has long been challenged by the emergence of antimicrobial resistance (AMR). Host defense peptides (HDPs), previously known as antimicrobial peptides, are emerging as a group of promising antimicrobial candidates for combatting AMR due to their rapid and unique antimicrobial action. Decades of research have advanced our understanding of the relationship between the physicochemical properties of HDPs and their underlying antimicrobial and non-antimicrobial functions, including immunomodulatory, anti-biofilm, and wound healing properties. However, the mission of translating novel HDP-derived molecules from bench to bedside has yet to be fully accomplished, primarily attributed to their intricate structure-activity relationship, toxicity, instability in host and microbial environment, lack of correlation between in vitro and in vivo efficacies, and dwindling interest from large pharmaceutical companies. Based on our previous experience and the expanding knowledge gleaned from the literature, this review aims to summarize the novel strategies that have been employed to enhance the antimicrobial efficacy, proteolytic stability, and cell selectivity, which are all crucial factors for bench-to-bedside translation of HDP-based treatment. Strategies such as residues substitution with natural and/or unnatural amino acids, hybridization, L-to-D heterochiral isomerization, C- and N-terminal modification, cyclization, incorporation with nanoparticles, and "smart design" using artificial intelligence technology, will be discussed. We also provide an overview of HDP-based treatment that are currently in the development pipeline.

Thrombocidin-1-derived antimicrobial peptide TC19 combats superficial multi-drug resistant bacterial wound infections

Antimicrobial peptides are considered promising candidates for the development of novel antimicrobial agents to combat infections by multi-drug-resistant (MDR) bacteria. Here, we describe the identification and characterization of the synthetic peptide TC19, derived from the human thrombocidin-1-derived peptide L3. Biophysical experiments into the interaction between TC19 and mimics of human and bacterial plasma membranes demonstrated that the peptide is highly selective for bacterial membranes. In agreement, TC19 combined low cytotoxicity towards human fibroblasts with efficient and rapid killing in human plasma of MDR strains of several bacterial species of the ESKAPE panel. In addition, TC19 induced minor resistance in vitro, neutralized pro-inflammatory activity of bacterial cell envelope components while displaying slight chemotactic activity for human neutrophils. Importantly, topical application of TC19-containing hypromellose gel significantly reduced numbers of viable methicillin-resistant Staphylococcus aureus (MRSA) and MDR Acinetobacter baumannii in a superficial wound infection in mice. Together, TC19 is an attractive candidate for further development as a novel agent against (MDR) bacterial skin wound infections.

Experimental concepts for linking the biological activities of antimicrobial peptides to their molecular modes of action

The search for novel compounds to combat multi-resistant bacterial infections includes exploring the potency of antimicrobial peptides and derivatives thereof. Complementary to high-throughput screening techniques, biophysical and biochemical studies of the biological activity of these compounds enable deep insight, which can be exploited in designing antimicrobial peptides with improved efficacy. This approach requires the combination of several techniques to study the effect of such peptides on both bacterial cells and simple mimics of their cell envelope, such as lipid-only vesicles. These efforts carry the challenge of bridging results across techniques and sample systems, including the proper choice of membrane mimics. This review describes some important concepts toward the development of potent antimicrobial peptides and how they translate to frequently applied experimental techniques, along with an outline of the biophysics pertaining to the killing mechanism of antimicrobial peptides.

Two small, cysteine-rich and cationic antifungal proteins from Penicillium chrysogenum: A comparative study of PAF and PAFB

The filamentous fungus Penicillium chrysogenum Q176 secretes the antimicrobial proteins (AMPs) PAF and PAFB, which share a compact disulfide-bond mediated, β-fold structure rendering them highly stable. These two AMPs effectively inhibit the growth of human pathogenic fungi in micromolar concentrations and exhibit antiviral potential without causing cytotoxic effects on mammalian cells in vitro and in vivo.

The antifungal mechanism of action of both AMPs is closely linked to - but not solely dependent on - the lipid composition of the fungal cell membrane and requires a strictly regulated protein uptake into the cell, indicating that PAF and PAFB are not canonical membrane active proteins. Variations in their antifungal spectrum and their killing dynamics point towards a divergent mode of action related to their physicochemical properties and surface charge distribution.

In this review, we relate characteristic features of PAF and PAFB to the current knowledge about other AMPs of different sources. In addition, we present original data that have never been published before to substantiate our assumptions and provide evidences that help to explain and understand better the mechanistic function of PAF and PAFB. Finally, we underline the promising potential of PAF and PAFB as future antifungal therapeutics.

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Penicillium chrysogenum secretes the small, cysteine-rich proteins PAF and PAFB.

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Both exhibit antifungal activity, but with differences in their mode of action.

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Structure, membrane interaction and cellular uptake determine their function.

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PAF and PAFB are well tolerated by mammalian cells.

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They promise applicability in medicine, plant protection and food industry.

The Role of Amylogenic Fiber Aggregation on the Elasticity of a Lipid Membrane

This work presents a systematic study of the swelling behavior of a lecithin lamellar phase incorporating different amounts of the short peptide sequence diphenylalanine (FF). Small- and wide-angle X-ray scattering assays provide relevant information about the structure and elasticity of the lamellar stacking. These data show that important changes occur at the interface of the lipid membrane dependent not only on the peptide content but also on the hydration of the lamellar structure. Multilamellar-to-unilamellar transitions, previously observed for an increasing number of peptides, are now observed to be dependent on the hydration of the lamellar phase. Wide-angle X-ray scattering and electron microscopy observations (TEM) provide experimental evidence of peptide aggregation into long amylogenic fibers. We argue that aggregates that partition in water may become large enough to destabilize the lamellar structure. It is also shown that, for a given peptide concentration, the lamellar structure can be rendered more flexible or more rigid, by tuning the hydration.

Selective Antibacterial Activity and Lipid Membrane Interactions of Arginine-Rich Amphiphilic Peptides

The self-assembly behavior and antimicrobial activity of two designed amphiphilic peptides, R3F3 and R4F4, containing short hydrophobic phenylalanine (F) and cationic arginine (R) sequences, are investigated. The conformation of the peptides was examined using circular dichroism and FTIR spectroscopy, which show that they have a disordered secondary structure. Concentration-dependent fluorescence assays show the presence of a critical aggregation concentration (cac) for each peptide. Above the cac, small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) reveal a population of twisted tapes for R3F3 and nanosheets for R4F4. The interaction of the peptides with model bacterial membranes comprising mixtures of the lipids DPPG [1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol] and DPPE [1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine], was studied using SAXS and cryogenic-TEM. Analysis of the SAXS structure factor indicates that R3F3 interacts with lipid bilayers by inducing correlation between bilayers, whereas R4F4 interacts with the bilayers causing an increase in polydispersity of the vesicle wall thickness. Both peptides break vesicles with a 1:3 DPPG:DPPE composition, which is close to the ratio of PG and PE lipids observed in the lipid membrane of Pseudomonas aeruginosa, a pathogen responsible for serious infections and which has developed antimicrobial resistant strains. Both peptides show activity against this bacterium in planktonic form. Peptide R4F4 shows particularly strong bioactivity against this microbe, with a minimum inhibitory concentration (MIC) value in the range of concentrations where the peptide is cytocompatible. It was further shown to have activity against other Pseudomonas species including the common plant pathogen Pseudomonas syringae. Finally, we show that R4F4 inhibits the development of P. aeruginosa biofilms. This was examined in detail and a proposed mechanism involving binding of the signaling molecule c-di-GMP is suggested, based on circular dichroism spectroscopy studies and Congo red assays of extracellular polysaccharides produced by the stressed bacteria. Thus, R4F4 is a promising candidate antimicrobial peptide with activity against Pseudomonas species.

Role and modulation of the secondary structure of antimicrobial peptides to improve selectivity

Helix is a two-edged sword for AMPs, and conformational modulation of AMPs can control the balance between antimicrobial activity and toxicity.

Current landscape in the discovery of novel antibacterial agents

BACKGROUND

Standard treatments against bacterial infections are becoming ineffective due to the rise of antibacterial resistance worldwide. Classical approaches to develop new antibacterial agents are not sufficient to fulfil the current pipeline, therefore new strategies are currently being devised in the field of antibacterial discovery.

OBJECTIVES

The objective of this narrative review is to compile the most successful strategies for drug discovery within the antibacterial context that are currently being pursued.

SOURCES

Peer-reviewed publications from the MEDLINE database with robust data addressing the discovery of new antibacterial agents in the current pipeline have been selected.

CONTENT

Several strategies to discover new antibacterials are described in this review: (i) derivatives of known antibacterial agents; the activity of a known antimicrobial agent can be improved through two strategies: (a) the modification of the original chemical structure of an antimicrobial agent to circumvent antibacterial resistance mechanisms and (b) the development of a compound that inhibits the mechanisms of resistance to an antibacterial agent; (ii) new antibacterial agents targeting new proteins; (iii) inhibitors of virulence factors; (iv) nanoparticles; (v) antimicrobial peptides and peptidomimetics; (vi) phage therapy and enzybiotics; and (vii) antisense oligonucleotides.

IMPLICATIONS

This review intends to provide a positive message affirming that several different strategies to design new antibacterial agents are currently being developed, and we are therefore confident that in the near future some of the most promising approaches will come to fruition.

Self-Assembly, Antimicrobial Activity, and Membrane Interactions of Arginine-Capped Peptide Bola-Amphiphiles

The self-assembly and antimicrobial activity of two novel arginine-capped bola-amphiphile peptides, namely RA6R and RA9R (R, arginine; A, alanine) are investigated. RA6R does not self-assemble in water due to its high solubility, but RA9R self-assembles above a critical aggregation concentration into ordered nanofibers due to the high hydrophobicity of the A9block. The structure of the RA9R nanofibers is studied by cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS). Circular dichroism spectroscopy shows that both RA6R and RA9R adopt coil conformations in water at low concentration, but only RA9R adopts a β-sheet conformation at high concentration. SAXS and differential scanning calorimetry are used to study RA6R and RA9R interactions with a mixed lipid membrane that models a bacterial cell wall, consisting of multilamellar 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol/1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine vesicles. Cytotoxicity studies show that RA6R is more cytocompatible than RA9R. RA6R has enhanced activity against the Gram-negative pathogen P. aeruginosa at a concentration where viability of mammalian cells is retained. RA9R has little antimicrobial activity, independently of concentration. Our results highlight the influence of the interplay between relative charge and hydrophobicity on the self-assembly, cytocompatibility, and bioactivity of peptide bola-amphiphiles.

The antimicrobial peptide SAAP-148 combats drug-resistant bacteria and biofilms

Development of novel antimicrobial agents is a top priority in the fight against multidrug-resistant (MDR) and persistent bacteria. We developed a panel of synthetic antimicrobial and antibiofilm peptides (SAAPs) with enhanced antimicrobial activities compared to the parent peptide, human antimicrobial peptide LL-37. Our lead peptide SAAP-148 was more efficient in killing bacteria under physiological conditions in vitro than many known preclinical- and clinical-phase antimicrobial peptides. SAAP-148 killed MDR pathogens without inducing resistance, prevented biofilm formation, and eliminated established biofilms and persister cells. A single 4-hour treatment with hypromellose ointment containing SAAP-148 completely eradicated acute and established, biofilm-associated infections with methicillin-resistant Staphylococcus aureus and MDR Acinetobacter baumannii from wounded ex vivo human skin and murine skin in vivo. Together, these data demonstrate that SAAP-148 is a promising drug candidate in the battle against antibiotic-resistant bacteria that pose a great threat to human health.

Strategically prolonged release of multiple antibacterial components from a thin film coating for synergistic effects against Staphylococcus aureus infections

Biomaterial-associated infections (BAIs) remain a major challenge in clinical surgery because they can potentially cause serious disabilities in patients.

Peptidoglycan potentiates the membrane disrupting effect of the carboxyamidated form of DMS-DA6, a Gram-positive selective antimicrobial peptide isolated from Pachymedusa dacnicolor skin

The occurrence of nosocomial infections has been on the rise for the past twenty years. Notably, infections caused by the Gram-positive bacteria Staphylococcus aureus represent a major clinical problem, as an increase in antibiotic multi-resistant strains has accompanied this rise. There is thus a crucial need to find and characterize new antibiotics against Gram-positive bacteria, and against antibiotic-resistant strains in general. We identified a new dermaseptin, DMS-DA6, produced by the skin of the Mexican frog Pachymedusa dacnicolor, with specific antibacterial activity against Gram-positive bacteria. This peptide is particularly effective against two multiple drug-resistant strains Enterococcus faecium BM4147 and Staphylococcus aureus DAR5829, and has no hemolytic activity. DMS-DA6 is naturally produced with the C-terminal carboxyl group in either the free or amide forms. By using Gram-positive model membranes and different experimental approaches, we showed that both forms of the peptide adopt an α-helical fold and have the same ability to insert into, and to disorganize a membrane composed of anionic lipids. However, the bactericidal capacity of DMS-DA6-NH2 was consistently more potent than that of DMS-DA6-OH. Remarkably, rather than resulting from the interaction with the negatively charged lipids of the membrane, or from a more stable conformation towards proteolysis, the increased capacity to permeabilize the membrane of Gram-positive bacteria of the carboxyamidated form of DMS-DA6 was found to result from its enhanced ability to interact with peptidoglycan.

Bioinspired Designs, Molecular Premise and Tools for Evaluating the Ecological Importance of Antimicrobial Peptides

This review article provides an overview of recent developments in antimicrobial peptides (AMPs), summarizing structural diversity, potential new applications, activity targets and microbial killing responses in general. The use of artificial and natural AMPs as templates for rational design of peptidomimetics are also discussed and some strategies are put forward to curtail cytotoxic effects against eukaryotic cells. Considering the heat-resistant nature, chemical and proteolytic stability of AMPs, we attempt to summarize their molecular targets, examine how these macromolecules may contribute to potential environmental risks vis-à-vis the activities of the peptides. We further point out the evolutional characteristics of the macromolecules and indicate how they can be useful in designing target-specific peptides. Methods are suggested that may help to assess toxic mechanisms of AMPs and possible solutions are discussed to promote the development and application of AMPs in medicine. Even if there is wide exposure to the environment like in the hospital settings, AMPs may instead contribute to prevent healthcare-associated infections so long as ecotoxicological aspects are considered.

Hybridization and antibiotic synergism as a tool for reducing the cytotoxicity of antimicrobial peptides

Introduction

As the development of new antimicrobial agents faces a historical decline, the issue of bacterial drug resistance has become a serious dilemma that threatens the human population worldwide. Antimicrobial peptides (AMPs) represent an attractive and a promising class of antimicrobial agents.

Aim

The hybridization of AMPs aimed at merging two individual active fragments of native peptides to generate a new AMP with altered physicochemical properties that translate into an enhanced safety profile.

Materials and methods

In this study, we have rationally designed a new hybrid peptide via combining two individual α-helical fragments of both BMAP-27 and OP-145. The resultant peptide, was evaluated for its antimicrobial and antibiofilm activity against a range of microbial strains. The resultant peptide was also evaluated for its toxicity against mammalian cells using hemolytic and anti proliferative assays.

Results

The antimicrobial activity of H4 revealed that the peptide is displaying a broad spectrum of activity against both Gram-positive and Gram-negative bacteria including standard and multidrug-resistant bacterial strains in the range of 2.5-25 μM. The new hybrid peptide displayed potent activity in eradicating biofilm-forming cells, and the reported minimum biofilm eradication concentrations were equal to the minimum inhibitory concentration values reported for planktonic cells. Additionally, H4 exhibited reduced toxicity profiles against eukaryotic cells. Combining H4 peptide with conventional antibiotics has led to a dramatic enhancement of the antimicrobial activity of both agents with synergistic or additive outcomes.

Conclusion

Overall, this study indicates the success of both the hybridization and synergism strategy in developing AMPs as potential antimicrobial therapeutics with reduced toxicity profiles that could be efficiently employed to eradicate resistant bacterial strains and enhance the selectivity and toxicity profiles of native AMPs.

Antimicrobial peptides with selective antitumor mechanisms: prospect for anticancer applications

In the last several decades, there have been significant advances in anticancer therapy. However, the development of resistance to cancer drugs and the lack of specificity related to actively dividing cells leading to toxic side effects have undermined these achievements. As a result, there is considerable interest in alternative drugs with novel antitumor mechanisms. In addition to the recent approach using immunotherapy, an effective but much cheaper therapeutic option of pharmaceutical drugs would still provide the best choice for cancer patients as the first line treatment. Ribosomally synthesized cationic antimicrobial peptides (AMPs) or host defense peptides (HDP) display broad-spectrum activity against bacteria based on electrostatic interactions with negatively charged lipids on the bacterial surface. Because of increased proportions of phosphatidylserine (negatively charged) on the surface of cancer cells compared to normal cells, cationic amphipathic peptides could be an effective source of anticancer agents that are both selective and refractory to current resistance mechanisms. We reviewed herein the prospect for AMP application to cancer treatment, with a focus on modes of action of cationic AMPs.

Multilamellar-to-Unilamellar Transition Induced by Diphenylalanine in Lipid Vesicles

In the present work, we investigate the effect of two short phenylalanine-based peptides on lipid membranes. A simplified model membrane composed of lecithin vesicles was used to incorporate different amounts of the two amino acid sequences, the dimmer l,l-diphenylallanine (FF) and the trimmer cysteine-diphenylallanine (CFF). Spectroscopic and scattering techniques were applied to probe in detail the structural behavior of lipid membranes in the presence of the peptides. The experimental results demonstrate that both peptides are located mainly at the interface of the membrane interacting with phosphate groups modifying membrane thickness and flexibility. The multilamellar structure of the vesicles is preserved with inclusion of small amounts of FF, accompanied by changes in membrane thickness and elasticity. Finally, a multi- to unilamellar transition is observed as a result of peptide self-association into a crystalline structure onto the membrane interface.

Structural remodeling and oligomerization of human cathelicidin on membranes suggest fibril-like structures as active species

Antimicrobial peptides as part of the mammalian innate immune system target and remove major bacterial pathogens, often through irreversible damage of their cellular membranes. To explore the mechanism by which the important cathelicidin peptide LL-37 of the human innate immune system interacts with membranes, we performed biochemical, biophysical and structural studies. The crystal structure of LL-37 displays dimers of anti-parallel helices and the formation of amphipathic surfaces. Peptide-detergent interactions introduce remodeling of this structure after occupation of defined hydrophobic sites at the dimer interface. Furthermore, hydrophobic nests are shaped between dimer structures providing another scaffold enclosing detergents. Both scaffolds underline the potential of LL-37 to form defined peptide-lipid complexes in vivo. After adopting the activated peptide conformation LL-37 can polymerize and selectively extract bacterial lipids whereby the membrane is destabilized. The supramolecular fibril-like architectures formed in crystals can be reproduced in a peptide-lipid system after nanogold-labelled LL-37 interacted with lipid vesicles as followed by electron microscopy. We suggest that these supramolecular structures represent the LL-37-membrane active state. Collectively, our study provides new insights into the fascinating plasticity of LL-37 demonstrated at atomic resolution and opens the venue for LL-37-based molecules as novel antibiotics.

The Antibacterial Effects of Antimicrobial Peptides OP-145 against Clinically Isolated Multi-Resistant Strains

OP-145 is a synthetic antimicrobial peptide developed from the human cathelicidin LL-37. The purpose of this investigation was to evaluate the effect of the antimicrobial peptide OP-145 against clinically isolated drug-resistant strains. Ten methicillin-resistant Staphylococcus aureus (MRSA) strains were obtained from our hospital's clinical inspection center, and the activity of OP-145 on growth and biofilm formation of these strains was evaluated by colony counts and scanning electron microscopy. The antimicrobial peptide OP-145 showed significant antibacterial activity against 9 MRSA strains. For the biofilm experiments, MRSA counts in the biofilms decreased significantly after 24 h (P ＜ 0.05). OP-145 strongly reduced growth and biofilm formation of clinically isolated drug-resistant strains in vitro, and the use of this class of antimicrobial agents may be an important new approach in controlling bacterial infections.

Advances in Development of Antimicrobial Peptidomimetics as Potential Drugs

The rapid emergence of multidrug-resistant pathogens has evolved into a global health problem as current treatment options are failing for infections caused by pan-resistant bacteria. Hence, novel antibiotics are in high demand, and for this reason antimicrobial peptides (AMPs) have attracted considerable interest, since they often show broad-spectrum activity, fast killing and high cell selectivity. However, the therapeutic potential of natural AMPs is limited by their short plasma half-life. Antimicrobial peptidomimetics mimic the structure and biological activity of AMPs, but display extended stability in the presence of biological matrices. In the present review, focus is on the developments reported in the last decade with respect to their design, synthesis, antimicrobial activity, cytotoxic side effects as well as their potential applications as anti-infective agents. Specifically, only peptidomimetics with a modular structure of residues connected via amide linkages will be discussed. These comprise the classes of α-peptoids (N-alkylated glycine oligomers), β-peptoids (N-alkylated β-alanine oligomers), β³-peptides, α/β³-peptides, α-peptide/β-peptoid hybrids, α/γ N-acylated N-aminoethylpeptides (AApeptides), and oligoacyllysines (OAKs). Such peptidomimetics are of particular interest due to their potent antimicrobial activity, versatile design, and convenient optimization via assembly by standard solid-phase procedures.

Antimicrobial peptide exposure selects for Staphylococcus aureus resistance to human defence peptides

Background

The clinical development of antimicrobial peptides (AMPs) is currently under evaluation to combat the rapid increase in MDR bacterial pathogens. However, many AMPs closely resemble components of the human innate immune system and the ramifications of prolonged bacterial exposure to AMPs are not fully understood.

Objectives

We show that in vitro serial passage of a clinical USA300 MRSA strain in a host-mimicking environment containing host-derived AMPs results in the selection of stable AMP resistance.

Methods

Serial passage experiments were conducted using steadily increasing concentrations of LL-37, PR-39 or wheat germ histones. WGS and proteomic analysis by MS were used to identify the molecular mechanism associated with increased tolerance of AMPs. AMP-resistant mutants were characterized by measuring in vitro fitness, AMP and antibiotic susceptibility, and virulence in a mouse model of sepsis.

Results

AMP-resistant Staphylococcus aureus mutants often displayed little to no fitness cost and caused invasive disease in mice. Further, this phenotype coincided with diminished susceptibility to both clinically prescribed antibiotics and human defence peptides.

Conclusions

These findings suggest that therapeutic use of AMPs could select for virulent mutants with cross-resistance to human innate immunity as well as antibiotic therapy. Thus, therapeutic use of AMPs and the implications of cross-resistance need to be carefully monitored and evaluated.

Antimicrobial Peptides Targeting Gram-Positive Bacteria

Antimicrobial peptides (AMPs) have remarkably different structures as well as biological activity profiles, whereupon most of these peptides are supposed to kill bacteria via membrane damage. In order to understand their molecular mechanism and target cell specificity for Gram-positive bacteria, it is essential to consider the architecture of their cell envelopes. Before AMPs can interact with the cytoplasmic membrane of Gram-positive bacteria, they have to traverse the cell wall composed of wall- and lipoteichoic acids and peptidoglycan. While interaction of AMPs with peptidoglycan might rather facilitate penetration, interaction with anionic teichoic acids may act as either a trap for AMPs or a ladder for a route to the cytoplasmic membrane. Interaction with the cytoplasmic membrane frequently leads to lipid segregation affecting membrane domain organization, which affects membrane permeability, inhibits cell division processes or leads to delocalization of essential peripheral membrane proteins. Further, precursors of cell wall components, especially the highly conserved lipid II, are directly targeted by AMPs. Thereby, the peptides do not inhibit peptidoglycan synthesis via binding to proteins like common antibiotics, but form a complex with the precursor molecule, which in addition can promote pore formation and membrane disruption. Thus, the multifaceted mode of actions will make AMPs superior to antibiotics that act only on one specific target.