Interaction of Cecropin B with Zwitterionic and Negatively Charged Lipid Bilayers Immobilized at Gold Electrode Surface

How do Antimicrobial Peptides Interact with the Outer Membrane of Gram-Negative Bacteria? Role of Lipopolysaccharides in Peptide Binding, Anchoring, and Penetration

Gram-negative bacteria possess a complex structural cell envelope that constitutes a barrier for antimicrobial peptides that neutralize the microbes by disrupting their cell membranes. Computational and experimental approaches were used to study a model outer membrane interaction with an antimicrobial peptide, melittin. The investigated membrane included di[3-deoxy-d-manno-octulosonyl]-lipid A (KLA) in the outer leaflet and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) in the inner leaflet. Molecular dynamics simulations revealed that the positively charged helical C-terminus of melittin anchors rapidly into the hydrophilic headgroup region of KLA, while the flexible N-terminus makes contacts with the phosphate groups of KLA, supporting melittin penetration into the boundary between the hydrophilic and hydrophobic regions of the lipids. Electrochemical techniques confirmed the binding of melittin to the model membrane. To probe the peptide conformation and orientation during interaction with the membrane, polarization modulation infrared reflection absorption spectroscopy was used. The measurements revealed conformational changes in the peptide, accompanied by reorientation and translocation of the peptide at the membrane surface. The study suggests that melittin insertion into the outer membrane affects its permeability and capacitance but does not disturb the membrane's bilayer structure, indicating a distinct mechanism of the peptide action on the outer membrane of Gram-negative bacteria.

Membrane lytic activity of antibacterial ionenes, critical role of phosphatidylcholine (PC) and cardiolipin (CL)

Understanding the mechanism by which an antibacterial agent interacts with a model membrane provides vital information for better design of future antibiotics. In this study, we investigated two antibacterial polymers, hydrophilic C0-T-p and hydrophobic C8-T-p ionenes, known for their potent antimicrobial activity and ability to disrupt the integrity of lipid bilayers. Our hypothesize is that the composition of a lipid bilayer alters the mechanism of ionenes action, potentially providing an explanation for the observed differences in their bioactivity and selectivity. Calcein release experiments utilizing a range of liposomes to examine the impact of (i) cardiolipin (CL) to phosphatidylglycerol (PG) ratio, (ii) overall vesicle charge, and (iii) phosphatidylethanolamine (PE) to phosphatidylcholine (PC) ratio on the activity of ionenes were performed. Additionally, polymer-bilayer interactions were also investigated through vesicle fusion assay and the black lipid membrane (BLM) technique The activity of C0-T-p is strongly influenced by the amount of cardiolipin, while the activity of C8-T-p primarily depends on the overall vesicle charge. Consequently, C0-T-p acts through interactions with CL, whereas C8-T-p modifies the bulk properties of the membrane in a less-specific manner. Moreover, the presence of a small amount of PC in the membrane makes the vesicle resistant to permeabilization by tested molecules. Intriguingly, more hydrophilic C0-T-p retains higher membrane activity compared to the hydrophobic C8-T-p. However, both ionenes induce vesicle fusion and increase lipid bilayer ion permeability.

Intervention of antimicrobial peptide usage on antimicrobial resistance in aquaculture

Antibiotic resistance genes (ARGs) widely exist in diverse matrices to pose potential risks and attract global attention. This study performed tilapia-cultivation experiment to discuss the intervention of antimicrobial peptide usage on antimicrobial resistance in aquaculture by analyzing 29 target ARGs in water, residue, and fish muscle samples. The sul1 gene was the dominant ARG in most of samples while absolute abundances of total ARGs were in the range of 1.5 × 10²-2.3 × 10⁷ copies/mL in water samples and 2.1 × 10⁸-3.9 × 10⁹ copies/g in the residue and fish muscle samples. Compared with antibiotic treatment, the cecropin treatment could significantly reduce the absolute abundance of ARGs and intI1 in all samples. Proteobacteria were the dominant phylum for most of samples while Aeromonas/Cetobacterium served as the dominant genus for samples with antibiotic/cecropin treatment. Network analysis showed that cecropin treatment could significantly reduce occurrence of multi-drug resistant bacteria and the correlation between genera and ARGs. Results of partial least squares structural equation modeling which was used to discuss the influential factors of ARGs exhibited that antibiotics could increase ARGs, further proving that cecropin peptide could effectively reduce ARGs in aquaculture. This study will provide important information on controlling ARGs in aquaculture in terms of antimicrobial peptide application.

Design, synthesis, and evaluation of the self-assembled antimicrobial peptides based on the ovalbumin-derived peptide TK913

The preparation, self-assembly, and antimicrobial activity of peptides based on TK913 is described. TK9Z4 incorporating a Pro-Pro motif exhibited self-assembly but no cytotoxicity. However, peptide TKZ3 (obtained by changing the amino acid sequence of TK9Z4) showed morphological changes at different concentrations, potent antimicrobial activity, low cytotoxicity, and trypsin resistance. Accordingly, TKZ3 is proposed as new AMP derived from ovalbumin-derived peptides.

Designing a Useful Lipid Raft Model Membrane for Electrochemical and Surface Analytical Studies

A model biomimetic system for the study of protein reconstitution or drug interactions should include lipid rafts in the mixed lipid monolayer, since they are usually the domains embedding membrane proteins and peptides. Four model lipid films composed of three components: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), cholesterol (Chol) and sphingomyelin (SM) mixed in different molar ratios were proposed and investigated using surface pressure measurements and thermodynamic analysis of the monolayers at the air-water interface and imaged by Brewster angle microscopy. The ternary monolayers were transferred from the air-water onto the gold electrodes to form bilayer films and were studied for the first time by electrochemical methods: alternative current voltammetry and electrochemical impedance spectroscopy and imaged by atomic force microscopy. In excess of DOPC, the ternary systems remained too liquid for the raft region to be stable, while in the excess of cholesterol the layers were too solid. The layers with SM in excess lead to the formation of Chol:SM complexes but the amount of the fluid matrix was very low. The equimolar content of the three components lead to the formation of a stable and well-organized assembly with well-developed raft microdomains of larger thickness, surrounded by the more fluid part of the bilayer. The latter is proposed as a convenient raft model membrane for further physicochemical studies of interactions with drugs or pollutants or incorporation of membrane proteins.

Interaction of LL-37 human cathelicidin peptide with a model microbial-like lipid membrane

The only representative of cathelicidin peptides in humans is LL-37, a multifunctional antimicrobial peptide (AMP) that is a part of the innate immune response. Details of the LL-37 direct activity against pathogens are not well understood at the molecular level. Here, we present research on the mechanism of interaction between LL-37 and a model multicomponent bilayer lipid membrane (BLM), mimicking microbial cell membrane. Electrochemical impedance spectroscopy (EIS), high-resolution atomic force microscopy (AFM) imaging, and polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) were applied to study the peptide influence on a model microbial-like membrane. We show that LL-37 causes changes in the phospholipid molecules conformation and orientation, leading to membrane disintegration, significantly affecting the membrane electrical parameters, such as capacitance and resistance. High-resolution AFM imaging shows topographical and mechanical effects of such disintegration, while PM-IRRAS data indicates that introduction of LL-37 causes changes in the phospholipid acyl chains from all-trans to gauche conformations. Moreover, the presence of LL-37 significantly alters the value of the phospholipid tilt angle. Altogether, our results suggest a "carpet" membrane dissolution followed by a detergent-like membrane disruption mechanism upon LL-37 activity. This research gives a novel insight into the understanding of LL-37 influence on multicomponent model membranes and a promising contribution to the development of LL-37-derived therapeutic agents against drug-resistant bacteria.

Interactions of Linear Analogues of Battacin with Negatively Charged Lipid Membranes

The increasing resistance of bacteria to available antibiotics has stimulated the search for new antimicrobial compounds with less specific mechanisms of action. These include the ability to disrupt the structure of the cell membrane, which in turn leads to its damage. In this context, amphiphilic lipopeptides belong to the class of the compounds which may fulfill this requirement. In this paper, we describe two linear analogues of battacin with modified acyl chains to tune the balance between the hydrophilic and hydrophobic portion of lipopeptides. We demonstrate that both compounds display antimicrobial activity with the lowest values of minimum inhibitory concentrations found for Gram-positive pathogens. Therefore, their mechanism of action was evaluated on a molecular level using model lipid films mimicking the membrane of Gram-positive bacteria. The surface pressure measurements revealed that both lipopeptides show ability to bind and incorporate into the lipid monolayers, resulting in decreased ordering of lipids and membrane fluidization. Atomic force microscopy (AFM) imaging demonstrated that the exposure of the model bilayers to lipopeptides leads to a transition from the ordered gel phase to disordered liquid crystalline phase. This observation was confirmed by attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) results, which revealed that lipopeptide action causes a substantial increase in the average tilt angle of lipid acyl chains with respect to the surface normal to compensate for lipopeptide insertion into the membrane. Moreover, the peptide moieties in both molecules do not adopt any well-defined secondary structure upon binding with the lipid membrane. It was also observed that a small difference in the structure of a lipophilic chain, altering the balance between hydrophobic and hydrophilic portion of the molecules, results in different insertion depth of the active compounds.

Physicochemical and Biological Characterization of Novel Membrane-Active Cationic Lipopeptides with Antimicrobial Properties

We have designed and synthesized new short lipopeptides composed of tetrapeptide conjugated to fatty acids with different chain lengths. The amino acid sequence of the peptide moiety included d-phenylalanine, two residues of l-2,4-diaminobutyric acid and l-leucine. To explore the possible mechanism of lipopeptide action, we have provided a physicochemical characterization of their interactions with artificial lipid membranes. For this purpose, we have used monolayers and bilayers composed of lipids representative of Gram-negative and Gram-positive bacterial membranes. Using surface pressure measurements and atomic force microscopy, we were able to monitor the changes occurring within the films upon exposure to lipopeptides. Our experiments revealed that all lipopeptides can penetrate the lipid membranes and affect their molecular ordering. The latter results in membrane thinning and fluidization. However, the effect is stronger in the lipid films mimicking Gram-positive bacterial membranes. The results of the physicochemical characterization were compared with the biological activity of lipopeptides. The effect of lipopeptides on bacterial growth was tested on several strains of bacteria. It was revealed that lipopeptides show stronger antimicrobial activity against Gram-positive bacteria. At the same time, all tested compounds display relatively low hemolytic activity.

Physicochemical Characterization of Daptomycin Interaction with Negatively Charged Lipid Membranes

Daptomycin is known as an effective antibiotic lipopeptide which shows activity against the number of Gram-positive pathogens. Its primary target is the bacterial cell membrane. However, the detailed mechanism of daptomycin action is still subject to debate. In this paper, we have investigated the interactions between lipopeptide and model lipid films composed of negatively charged phosphatidylglycerols and cardiolipin. In order to evaluate the effect of daptomycin on the molecular organization and the properties of lipid assemblies, we have used surface pressure measurements and electrochemical methods combined with atomic force microscopy, quartz crystal microbalance, and surface-enhanced infrared absorption spectroscopy. Our results indicate that daptomycin interaction with the lipid membrane is complex. It involves daptomycin aggregation and partial insertion, which in turn affect the charge distribution on both sides of the membrane and may result in a gradient of water chemical potential. The latter can drive the flux of water across the membrane.

Novel Platform for Surface-Mediated Gene Delivery Assisted with Visible-Light Illumination

Surface-mediated gene delivery has attracted more and more attentions in biomedical research and applications because of its characteristics of low toxicity and localized delivery. Herein, a novel visible-light-regulated, surface-mediated gene-delivery platform is exhibited, arising from the photoinduced surface-charge accumulation on silicon. Silicon with a pn junction is used and tested subsequently for the behavior of surface-mediated gene delivery under visible-light illumination. It is found that positive-charge accumulation under light illumination changes the surface potential and then facilitates the delivery of gene-loaded carriers. As a result, the gene-expression efficiency shows a significant improvement from 6% to 28% under a 10 min visible-light illumination. Such improvement is ascribed to the increase in surface potential caused by light illumination, which promotes both the release of gene-loaded carriers and the cellular uptake. This work suggests that silicon with photovoltaic effect could offer a new strategy for surface-mediated, gene-delivery-related biomedical research and applications.

The shape of lipid molecules affects potential-driven molecular-scale rearrangements in model cell membranes on electrodes

Planar asymmetric lipid bilayers composed of phosphatidylethanolamine and phosphatidylglycerol lipids are transferred onto a gold electrode surface. Lipids containing two saturated, one monounsaturated and two monounsaturated hydrocarbon chains compose the model membranes. Results of electrochemically controlled polarization modulation infrared reflection absorption spectroscopy and quartz crystal microbalance with energy dissipation studies reveal two different types of electric potential-dependent structural rearrangements in the bilayers. They are correlated with the geometry of the lipid molecule. Packing parameter correlates the cross-section area of the hydrophobic and hydrophilic parts of amphiphilic molecules. In bilayers composed of lipids with the packing parameter <1, the hydrocarbon chains are tilted with respect to the bilayer plane and the polar head groups are well hydrated. At a threshold potential an abrupt flow of water through the bilayer is connected with membrane dehydration and upward orientation of the chains. In bilayers composed of lipids with packing parameter ≥1, electric potentials have negligible effect on the membrane structure. A simple rule correlating the packing parameter with molecular scale changes occurring at electrified membranes has a large diagnostic implication for biomimetic studies and our understanding of molecular processes occurring in biological cell membranes.

Modulation of Activity of Ultrashort Lipopeptides toward Negatively Charged Model Lipid Films

Because of the increasing resistance of pathogens to commonly used antibiotics, there is an urgent need to find alternative antimicrobial compounds with different mechanisms of action. Among them, lipopeptides are recognized as promising candidates. In this work, the Langmuir technique and atomic force microscopy were employed to investigate the interactions of two novel lipopeptides with negatively charged phospholipid membranes, which served as a simplified model of inner membrane of Gram-negative bacteria. Lipid films contained phosphatidylethanolamine and phosphatidylglycerol extracts from E. coli bacteria. Lipopeptides were composed of palmitoyl chain covalently coupled to N-terminus of peptide with Trp-Lys-Leu-Lys amino acid sequence and the conformation of third residue was either d-Leu or l-Leu. It was found that chirality of leucine strongly affects interfacial behavior of these compounds, which was ascribed to the difference in effective size of the peptide portion of the molecules. Although the lipopeptides were the same in terms of amino acid sequence, charge, and identity of lipophilic chain, the experiments revealed that the barrier for their insertion into the lipid membrane is significantly different. Namely, it was lower for lipopeptide containing d-Leu residue. We have also found that insertion of the lipopeptides into the model membranes strongly alters lateral distribution of the membrane components and leads to its substantial fluidization. The dynamics of reorganization was noticeably faster in the presence of lipopeptide with smaller size of peptide moiety, i.e., containing d-Leu. It proves that effective size of the peptide headgroup is an important factor determining lipopeptide activity toward the lipid membranes.