Could Cardiolipin Protect Membranes against the Action of Certain Antimicrobial Peptides? Aurein 1.2, a Case Study

Exploring the Membrane-Active Interactions of Antimicrobial Long-Chain Fatty Acids Using a Supported Lipid Bilayer Model for Gram-Positive Bacterial Membranes

The dynamic nature of bacterial lipid membranes significantly impacts the efficacy of antimicrobial therapies. However, traditional assay methods often fall short in replicating the complexity of these membranes, necessitating innovative approaches. Herein, we successfully fabricated model bacterially supported lipid bilayers (SLBs) that closely mimic the characteristics of Gram-positive bacteria using the solvent-assisted lipid bilayer (SALB) technique. By employing a quartz crystal microbalance with dissipation and fluorescence microscopy, we investigated the interactions between these bacterial mimetic membranes and long-chain unsaturated fatty acids. Specifically, linolenic acid (LNA) and linoleic acid (LLA) demonstrated interaction behaviors correlated with the critical micelle concentration (CMC) on Gram-positive membranes, resulting in membrane remodeling and removal at concentrations above their respective CMC values. In contrast, oleic acid (OA), while showing similar membrane remodeling patterns to LNA and LLA, exhibited membrane insertion and CMC-independent activity on the Gram-positive membranes. Particularly, LNA and LLA demonstrated bactericidal effects and promoted membrane permeability and ATP leakage in the bacterial membranes. OA, characterized by a CMC-independent activity profile, exhibited potent bactericidal effects due to its robust penetration into the SLBs, also enhancing membrane permeability and ATP leakage. These findings shed light on the intricate molecular mechanisms governing the interactions between long-chain unsaturated fatty acids and bacterial membranes. Importantly, this study underscores the potential of using biologically relevant model bacterial membrane systems to develop innovative strategies for combating bacterial infections and designing effective therapeutic agents.

Molecular mechanism of action of imidazolium carbosilane dendrimers on the outer bacterial membrane - From membrane damage to permeability to antimicrobial endolysin

The outer bacterial membrane of drug-resistant bacteria is a significant barrier to many antimicrobials. Therefore, the development of new antibacterials primarily focuses on damaging the outer bacterial membrane of Gram-negative bacteria. Among many membrane-disrupting substances, the most promising are cationic dendritic systems. However, the mode of action may vary among different strains due to variations in the lipid compositions of the membrane. Here, we investigated the interaction of two types of cationic imidazolium carbosilane dendrimers: one with a single cationic group (methyl imidazolium) and the other with the same cationic group but attached to a functional group (a pendant pyridyl moiety), capable of establishing interactions with membranes through H-bonding or ion-dipole electrostatic interactions. We used different models of the outer membrane of Gram-negative bacteria - Escherichia coli, Pseudomonas aeruginosa, and Acinetobacter baumannii. Additionally, we assessed the combined effect of the dendrimers and the antibacterial endolysin on P. aeruginosa. Our results show that the mechanism of action depends on the type of dendrimer and the lipid composition of the membrane. We also demonstrate that the alteration of membrane fluidity and permeability to endolysin by the methyl imidazolium and pyridyl imidazolium dendrimers may play a more significant role in antimicrobial activity compared to membrane damage caused by positively charged dendrimers.

Molecular Mechanisms of Bacterial Resistance to Antimicrobial Peptides in the Modern Era: An Updated Review

Antimicrobial resistance (AMR) poses a serious global health concern, resulting in a significant number of deaths annually due to infections that are resistant to treatment. Amidst this crisis, antimicrobial peptides (AMPs) have emerged as promising alternatives to conventional antibiotics (ATBs). These cationic peptides, naturally produced by all kingdoms of life, play a crucial role in the innate immune system of multicellular organisms and in bacterial interspecies competition by exhibiting broad-spectrum activity against bacteria, fungi, viruses, and parasites. AMPs target bacterial pathogens through multiple mechanisms, most importantly by disrupting their membranes, leading to cell lysis. However, bacterial resistance to host AMPs has emerged due to a slow co-evolutionary process between microorganisms and their hosts. Alarmingly, the development of resistance to last-resort AMPs in the treatment of MDR infections, such as colistin, is attributed to the misuse of this peptide and the high rate of horizontal genetic transfer of the corresponding resistance genes. AMP-resistant bacteria employ diverse mechanisms, including but not limited to proteolytic degradation, extracellular trapping and inactivation, active efflux, as well as complex modifications in bacterial cell wall and membrane structures. This review comprehensively examines all constitutive and inducible molecular resistance mechanisms to AMPs supported by experimental evidence described to date in bacterial pathogens. We also explore the specificity of these mechanisms toward structurally diverse AMPs to broaden and enhance their potential in developing and applying them as therapeutics for MDR bacteria. Additionally, we provide insights into the significance of AMP resistance within the context of host-pathogen interactions.

Structure of the Bacterial Cell Envelope and Interactions with Antimicrobials: Insights from Molecular Dynamics Simulations

Bacteria have evolved over 3 billion years, shaping our intrinsic and symbiotic coexistence with these single-celled organisms. With rising populations of drug-resistant strains, the search for novel antimicrobials is an ongoing area of research. Advances in high-performance computing platforms have led to a variety of molecular dynamics simulation strategies to study the interactions of antimicrobial molecules with different compartments of the bacterial cell envelope of both Gram-positive and Gram-negative species. In this review, we begin with a detailed description of the structural aspects of the bacterial cell envelope. Simulations concerned with the transport and associated free energy of small molecules and ions through the outer membrane, peptidoglycan, inner membrane and outer membrane porins are discussed. Since surfactants are widely used as antimicrobials, a section is devoted to the interactions of surfactants with the cell wall and inner membranes. The review ends with a discussion on antimicrobial peptides and the insights gained from the molecular simulations on the free energy of translocation. Challenges involved in developing accurate molecular models and coarse-grained strategies that provide a trade-off between atomic details with a gain in sampling time are highlighted. The need for efficient sampling strategies to obtain accurate free energies of translocation is also discussed. Molecular dynamics simulations have evolved as a powerful tool that can potentially be used to design and develop novel antimicrobials and strategies to effectively treat bacterial infections.

Lipidomics Characterization of the Microbiome in People with Diabetic Foot Infection Using MALDI-TOF MS

Lipidomic profiling has emerged as a powerful tool for the comprehensive characterization of bacterial species, particularly in the context of clinical diagnostics. Utilizing matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), this study aims to elucidate the lipidomic landscapes of bacterial strains isolated from diabetic foot infections (DFI). Our analysis successfully identified a diverse array of lipids in the cellular membranes of both Gram-positive and Gram-negative bacteria, revealing a total of 108 unique fatty acid combinations. Specifically, we identified 26 LPG, 33 LPE, 43 PE, 114 PG, 89 TAG, and 120 CLP in Gram-positive bacteria and 10 LPG, 14 LPE, 124 PE, 37 PG, 13 TAG, and 22 CLP in Gram-negative strains. Key fatty acids, such as palmitic acid, palmitoleic acid, stearic acid, and oleic acid, were prominently featured. Univariate analysis further highlighted distinct lipidomic signatures among the bacterial strains, revealing elevated levels of phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) in Gram-negative bacteria associated with DFI. In contrast, Gram-positive strains demonstrated increased or uniquely fluctuating levels of triglyceride (TAG) and cardiolipin (CLP). These findings not only underscore the utility of MALDI-TOF MS in bacterial lipidomics but also provide valuable insights into the lipidomic adaptations of bacteria in diabetic foot infections, thereby laying the groundwork for future studies aimed at constructing microbial lipid libraries for enhanced bacterial identification.

Bacterial susceptibility and resistance to modelin-5

Modelin-5 (M5-NH2) killed Pseudomonas aeruginosa with a minimum lethal concentration (MLC) of 5.86 μM and strongly bound its cytoplasmic membrane (CM) with a Kd of 23.5 μM. The peptide adopted high levels of amphiphilic α-helical structure (75.0%) and penetrated the CM hydrophobic core (8.0 mN m-1). This insertion destabilised CM structure via increased lipid packing and decreased fluidity (ΔGmix < 0), which promoted high levels of lysis (84.1%) and P. aeruginosa cell death. M5-NH2 showed a very strong affinity (Kd = 3.5 μM) and very high levels of amphiphilic α-helical structure with cardiolipin membranes (96.0%,) which primarily drove the peptide's membranolytic action against P. aeruginosa. In contrast, M5-NH2 killed Staphylococcus aureus with an MLC of 147.6 μM and weakly bound its CM with a Kd of 117.6 μM, The peptide adopted low levels of amphiphilic α-helical structure (35.0%) and only penetrated the upper regions of the CM (3.3 mN m-1). This insertion stabilised CM structure via decreased lipid packing and increased fluidity (ΔGmix > 0) and promoted only low levels of lysis (24.3%). The insertion and lysis of the S. aureus CM by M5-NH2 showed a strong negative correlation with its lysyl phosphatidylglycerol (Lys-PG) content (R2 > 0.98). In combination, these data suggested that Lys-PG mediated mechanisms inhibited the membranolytic action of M5-NH2 against S. aureus, thereby rendering the organism resistant to the peptide. These results are discussed in relation to structure/function relationships of M5-NH2 and CM lipids that underpin bacterial susceptibility and resistance to the peptide.

Dendritic systems for bacterial outer membrane disruption as a method of overcoming bacterial multidrug resistance

The alarming rise of multi-drug resistant microorganisms has increased the need for new approaches through the development of innovative agents that are capable of attaching to the outer layers of bacteria and causing permanent damage by penetrating the bacterial outer membrane. The permeability (disruption) of the outer membrane of Gram-negative bacteria is now considered to be one of the main ways to overcome multidrug resistance in bacteria. Natural and synthetic permeabilizers such as AMPs and dendritic systems seem promising. However, due to their advantages in terms of biocompatibility, antimicrobial capacity, and wide possibilities for modification and synthesis, highly branched polymers and dendritic systems have gained much more interest in recent years. Various forms of arrangement, and structure of the skeleton, give dendritic systems versatile applications, especially the possibility of attaching other ligands to their surface. This review will focus on the mechanisms used by different types of dendritic polymers, and their complexes with macromolecules to enhance their antimicrobial effect, and to permeabilize the bacterial outer membrane. In addition, future challenges and potential prospects are illustrated in the hope of accelerating the advancement of nanomedicine in the fight against resistant pathogens.

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.

Cardiolipin Strongly Inhibits the Leakage Activity of the Short Antimicrobial Peptide ATRA-1 in Comparison to LL-37, in Model Membranes Mimicking the Lipid Composition of Staphylococcus aureus

Cardiolipin is one of the main phospholipid components of Staphylococcus aureus membranes. This lipid is found at varying concentrations in the bilayer, depending on the growth stage of the bacteria, and as a response to environmental stress. Cardiolipin is an anionic phospholipid with four acyl chains, which modulates the bending properties of the membrane due to its inverted conical shape. It has been shown to inhibit the pore forming activity of several antimicrobial peptides, in general doubling the peptide concentration needed to induce leakage. Here we find that the short snake-derived antimicrobial peptide ATRA-1 is inhibited by several orders of magnitude in the presence of cardiolipin in saturated membranes (DMPG) compared to the human cathelicidin LL-37, which is only inhibited two-fold in its leakage-inducing concentration. The ATRA-1 is too short to span the membrane and its leakage activity is likely related to detergent-like alterations of bilayer structure. Fluorescence spectroscopy shows only a minor effect on ATRA-1 binding to DMPG membranes due to the presence of cardiolipin. However, FTIR spectroscopy shows that the acyl chain structure of DMPG membranes, containing cardiolipin, become more organized in the presence of ATRA-1, as reflected by an increase in the gel to liquid-crystalline phase transition temperature. Instead, a depression in the melting temperature is induced by ATRA-1 in DMPG in the absence of cardiolipin. In comparison, LL-37 induces a depression of the main phase transition of DMPG even in the presence of cardiolipin. These data suggest that cardiolipin inhibits the penetration of ATRA-1 into the membrane core, impeding its capacity to disrupt lipid packing.

Lipid-A-dependent and cholesterol-dependent dynamics properties of liposomes from gram-negative bacteria in ESKAPE

AntiMicrobial Resistance (AMR) is a worldwide health emergency. ESKAPE pathogens include the most relevant AMR bacterial families. In particular, Gram-negative bacteria stand out due to their cell envelope complexity which exhibits strong resistance to antimicrobials. A key element for AMR is the chemical structure of lipid A, modulating the physico-chemical properties of the membrane and permeability to antibiotics. Liposomes are used as models of bacterial membrane infective vesicles. In this work, coarse-grained molecular dynamics simulations were used to model liposomes from ESKAPE Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa). We captured the role of lipid A, cardiolipin and cholesterol on liposome morphology and physico-chemical properties. Additionally, the reported antimicrobial peptides Cecropin B1, JB95, and PTCDA1-kf, were used to unveil their implications on membrane disruption. This study opens a promising starting point to understand molecular keys of bacterial membranes and to promote the discovery of new antimicrobials to overcome AMR.

Harnessing the Role of Bacterial Plasma Membrane Modifications for the Development of Sustainable Membranotropic Phytotherapeutics

Membrane-targeted molecules such as cationic antimicrobial peptides (CAMPs) are amongst the most advanced group of antibiotics used against drug-resistant bacteria due to their conserved and accessible targets. However, multi-drug-resistant bacteria alter their plasma membrane (PM) lipids, such as lipopolysaccharides (LPS) and phospholipids (PLs), to evade membrane-targeted antibiotics. Investigations reveal that in addition to LPS, the varying composition and spatiotemporal organization of PLs in the bacterial PM are currently being explored as novel drug targets. Additionally, PM proteins such as Mla complex, MPRF, Lpts, lipid II flippase, PL synthases, and PL flippases that maintain PM integrity are the most sought-after targets for development of new-generation drugs. However, most of their structural details and mechanism of action remains elusive. Exploration of the role of bacterial membrane lipidome and proteome in addition to their organization is the key to developing novel membrane-targeted antibiotics. In addition, membranotropic phytochemicals and their synthetic derivatives have gained attractiveness as popular herbal alternatives against bacterial multi-drug resistance. This review provides the current understanding on the role of bacterial PM components on multidrug resistance and their targeting with membranotropic phytochemicals.

Generation and Computational Characterization of a Complex Staphylococcus aureus Lipid Bilayer

Studies indicate a crucial cell membrane role in the antibiotic resistance of Staphylococcus aureus. To simulate its membrane structure and dynamics, a complex molecular-scale computational representation of the S. aureus lipid bilayer was developed. Phospholipid types and their amounts were optimized by reverse Monte Carlo to represent characterization data from the literature, leading to 19 different phospholipid types that combine three headgroups [phosphatidylglycerol, lysyl-phosphatidylglycerol (LPG), and cardiolipin] and 10 tails, including iso- and anteiso-branched saturated chains. The averaged lipid bilayer thickness was 36.7 Å, and area per headgroup was 67.8 Ų. Phosphorus and nitrogen density profiles showed that LPG headgroups tended to be bent and oriented more parallel to the bilayer plane. The water density profile showed that small amounts reached the membrane center. Carbon density profiles indicated hydrophobic interactions for all lipids in the middle of the bilayer. Bond vector order parameters along each tail demonstrated different C-H ordering even within distinct lipids of the same type; however, all tails followed similar trends in average order parameter. These complex simulations further revealed bilayer insights beyond those attainable with monodisperse, unbranched lipids. Longer tails often extended into the opposite leaflet. Carbon at and beyond a branch showed significantly decreased ordering compared to carbon in unbranched tails; this feature arose in every branched lipid. Diverse tail lengths distributed these disordered methyl groups throughout the middle third of the bilayer. Distributions in mobility and ordering reveal diverse properties that cannot be obtained with monodisperse lipids.

Infrared Spectroscopic Study of Multi-Component Lipid Systems: A Closer Approximation to Biological Membrane Fluidity

Membranes are essential to cellular organisms, and play several roles in cellular protection as well as in the control and transport of nutrients. One of the most critical membrane properties is fluidity, which has been extensively studied, using mainly single component systems. In this study, we used Fourier transform infrared spectroscopy to evaluate the thermal behavior of multi-component supported lipid bilayers that mimic the membrane composition of tumoral and non-tumoral cell membranes, as well as microorganisms such as Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus. The results showed that, for tumoral and non-tumoral membrane models, the presence of cholesterol induced a loss of cooperativity of the transition. However, in the absence of cholesterol, the transitions of the multi-component lipid systems had sigmoidal curves where the gel and fluid phases are evident and where main transition temperatures were possible to determine. Additionally, the possibility of designing multi-component lipid systems showed the potential to obtain several microorganism models, including changes in the cardiolipin content associated with the resistance mechanism in Staphylococcus aureus. Finally, the potential use of multi-component lipid systems in the determination of the conformational change of the antimicrobial peptide LL-37 was studied. The results showed that LL-37 underwent a conformational change when interacting with Staphylococcus aureus models, instead of with the erythrocyte membrane model. The results showed the versatile applications of multi-component lipid systems studied by Fourier transform infrared spectroscopy.

Multi-Omic Analysis to Characterize Metabolic Adaptation of the E. coli Lipidome in Response to Environmental Stress

As an adaptive survival response to exogenous stress, bacteria undergo dynamic remodelling of their lipid metabolism pathways to alter the composition of their cellular membranes. Here, using Escherichia coli as a well characterised model system, we report the development and application of a ‘multi-omics’ strategy for comprehensive quantitative analysis of the temporal changes in the lipidome and proteome profiles that occur under exponential growth phase versus stationary growth phase conditions i.e., nutrient depletion stress. Lipidome analysis performed using ‘shotgun’ direct infusion-based ultra-high resolution accurate mass spectrometry revealed a quantitative decrease in total lipid content under stationary growth phase conditions, along with a significant increase in the mol% composition of total cardiolipin, and an increase in ‘odd-numbered’ acyl-chain length containing glycerophospholipids. The inclusion of field asymmetry ion mobility spectrometry was shown to enable the enrichment and improved depth of coverage of low-abundance cardiolipins, while ultraviolet photodissociation-tandem mass spectrometry facilitated more complete lipid structural characterisation compared with conventional collision-induced dissociation, including unambiguous assignment of the odd-numbered acyl-chains as containing cyclopropyl modifications. Proteome analysis using data-dependent acquisition nano-liquid chromatography mass spectrometry and tandem mass spectrometry analysis identified 83% of the predicted E. coli lipid metabolism enzymes, which enabled the temporal dependence associated with the expression of key enzymes responsible for the observed adaptive lipid metabolism to be determined, including those involved in phospholipid metabolism (e.g., ClsB and Cfa), fatty acid synthesis (e.g., FabH) and degradation (e.g., FadA/B,D,E,I,J and M), and proteins involved in the oxidative stress response resulting from the generation of reactive oxygen species during β-oxidation or lipid degradation.

The effects of magainin 2-derived and rationally designed antimicrobial peptides on Mycoplasma pneumoniae

Combating the spread of antimicrobial resistance (AMR) among bacteria requires a new class of antimicrobials, which desirably have a narrow spectrum because of their low propensity for the spread of AMR. Antimicrobial peptides (AMPs), which target the bacterial cell membrane, are promising seeds for novel antimicrobials because the cell membrane is essential for all cells. Previously, we reported the antimicrobial and haemolytic effects of a natural AMP, magainin 2 (Mag2), isolated from the skin of Xenopus laevis (the African clawed frog), four types of synthesised Mag2 derivatives, and three types of rationally designed AMPs on gram-positive and gram-negative bacteria. To identify novel antimicrobial seeds, we evaluated the effect of AMPs on Mycoplasma pneumoniae, which also exhibits AMR. We also evaluated the antimicrobial effects of an AMP, NK2A, which has been reported to have antimicrobial effects on Mycoplasma bovis, in addition to Mag2 and previously synthesised seven AMPs, on four strains of M. pneumoniae using colorimetric, biofilm, and killing assays. We found that three synthesised AMPs, namely 17base-Ac6c, 17base-Hybrid, and Block, had anti-M. pneumoniae (anti-Mp) effect at 8–30 μM, whereas others, including NK2A, did not have any such effect. For the further analysis, the membrane disruption activities of AMPs were measured by propidium iodide (PI) uptake assays, which suggested the direct interaction of AMPs to the cell membrane basically following the colorimetric, biofilm, and killing assay results. PI uptake assay, however, also showed the NK2A strong interaction to cell membrane, indicating unknown anti-Mp determinant factors related to the peptide sequences. Finally, we conclude that anti-Mp effect was not simply determined by the membrane disruption activities of AMPs, but also that the sequence of AMPs were important for killing of M. pneumoniae. These findings would be helpful for the development of AMPs for M. pneumoniae.

Cardiolipin prevents pore formation in phosphatidylglycerol bacterial membrane models

Several antimicrobial peptides, including magainin and the human cathelicidin LL-37, act by forming pores in bacterial membranes. Bacteria such as Staphylococcus aureus modify their membrane's cardiolipin composition to resist such types of perturbations that compromise their membrane stability. Here, we used molecular dynamic simulations to quantify the role of cardiolipin on the formation of pores in simple bacterial-like membrane models composed of phosphatidylglycerol and cardiolipin mixtures. Cardiolipin modified the structure and ordering of the lipid bilayer, making it less susceptible to mechanical changes. Accordingly, the free-energy barrier for the formation of a transmembrane pore and its kinetic instability augmented by increasing the cardiolipin concentration. This is attributed to the unfavorable positioning of cardiolipin near the formed pore, due to its small polar head and bulky hydrophobic body. Overall, our study demonstrates how cardiolipin prevents membrane-pore formation and this constitutes a plausible mechanism used by bacteria to act against stress perturbations and, thereby, gain resistance to antimicrobial agents.

Roles of histidine charge and cardiolipin in membrane disruption by antimicrobial peptides Gaduscidin-1 and Gaduscidin-2

Gad-1 and Gad-2 are helical, histidine-rich antimicrobial peptides (AMPs) from paralogous genes in cod. ¹⁵N and ²H solid state nuclear magnetic resonance (NMR) were used to characterize their lipid-bound structures and lipid interactions. Gad-1 was found to position in-plane in POPC: POPG bilayers. Gad-1 displayed greater effects than Gad-2 on lipid acyl chain order of POPE: POPG and POPE: POPG: CL bilayers, in keeping with its greater activity against E. coli. The effect of Gad-1 and Gad-2 on lipid bilayer order was only weakly affected by changes in pH, and hence changes in histidine charge. This was somewhat surprising for Gad-2 as this peptide's biological activity has been shown to be greater at low pH and thus the finding may point to the existence of functional interactions with non-lipid components of bacteria. The incorporation of cardiolipin into POPE: POPG bilayers in such a way as to preserve the overall charge of the bilayers did not alter Gad-1's effects on lipid acyl chain order parameters, which report on motions on the 10^-5 s timescale. When cardiolipin and Gad-1 were both present, there were subtle changes on membrane dynamics at other timescales.

Differential Stability of Aurein 1.2 Pores in Model Membranes of Two Probiotic Strains

Aurein 1.2 is an antimicrobial peptide from the skin secretion of an Australian frog. In the previous experimental work, we reported a differential action of aurein 1.2 on two probiotic strains Lactobacillus delbrueckii subsp. bulgaricus (CIDCA 331) and Lactobacillus delbrueckii subsp. lactis (CIDCA 133). The differences found were attributed to the bilayer compositions. Cell cultures and CIDCA 331-derived liposomes showed higher susceptibility than the ones derived from the CIDCA 133 strain, leading to content leakage and structural disruption. Here, we used molecular dynamics simulations to explore these systems at the atomistic level. We hypothesize that if the antimicrobial peptides organized themselves to form a pore, it will be more stable in membranes that emulate the CIDCA 331 strain than in those of the CIDCA 133 strain. To test this hypothesis, we simulated preassembled aurein 1.2 pores embedded into bilayer models that emulate the two probiotic strains. It was found that the general behavior of the systems depends on the composition of the membrane rather than the preassemble system characteristics. Overall, it was observed that aurein 1.2 pores are more stable in the CIDCA 331 model membranes. This fact coincides with the high susceptibility of this strain against antimicrobial peptide. In contrast, in the case of the CIDCA 133 model membranes, peptides migrate to the water-lipid interphase, the pore shrinks, and the transport of water through the pore is reduced. The tendency of glycolipids to make hydrogen bonds with peptides destabilizes the pore structures. This feature is observed to a lesser extent in CIDCA 331 due to the presence of anionic lipids. Glycolipid transverse diffusion (flip-flop) between monolayers occurs in the pore surface region in all the cases considered. These findings expand our understanding of the antimicrobial peptide resistance properties of probiotic strains.

Understanding and modelling the interactions of peptides with membranes: from partitioning to self-assembly

Atomic detail simulations are starting to reveal how flexible polypeptides interact with fluid lipid bilayers. These insights are transforming our understanding of one of the fundamental processes in biology: membrane protein folding and assembly. Advanced molecular dynamics (MD) simulation techniques enable accurate prediction of protein structure, folding pathways and assembly in microsecond-timescales. Such simulations show how membrane-active peptides self-assemble in cell membranes, revealing their binding, folding, insertion, and aggregation, while at the same time providing atomic resolution details of peptide-lipid interactions. Essential to the impact of simulations are experimental approaches that enable calibration and validation of the computational models and techniques. In this review, we summarize the current development of applying unbiased atomic detail MD simulations and the relation to experimental techniques, to study peptide folding and provide our perspective of the field.

Structural Plasticity of EAK-16 Peptide Inducing Vesicle Membrane Leakage

BACKGROUND

Ionic complementary peptide EAK-16 has been studies for anticancer drug delivery application. This is a 16 residues, short sequence peptide has ability to trosnform into micro/nanoparticle via self-assembly. However, it is still not clear that how this can bind with cell membrane to induce membrane leakage or delivering their cargo inside cell membrane.

OBJECTIVE

The main objective of this work was to understand behaviour of secondary structure conformation of peptide in solution and at lipid membrane interfaces and membrane permeability of synthetic ionic complementary peptide EAK-16. The corresponding secondary structure conformation was evaluated.

METHODS

We performed biophysical investigation to probe the interaction of synthesised ionic complementary peptide (EAK-16) with dimyristoylphospholcholine (DMPC) and dimyristoylphosphoserine (DMPS) membrane interfaces. The folding behaviours of EAK-16 were studied with Circular Dichroism (CD) spectroscopy. Membrane leakage with peptide was confirmed with calcein leakage assay.

RESULTS

Our finding of this study showed that in aqueous phase EAK-16 was predominantly folded into β-sheets. The temperature could alter the β-sheets. However, in DMPC and DMPS membrane interfaces, EAK-16 adopted helical conformation. EAK-16 has preference in perturbing anionic compared Zwitterionic lipid vesicles. This study proposed that hydrophobic grooves of EAK-16 might be a key in the association with lipid bilayers. Secondly, a charge distribution of ionic residues would also support the orientation at lipid bilayers. This peptide membrane association would facilitate the membrane destabilisation.

CONCLUSION

This study demonstrated the supporting evidence that EAK-16 could interact with lipid membranes and conforming to helical structure, while the helical conformation induced the lipid membrane leakage. Overall, this study provides a physical rationale that ionic complementary peptide can be a useful tool for designing and development of novel antibiotics and anticancer agents along its previous drug delivery applications.

Biophysical studies on the antimicrobial activity of linearized esculentin 2EM

Linearized esculentin 2 EM (E2EM-lin) from the frog, Glandirana emeljanovi was highly active against Gram-positive bacteria (minimum lethal concentration ≤ 5.0 μM) and strongly α-helical in the presence of lipid mimics of their membranes (>55.0%). The N-terminal α-helical structure adopted by E2EM-lin showed the potential to form a membrane interactive, tilted peptide with an hydrophobicity gradient over residues 9 to 23. E2EM-lin inserted strongly into lipid mimics of membranes from Gram-positive bacteria (maximal surface pressure changes ≥5.5 mN m^-1), inducing increased rigidity (C_s^-1 ↑), thermodynamic instability (ΔG_mix < 0 → ΔG_mix > 0) and high levels of lysis (>50.0%). These effects appeared to be driven by the high anionic lipid content of membranes from Gram-positive bacteria; namely phosphatidylglycerol (PG) and cardiolipin (CL) species. The high levels of α-helicity (60.0%), interaction (maximal surface pressure change = 6.7 mN m^-1) and lysis (66.0%) shown by E2EM-lin with PG species was a major driver in the ability of the peptide to lyse and kill Gram-positive bacteria. E2EM-lin also showed high levels of α-helicity (62.0%) with CL species but only low levels of interaction (maximal surface pressure change = 2.9 mN m^-1) and lysis (21.0%) with the lipid. These combined data suggest that E2EM-lin has a specificity for killing Gram-positive bacteria that involves the formation of tilted structure and appears to be primarily driven by PG-mediated membranolysis. These structure/function relationships are used to help explain the pore forming process proposed to describe the membranolytic, antibacterial action of E2EM-lin.