Cationic Antiseptics Facilitate Pore Formation in Model Bacterial Membranes

The influence of cationic antiseptics on the processes of light energy conversion in various photosynthetic pigment-protein complexes

The widespread use of disinfectants and antiseptics, and consequently their release into the environment, determines the relevance of studying their potential impact on the main producers of organic matter on the planet-photosynthetic organisms. The review examines the effects of some biguanides and quaternary ammonium compounds, octenidine, miramistin, chlorhexidine, and picloxidine, on the functioning of the photosynthetic apparatus of various organisms (Strakhovskaya et al. in Photosynth Res 147:197-209, 2021; Knox et al. in Photosynth Res 153:103, 2022; Paschenko et al. in Photosynth Res 155:93-105, 2023a, Photosynth Res 2023b). A common feature of these antiseptics is the combination of hydrophobic and hydrophilic regions in the molecules, the latter carrying a positive charge(s). The comparison of the results obtained with intact bacterial membrane vesicles (chromatophores) and purified pigment-protein complexes (photosystem II and I) of oxygenic organisms allows us to draw conclusions about the mechanisms of the cationic antiseptic action on the functional properties of the components of the photosynthetic apparatus.

Effect of cationic antiseptics on fluorescent characteristics and electron transfer in cyanobacterial photosystem I complexes

In this study, the effects of cationic antiseptics such as chlorhexidine, picloxidine, miramistin, and octenidine at concentrations up to 150 µM on fluorescence spectra and its lifetimes, as well as on light-induced electron transfer in protein-pigment complexes of photosystem I (PSI) isolated from cyanobacterium Synechocystis sp. PCC 6803 have been studied. In doing so, octenidine turned out to be the most "effective" in terms of its influence on the spectral and functional characteristics of PSI complexes. It has been shown that the rate of energy migration from short-wavelength forms of light-harvesting chlorophyll to long-wavelength ones slows down upon addition of octenidine to the PSI suspension. After photo-separation of charges between the primary electron donor P700 and the terminal iron-sulfur center(s) FA/FB, the rate of forward electron transfer from (FA/FB)- to the external medium slows down while the rate of charge recombination between reduced FA/FB- and photooxidized P700+ increases. The paper considers the possible causes of the observed action of the antiseptic.

The Mechanism of Antimicrobial Small-Cationic Peptides from Coarse-Grained Simulations

Antimicrobial cationic peptides (AMPs) are excellent candidates for use as therapeutic antimicrobial agents. Among them, short peptides possessing sequences of 9-11 amino acids have some advantages over long-sequence peptides. However, one of the main limitations of short peptides is that their mechanism of action at the molecular level is not well-known. In this article, we report a model based on multiscale molecular dynamics simulations of short peptides interacting with vesicles containing palmitoyl-oleoyl-phosphatidylglycerol (POPG)/palmitoyl-oleoyl-phosphatidylethanolamine (POPE). Simulations using this approach have allowed us to understand the different behaviors of peptides with antimicrobial activity with respect to those that do not produce this effect. We found remarkable agreement with a series of experimental results directly supporting our model. Moreover, these results allow us to understand the mechanism of action at the molecular level of these short peptides. Our simulations suggest that mechanical inhomogeneities appear in the membrane, promoting membrane rupture when a threshold concentration of peptides adsorbed on the membrane is achieved. These results explain the high structural demand for these peptides to maintain a delicate balance between the affinity for the bilayer surface, a low peptide-peptide repulsion (in order to reach the threshold concentration), and an acceptable tendency to penetrate into the bilayer. This mechanism is different from those proposed for peptides with long amino acid sequences. Such information is very useful from the medicinal chemistry point of view for the design of new small antimicrobial peptides.

Insights into the Formation of Intermolecular Complexes of Fluorescent Probe 10-N-Nonyl Acridine Orange with Cardiolipin and Phosphatidylglycerol in Bacterial Plasma Membrane by Molecular Modeling

In this article, we used molecular dynamics (MD), one of the most common methods for simulations of membranes, to study the interaction of fluorescent membranotropic biological probe 10-N-nonyl acridine orange (NAO) with the bilayer, mimicking a plasma membrane of Gram-negative bacteria. Fluorescent probes serve as an effective tool to study the localization of different components in biological membranes. Revealing the molecular details of their interaction with membrane phospholipids is important both for the interpretation of experimental results and future design of lipid-specific stains. By means of coarse-grained (CG) MD, we studied the interactions of NAO with a model membrane, imitating the plasma membrane of Gram-negative bacteria. In our simulations, we detected different NAO forms: monomers, dimers, and stacks. NAO dimers had the central cardiolipin (CL) molecule in a sandwich-like structure. The stacks were formed by NAO molecules interlayered with anionic lipids, predominantly CL. Use of the CG approach allowed to confirm the ability of NAO to bind to both major negatively charged phospholipids, phosphatidylglycerol (PG) and CL, and to shed light on the exact structure of previously proposed NAO-lipid complexes. Thus, CG modeling can be useful for the development of new effective and highly specific molecular probes.

Influence of the antiseptic octenidine on spectral characteristics and energy migration processes in photosystem II core complexes

Herein, the effect of cationic antiseptics (chlorhexidine, picloxidine, miramistin, octenidine) on the initial processes of the delivery of light energy and its efficient use by the reaction centers in intact spinach photosystem II core complexes has been investigated. The characteristic effects-an increase in the fluorescence yield of light-harvesting pigments and a slowdown in the rate of energy migration in bacterial photosynthetic chromatophores has been recently demonstrated mainly in the presence of octenidine (Strakhovskaya et al., in Photosynth Res 147:197-209, 2021; Knox et al., in Photosynth Res, https://doi.org/10.1007/s11120-022-00909-8 , 2022). In this study, we also observed that in the presence of octenidine, the fluorescence intensity of photosystem II core complexes increases by 5-10 times, and the rate of energy migration from antennae to the reaction centers decreases by 3 times. In addition, with an increase in the concentration of this antiseptic, a new effect related to a shift of the spectrum, absorption and fluorescence to the short-wavelength region has been found. Similar effects were observed when detergent Triton X-100 was added to photosystem II samples. We concluded that the antiseptic primarily affects the structure of the internal light-harvesting antenna (CP43 and CP47), through which the excitation energy is delivered to the reaction center. As a result of such an impact, the chlorophyll molecules in this structure are destabilized and their optical and functional characteristics change.

Efflux-mediated tolerance to cationic biocides, a cause for concern?

AbstractWith an increase in the number of isolates resistant to multiple antibiotics, infection control has become increasingly important to help combat the spread of multi-drug-resistant pathogens. An important component of this is through the use of disinfectants and antiseptics (biocides). Antibiotic resistance has been well studied in bacteria, but little is known about potential biocide resistance genes and there have been few reported outbreaks in hospitals resulting from a breakdown in biocide effectiveness. Development of increased tolerance to biocides has been thought to be more difficult due to the mode of action of biocides which affect multiple cellular targets compared with antibiotics. Very few genes which contribute towards increased biocide tolerance have been identified. However, the majority of those that have are components or regulators of different efflux pumps or genes which modulate membrane function/modification. This review will examine the role of efflux in increased tolerance towards biocides, focusing on cationic biocides and heavy metals against Gram-negative bacteria. As many efflux pumps which are upregulated by biocide presence also contribute towards an antimicrobial resistance phenotype, the role of these efflux pumps in cross-resistance to both other biocides and antibiotics will be explored.

Disproportionate effect of cationic antiseptics on the quantum yield and fluorescence lifetime of bacteriochlorophyll molecules in the LH1-RC complex of R. rubrum chromatophores

Photosynthetic membrane complexes of purple bacteria are convenient and informative macromolecular systems for studying the mechanisms of action of various physicochemical factors on the functioning of catalytic proteins both in an isolated state and as part of functional membranes. In this work, we studied the effect of cationic antiseptics (chlorhexidine, picloxydine, miramistin, and octenidine) on the fluorescence intensity and the efficiency of energy transfer from the light-harvesting LH1 complex to the reaction center (RC) of Rhodospirillum rubrum chromatophores. The effect of antiseptics on the fluorescence intensity and the energy transfer increased in the following order: chlorhexidine, picloxydine, miramistin, octenidine. The most pronounced changes in the intensity and lifetime of fluorescence were observed with the addition of miramistin and octenidine. At the same concentration of antiseptics, the increase in fluorescence intensity was 2-3 times higher than the increase in its lifetime. It is concluded that the addition of antiseptics decreases the efficiency of the energy migration LH1 → RC and increases the fluorescence rate constant k_fl. We associate the latter with a change in the polarization of the microenvironment of bacteriochlorophyll molecules upon the addition of charged antiseptic molecules. A possible mechanism of antiseptic action on R. rubrum chromatophores is considered. This work is a continuation of the study of the effect of antiseptics on the energy transfer and fluorescence intensity in chromatophores of purple bacteria published earlier in Photosynthesis Research (Strakhovskaya et al. in Photosyn Res 147:197-209, 2021).

Comparable Studies on Nanoscale Antibacterial Polymer Coatings Based on Different Coating Procedures

The antibacterial activity of different antibiotic and metal-free thin polymer coatings was investigated. The films comprised quaternary ammonium compounds (QAC) based on a vinyl benzyl chloride (VBC) building block. Two monomeric QAC of different alkyl chain lengths were prepared, and then polymerized by two different polymerization processes to apply them onto Ti surfaces. At first, the polymeric layer was generated directly on the surface by atom transfer radical polymerization (ATRP). For comparison purposes, in a classical route a copolymerization of the QAC-containing monomers with a metal adhesion mediating phosphonate (VBPOH) monomers was carried out and the Ti surfaces were coated via drop coating. The different coatings were characterized by X-ray photoelectron spectroscopy (XPS) illustrating a thickness in the nanomolecular range. The cytocompatibility in vitro was confirmed by both live/dead and WST-1 assay. The antimicrobial activity was evaluated by two different assays (CFU and BTG, resp.,), showing for both coating processes similar results to kill bacteria on contact. These antibacterial coatings present a simple method to protect metallic devices against microbial contamination.

Model architectures for bacterial membranes

The complex composition of bacterial membranes has a significant impact on the understanding of pathogen function and their development towards antibiotic resistance. In addition to the inherent complexity and biosafety risks of studying biological pathogen membranes, the continual rise of antibiotic resistance and its significant economical and clinical consequences has motivated the development of numerous in vitro model membrane systems with tuneable compositions, geometries, and sizes. Approaches discussed in this review include liposomes, solid-supported bilayers, and computational simulations which have been used to explore various processes including drug-membrane interactions, lipid-protein interactions, host-pathogen interactions, and structure-induced bacterial pathogenesis. The advantages, limitations, and applicable analytical tools of all architectures are summarised with a perspective for future research efforts in architectural improvement and elucidation of resistance development strategies and membrane-targeting antibiotic mechanisms.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-021-00913-7.

An evidence of pores in phospholipid membrane induced by an antimicrobial peptide NK-2

NK-2, a peptide derived from a cationic core region of NK-lysin, has emerged as a promising candidate for new antibiotics. In contrast to classical antibiotics, antimicrobial peptides target bacterial membranes and disintegrate the membrane by forming the transmembrane pores. However, complete understanding of the precise mechanisms of cellular apoptosis and molecular basis of membrane selectivity is still in dispute. In the present study, we have shown that NK-2 forms trans-membrane pores on negatively charged phospholipid membranes using phase contrast microscopy. As bacteria mimicking membranes, we have chosen large unilamellar vesicles (LUV) and giant unilamellar vesicles (GUV) composed of negatively charged phospholipid, dioleoyl phosphatidyl glycerol (DOPG) and neutral phospholipid, dioleoyl phophatidylcholine (DOPC). Leakage of internal fluid of giant unilamellar vesicles (GUV), leading to decrease in intensity in the halo region of phase contrast micrographs, suggests the formation of transmembrane pores. No such reduction of intensity in the halo region of DOPC was observed, indicating, neutral vesicles does not exhibit pores. Rate constant reckoned from the decaying intensity in the halo region was found to be 0.007 s^-1. Further, significant interaction of NK-2 with anionic membranes has been envisaged from zeta potential and dynamic light scattering. Binding free energy and other interaction parameters have been delineated using theoretical ansatz. A proliferation of average Size of anionic LUV on increasing NK-2 concentration indicates membrane-membrane interaction leading to peptide induced large aggregates of vesicles.

[Antibacterial effect of the antiseptic picloxydine dihydrochloride on conjunctival isolates of gram-negative bacteria]

The preoperative and postoperative use of antiseptics can be an alternative to antibiotics in repeated courses of anti-VEGF therapy for reducing the risk of developing antibiotic resistance in eye microflora. Among gram-negative bacteria, the most frequently isolated pathogen that causes eye infections is Pseudomonas aeruginosa, which is characterized by reduced sensitivity to antibiotics and disinfectants.

PURPOSE

To study the effect of the antiseptic picloxydine dihydrochloride on the gram-negative bacteria Escherichia coli, Pseudomonas luteola and P. aeruginosa isolated from the conjunctiva.

MATERIAL AND METHODS

The identification of bacterial isolates and study of their sensitivity to antibiotics were carried out using the automated bacteriological analyzer BD Phoenix 100. To determine the bactericidal concentration, the method of serial dilutions of the antiseptic in a liquid nutrient medium was used. The binding of cationic molecules of picloxydine dihydrochloride to bacterial cells was detected by neutralizing the bacterial surface with increasing amounts of antiseptic, and measuring the zeta potential on the Zetasizer Nano ZS analyzer. The ultrastructure of bacterial cells was studied using the two-beam scanning ion-electron microscope Quanta 200 3D.

RESULTS

The most resistant was P. aeruginosa. The interaction mechanism of picloxydine dihydrochloride with bacterial cells includes electrostatic binding of positively charged antiseptic molecules to negatively charged cell walls. Picloxydine dihydrochloride has a destructive effect on the bacterial cell wall and plasma membrane, which leads to cell lysis and release of intracellular components.

CONCLUSION

Picloxydine dihydrochloride exhibits bactericidal activity against gram-negative conjunctival isolates and is promising for preventive use during repeated courses of intravitreal injections.

Mechanistic Understanding from Molecular Dynamics in Pharmaceutical Research 2: Lipid Membrane in Drug Design

We review the use of molecular dynamics (MD) simulation as a drug design tool in the context of the role that the lipid membrane can play in drug action, i.e., the interaction between candidate drug molecules and lipid membranes. In the standard "lock and key" paradigm, only the interaction between the drug and a specific active site of a specific protein is considered; the environment in which the drug acts is, from a biophysical perspective, far more complex than this. The possible mechanisms though which a drug can be designed to tinker with physiological processes are significantly broader than merely fitting to a single active site of a single protein. In this paper, we focus on the role of the lipid membrane, arguably the most important element outside the proteins themselves, as a case study. We discuss work that has been carried out, using MD simulation, concerning the transfection of drugs through membranes that act as biological barriers in the path of the drugs, the behavior of drug molecules within membranes, how their collective behavior can affect the structure and properties of the membrane and, finally, the role lipid membranes, to which the vast majority of drug target proteins are associated, can play in mediating the interaction between drug and target protein. This review paper is the second in a two-part series covering MD simulation as a tool in pharmaceutical research; both are designed as pedagogical review papers aimed at both pharmaceutical scientists interested in exploring how the tool of MD simulation can be applied to their research and computational scientists interested in exploring the possibility of a pharmaceutical context for their research.

Unraveling the mechanism of octenidine and chlorhexidine on membranes: Does electrostatics matter?

The increasing problem of antibiotic resistance in bacteria requires the development of new antimicrobial candidates. There are several well-known substances with commercial use, but their molecular mode of action is not fully understood. In this work, we focus on two commonly used antimicrobial agents from the detergent family—octenidine dichloride (OCT) and chlorhexidine digluconate (CHX). Both of them are reported to be agents selectively attacking the cell membrane through interaction inducing membrane disruption by emulsification. They are believed to present electrostatic selectivity toward charged lipids. In this study, we tested this hypothesis and revised previously proposed molecular mechanisms of action. Employing a variety of techniques such as molecular dynamics, ζ potential with dynamic light scattering, vesicle fluctuation spectroscopy, carboxyfluorescein leakage measurement, and fluorescence trimethylammonium-diphenylhexatriene- and diphenylhexatriene-based studies for determination of OCT and CHX membrane location, we performed experimental studies using two model membrane systems—zwitterionic PC and negatively charged PG (18:1/18:1):PC (16:0/18:1) 3:7, respectively. These studies were extended by molecular dynamics simulations performed on a three-component bacterial membrane model system to further test interactions with another negatively charged lipid, cardiolipin. In summary, our study demonstrated that detergent selectivity is far more complicated than supposed simple electrostatic interactions. Although OCT does disrupt the membrane, our results suggest that its primary selectivity was more linked to mechanical properties of the membrane. On the other hand, CHX did not disrupt membranes as a primary activity, nor did it show any sign of electrostatic selectivity toward negatively charged membranes at any stage of interactions, which suggests membrane disruption by influencing more discrete membrane properties.

Synergistic Effect of Chemical Penetration Enhancers on Lidocaine Permeability Revealed by Coarse-Grained Molecular Dynamics Simulations

The search for new formulations for transdermal drug delivery (TDD) is an important field in medicine and cosmetology. Molecules with specific physicochemical properties which can increase the permeability of active ingredients across the stratum corneum (SC) are called chemical penetration enhancers (CPEs), and it was shown that some CPEs can act synergistically. In this study, we performed coarse-grained (CG) molecular dynamics (MD) simulations of the lidocaine delivery facilitated by two CPEs-linoleic acid (LA) and ethanol-through the SC model membrane containing cholesterol, N-Stearoylsphingosine (DCPE), and behenic acid. In our simulations, we probed the effects of individual CPEs as well as their combination on various properties of the SC membrane and the lidocaine penetration across it. We demonstrated that the addition of both CPEs decreases the membrane thickness and the order parameters of the DPCE hydrocarbon chains. Moreover, LA also enhances diffusion of the SC membrane components, especially cholesterol. The estimated potential of mean force (PMF) profiles for the lidocaine translocation across SC in the presence/absence of two individual CPEs and their combination demonstrated that while ethanol lowers the free energy barrier for lidocaine to enter SC, LA decreases the depth of the free energy minima for lidocaine inside SC. These two effects supposedly result in synergistic penetration enhancement of drugs. Altogether, the present simulations provide a detailed molecular picture of CPEs' action and their synergistic effect on the penetration of small molecular weight therapeutics that can be beneficial for the design of novel drug and cosmetics formulations.

Biguanides: Species with versatile therapeutic applications

Biguanides are compounds in which two guanidine moieties are fused to form a highly conjugated system. Biguanides are highly basic and hence they are available as salts mostly hydrochloride salts, these cationic species have been found to exhibit many therapeutic properties. This review covers the research and development carried out on biguanides and accounts the various therapeutic applications of drugs containing biguanide group-such as antimalarial, antidiabetic, antiviral, anticancer, antibacterial, antifungal, anti-tubercular, antifilarial, anti-HIV, as well as other biological activities. The aim of this review is to compile all the medicinal chemistry applications of this class of compounds so as to pave way for the accelerated efforts in finding the drug action mechanisms associated with this class of compounds. Importance has been given to the organic chemistry of these biguanide derivatives also.

Lipid Dynamics in Diisobutylene-Maleic Acid (DIBMA) Lipid Particles in Presence of Sensory Rhodopsin II

Amphiphilic diisobutylene/maleic acid (DIBMA) copolymers extract lipid-encased membrane proteins from lipid bilayers in a detergent-free manner, yielding nanosized, discoidal DIBMA lipid particles (DIBMALPs). Depending on the DIBMA/lipid ratio, the size of DIBMALPs can be broadly varied which makes them suitable for the incorporation of proteins of different sizes. Here, we examine the influence of the DIBMALP sizes and the presence of protein on the dynamics of encased lipids. As shown by a set of biophysical methods, the stability of DIBMALPs remains unaffected at different DIBMA/lipid ratios. Coarse-grained molecular dynamics simulations confirm the formation of viable DIBMALPs with an overall size of up to 35 nm. Electron paramagnetic resonance spectroscopy of nitroxides located at the 5th, 12th or 16th carbon atom positions in phosphatidylcholine-based spin labels reveals that the dynamics of enclosed lipids are not altered by the DIBMALP size. The presence of the membrane protein sensory rhodopsin II from Natronomonas pharaonis (NpSRII) results in a slight increase in the lipid dynamics compared to empty DIBMALPs. The light-induced photocycle shows full functionality of DIBMALPs-embedded NpSRII and a significant effect of the protein-to-lipid ratio during preparation on the NpSRII dynamics. This study indicates a possible expansion of the applicability of the DIBMALP technology on studies of membrane protein–protein interaction and oligomerization in a constraining environment.

The effect of some antiseptic drugs on the energy transfer in chromatophore photosynthetic membranes of purple non-sulfur bacteria Rhodobacter sphaeroides

Chromatophores of purple non-sulfur bacteria (PNSB) are invaginations of the cytoplasmic membrane that contain a relatively simple system of light-harvesting protein-pigment complexes, a photosynthetic reaction center (RC), a cytochrome complex, and ATP synthase, which transform light energy into the energy of synthesized ATP. The high content of negatively charged phosphatidylglycerol (PG) and cardiolipin (CL) in PNSB chromatophore membranes makes these structures potential targets that bind cationic antiseptics. We used the methods of stationary and kinetic fluorescence spectroscopy to study the effect of some cationic antiseptics (chlorhexidine, picloxydine, miramistin, and octenidine at concentrations up to 100 μM) on the spectral and kinetic characteristics of the components of the photosynthetic apparatus of Rhodobacter sphaeroides chromatophores. Here we present the experimental data on the reduced efficiency of light energy conversion in the chromatophore membranes isolated from the photosynthetic bacterium Rb. sphaeroides in the presence of cationic antiseptics. The addition of antiseptics did not affect the energy transfer between the light-harvesting LH1 complex and reaction center (RC). However, it significantly reduced the efficiency of the interaction between the LH2 and LH1 complexes. The effect was maximal with 100 μM octenidine. It has been proved that molecules of cationic antiseptics, which apparently bind to the heads of negatively charged cardiolipin molecules located in the rings of light-harvesting pigments on the cytoplasmic surface of the chromatophores, can disturb the optimal conditions for efficient energy migration in chromatophore membranes.