Mechanistic Evaluation of Antimicrobial Lipid Interactions with Tethered Lipid Bilayers by Electrochemical Impedance Spectroscopy

Unraveling the Biophysical Mechanisms of How Antiviral Detergents Disrupt Supported Lipid Membranes: Toward Replacing Triton X-100

Triton X-100 (TX-100) is a membrane-disrupting detergent that is widely used to inactivate membrane-enveloped viral pathogens, yet is being phased out due to environmental safety concerns. Intense efforts are underway to discover regulatory acceptable detergents to replace TX-100, but there is scarce mechanistic understanding about how these other detergents disrupt phospholipid membranes and hence which ones are suitable to replace TX-100 from a biophysical interaction perspective. Herein, using the quartz crystal microbalance-dissipation (QCM-D) and electrochemical impedance spectroscopy (EIS) techniques in combination with supported lipid membrane platforms, we characterized the membrane-disruptive properties of a panel of TX-100 replacement candidates with varying antiviral activities and identified two distinct classes of membrane-interacting detergents with different critical micelle concentration (CMC) dependencies and biophysical mechanisms. While all tested detergents formed micelles, only a subset of the detergents caused CMC-dependent membrane solubilization similarly to that of TX-100, whereas other detergents adsorbed irreversibly to lipid membrane interfaces in a CMC-independent manner. We compared these biophysical results to virus inactivation data, which led us to identify that certain membrane-interaction profiles contribute to greater antiviral activity and such insights can help with the discovery and validation of antiviral detergents to replace TX-100.

Design and Construction of a Multi-Tiered Minimal Actin Cortex for Structural Support in Lipid Bilayer Applications

Artificial lipid bilayers have revolutionized biochemical and biophysical research by providing a versatile interface to study aspects of cell membranes and membrane-bound processes in a controlled environment. Artificial bilayers also play a central role in numerous biosensing applications, form the foundational interface for liposomal drug delivery, and provide a vital structure for the development of synthetic cells. But unlike the envelope in many living cells, artificial bilayers can be mechanically fragile. Here, we develop prototype scaffolds for artificial bilayers made from multiple chemically linked tiers of actin filaments that can be bonded to lipid headgroups. We call the interlinked and layered assembly a multiple minimal actin cortex (multi-MAC). Construction of multi-MACs has the potential to significantly increase the bilayer's resistance to applied stress while retaining many desirable physical and chemical properties that are characteristic of lipid bilayers. Furthermore, the linking chemistry of multi-MACs is generalizable and can be applied almost anywhere lipid bilayers are important. This work describes a filament-by-filament approach to multi-MAC assembly that produces distinct 2D and 3D architectures. The nature of the structure depends on a combination of the underlying chemical conditions. Using fluorescence imaging techniques in model planar bilayers, we explore how multi-MACs vary with electrostatic charge, assembly time, ionic strength, and type of chemical linker. We also assess how the presence of a multi-MAC alters the underlying lateral diffusion of lipids and investigate the ability of multi-MACs to withstand exposure to shear stress.

Unraveling How Antimicrobial Lipid Mixtures Disrupt Virus-Mimicking Lipid Vesicles: A QCM-D Study

Single-chain lipid amphiphiles such as fatty acids and monoglycerides are promising antimicrobial alternatives to replace industrial surfactants for membrane-enveloped pathogen inhibition. Biomimetic lipid membrane platforms in combination with label-free biosensing techniques offer a promising route to compare the membrane-disruptive properties of different fatty acids and monoglycerides individually and within mixtures. Until recently, most related studies have utilized planar model membrane platforms, and there is an outstanding need to investigate how antimicrobial lipid mixtures disrupt curved model membrane platforms such as intact vesicle adlayers that are within the size range of membrane-enveloped virus particles. This need is especially evident because certain surfactants that completely disrupt planar/low-curvature membranes are appreciably less active against high-curvature membranes. Herein, we conducted quartz crystal microbalance-dissipation (QCM-D) measurements to investigate the membrane-disruptive properties of glycerol monolaurate (GML) monoglyceride and lauric acid (LA) fatty acid mixtures to rupture high-curvature, ~75 nm diameter lipid vesicle adlayers. We identified that the vesicle rupture activity of GML/LA mixtures mainly occurred above the respective critical micelle concentration (CMC) of each mixture, and that 25/75 mol% GML/LA micelles exhibited the greatest degree of vesicle rupture activity with ~100% efficiency that exceeded the rupture activity of other tested mixtures, individual compounds, and past reported values with industrial surfactants. Importantly, 25/75 GML/LA micelles outperformed 50/50 GML/LA micelles, which were previously reported to have the greatest membrane-disruptive activity towards planar model membranes. We discuss the mechanistic principles behind how antimicrobial lipid engineering can influence membrane-disruptive activity in terms of optimizing the balance between competitive membrane remodeling processes and inducing anisotropic vs. isotropic spontaneous curvature in lipid membrane systems.

Membrane-Disruptive Effects of Fatty Acid and Monoglyceride Mitigants on E. coli Bacteria-Derived Tethered Lipid Bilayers

We report electrochemical impedance spectroscopy measurements to characterize the membrane-disruptive properties of medium-chain fatty acid and monoglyceride mitigants interacting with tethered bilayer lipid membrane (tBLM) platforms composed of E. coli bacterial lipid extracts. The tested mitigants included capric acid (CA) and monocaprin (MC) with 10-carbon long hydrocarbon chains, and lauric acid (LA) and glycerol monolaurate (GML) with 12-carbon long hydrocarbon chains. All four mitigants disrupted E. coli tBLM platforms above their respective critical micelle concentration (CMC) values; however, there were marked differences in the extent of membrane disruption. In general, CA and MC caused larger changes in ionic permeability and structural damage, whereas the membrane-disruptive effects of LA and GML were appreciably smaller. Importantly, the distinct magnitudes of permeability changes agreed well with the known antibacterial activity levels of the different mitigants against E. coli, whereby CA and MC are inhibitory and LA and GML are non-inhibitory. Mechanistic insights obtained from the EIS data help to rationalize why CA and MC are more effective than LA and GML at disrupting E. coli membranes, and these measurement capabilities support the potential of utilizing bacterial lipid-derived tethered lipid bilayers for predictive assessment of antibacterial drug candidates and mitigants.

Glycerol Monolaurate Inhibits Wild-Type African Swine Fever Virus Infection in Porcine Macrophages

Naturally abundant antimicrobial lipids, such as fatty acids and monoglycerides, that disrupt membrane-enveloped viruses are promising mitigants to inhibit African swine fever virus (ASFV). Among mitigant candidates in this class, glycerol monolaurate (GML) has demonstrated particularly high antiviral activity against laboratory-adapted ASFV strains. However, there is an outstanding need to further determine the effects of GML on wild-type ASFV strains, which can have different virulence levels and sensitivities to membrane-disrupting compounds as compared to laboratory-adapted strains. Herein, we investigated the antiviral effects of GML on a highly virulent strain of a wild-type ASFV isolate (Armenia/07) in an in vitro porcine macrophage model. GML treatment caused a concentration-dependent reduction in viral infectivity, and there was a sharp transition between inactive and active GML concentrations. Low GML concentrations had negligible effect on viral infectivity, whereas sufficiently high GML concentrations caused a >99% decrease in viral infectivity. The concentration onset of antiviral activity matched the critical micelle concentration (CMC) value of GML, reinforcing that GML micelles play a critical role in enabling anti-ASFV activity. These findings validate that GML can potently inhibit wild-type ASFV infection of porcine macrophages and support a biophysical explanation to guide antimicrobial lipid performance optimization for pathogen mitigation applications.

Development and Analytical Evaluation of a Point-of-Care Electrochemical Biosensor for Rapid and Accurate SARS-CoV-2 Detection

The COVID-19 pandemic has underscored the critical need for rapid and accurate screening and diagnostic methods for potential respiratory viruses. Existing COVID-19 diagnostic approaches face limitations either in terms of turnaround time or accuracy. In this study, we present an electrochemical biosensor that offers nearly instantaneous and precise SARS-CoV-2 detection, suitable for point-of-care and environmental monitoring applications. The biosensor employs a stapled hACE-2 N-terminal alpha helix peptide to functionalize an in situ grown polypyrrole conductive polymer on a nitrocellulose membrane backbone through a chemical process. We assessed the biosensor's analytical performance using heat-inactivated omicron and delta variants of the SARS-CoV-2 virus in artificial saliva (AS) and nasal swab (NS) samples diluted in a strong ionic solution, as well as clinical specimens with known Ct values. Virus identification was achieved through electrochemical impedance spectroscopy (EIS) and frequency analyses. The assay demonstrated a limit of detection (LoD) of 40 TCID50/mL, with 95% sensitivity and 100% specificity. Notably, the biosensor exhibited no cross-reactivity when tested against the influenza virus. The entire testing process using the biosensor takes less than a minute. In summary, our biosensor exhibits promising potential in the battle against pandemic respiratory viruses, offering a platform for the development of rapid, compact, portable, and point-of-care devices capable of multiplexing various viruses. The biosensor has the capacity to significantly bolster our readiness and response to future viral outbreaks.

Unraveling Membrane-Disruptive Properties of Sodium Lauroyl Lactylate and Its Hydrolytic Products: A QCM-D and EIS Study

Membrane-disrupting lactylates are an important class of surfactant molecules that are esterified adducts of fatty acid and lactic acid and possess industrially attractive properties, such as high antimicrobial potency and hydrophilicity. Compared with antimicrobial lipids such as free fatty acids and monoglycerides, the membrane-disruptive properties of lactylates have been scarcely investigated from a biophysical perspective, and addressing this gap is important to build a molecular-level understanding of how lactylates work. Herein, using the quartz crystal microbalance-dissipation (QCM-D) and electrochemical impedance spectroscopy (EIS) techniques, we investigated the real-time, membrane-disruptive interactions between sodium lauroyl lactylate (SLL)-a promising lactylate with a 12-carbon-long, saturated hydrocarbon chain-and supported lipid bilayer (SLB) and tethered bilayer lipid membrane (tBLM) platforms. For comparison, hydrolytic products of SLL that may be generated in biological environments, i.e., lauric acid (LA) and lactic acid (LacA), were also tested individually and as a mixture, along with a structurally related surfactant (sodium dodecyl sulfate, SDS). While SLL, LA, and SDS all had equivalent chain properties and critical micelle concentration (CMC) values, our findings reveal that SLL exhibits distinct membrane-disruptive properties that lie in between the rapid, complete solubilizing activity of SDS and the more modest disruptive properties of LA. Interestingly, the hydrolytic products of SLL, i.e., the LA + LacA mixture, induced a greater degree of transient, reversible membrane morphological changes but ultimately less permanent membrane disruption than SLL. These molecular-level insights support that careful tuning of antimicrobial lipid headgroup properties can modulate the spectrum of membrane-disruptive interactions, offering a pathway to design surfactants with tailored biodegradation profiles and reinforcing that SLL has attractive biophysical merits as a membrane-disrupting antimicrobial drug candidate.

Tethered Bilayer Lipid Membrane Platform for Screening Triton X-100 Detergent Replacements by Electrochemical Impedance Spectroscopy

In light of regulatory considerations, there are ongoing efforts to identify Triton X-100 (TX-100) detergent alternatives for use in the biological manufacturing industry to mitigate membrane-enveloped pathogen contamination. Until now, the efficacy of antimicrobial detergent candidates to replace TX-100 has been tested regarding pathogen inhibition in endpoint biological assays or probing lipid membrane disruption in real-time biophysical testing platforms. The latter approach has proven especially useful to test compound potency and mechanism of action, however, existing analytical approaches have been limited to studying indirect effects of lipid membrane disruption such as membrane morphological changes. A direct readout of lipid membrane disruption by TX-100 detergent alternatives would be more practical to obtain biologically relevant information to guide compound discovery and optimization. Herein, we report the use of electrochemical impedance spectroscopy (EIS) to investigate how TX-100 and selected replacement candidates-Simulsol SL 11W (Simulsol) and cetyltrimethyl ammonium bromide (CTAB)-affect the ionic permeability of tethered bilayer lipid membrane (tBLM) platforms. The EIS results revealed that all three detergents exhibited dose-dependent effects mainly above their respective critical micelle concentration (CMC) values while displaying distinct membrane-disruptive behaviors. TX-100 caused irreversible membrane disruption leading to complete solubilization, whereas Simulsol caused reversible membrane disruption and CTAB induced irreversible, partial membrane defect formation. These findings establish that the EIS technique is useful for screening the membrane-disruptive behaviors of TX-100 detergent alternatives with multiplex formatting possibilities, rapid response, and quantitative readouts relevant to antimicrobial functions.

DNA-Tethered Lipid Membrane Formation via Solvent-Assisted Self-Assembly

DNA-tethered lipid bilayers have been used in many studies, based on the controllable and well-defined properties of DNA tethers. However, their application has been limited, because it is difficult to cover a wide range of surfaces and achieve electrical insulation. We implemented an existing method, where a DNA hybrid chip on a silica or glass surface supports a lipid membrane using solvent-assisted self-assembly. The formation of a continuous lipid bilayer was confirmed through the change in quartz crystal microbalance dissipation results, depending on the presence or absence of DNA hybrids. The fluidity of the DNA-tethered lipid membranes was analyzed using a fluorescence microscope. The electrochemical analysis demonstrated the versatility of this new technique, which can be used for sensor or electrode surface modification for biosensors or bioelectronics.

Nanoarchitectonics-based model membrane platforms for probing membrane-disruptive interactions of odd-chain antimicrobial lipids

The use of nanoscience tools to investigate how antimicrobial lipids disrupt phospholipid membranes has greatly advanced molecular-level biophysical understanding and opened the door to new application possibilities. Until now, relevant studies have focused on even-chain antimicrobial lipids while there remains an outstanding need to investigate the membrane-disruptive properties of odd-chain antimicrobial lipids that are known to be highly biologically active. Herein, using the quartz crystal microbalance-dissipation (QCM-D) and electrochemical impedance spectroscopy (EIS) techniques, we investigated how an 11-carbon, saturated fatty acid and its corresponding monoglyceride-termed undecanoic acid and monoundecanoin, respectively-disrupt membrane-mimicking phospholipid bilayers with different nanoarchitectures. QCM-D tracking revealed that undecanoic acid and monoundecanoin caused membrane tubulation and budding from supported lipid bilayers, respectively, and were only active above their experimentally determined critical micelle concentration (CMC) values. Monoundecanoin was more potent due to a lower CMC and electrochemical impedance spectroscopy (EIS) characterization demonstrated that monoundecanoin caused irreversible membrane disruption of a tethered lipid bilayer platform at sufficiently high compound concentrations, whereas undecanoic acid only induced transient membrane disruption. This integrated biophysical approach also led us to identify that the tested 11-carbon antimicrobial lipids cause more extensive membrane disruption than their respective 12-carbon analogues at 2 × CMC, which suggests that they could be promising molecular components within next-generation antimicrobial nanomedicine strategies.

Biophysical Characterization of LTX-315 Anticancer Peptide Interactions with Model Membrane Platforms: Effect of Membrane Surface Charge

LTX-315 is a clinical-stage, anticancer peptide therapeutic that disrupts cancer cell membranes. Existing mechanistic knowledge about LTX-315 has been obtained from cell-based biological assays, and there is an outstanding need to directly characterize the corresponding membrane-peptide interactions from a biophysical perspective. Herein, we investigated the membrane-disruptive properties of the LTX-315 peptide using three cell-membrane-mimicking membrane platforms on solid supports, namely the supported lipid bilayer, intact vesicle adlayer, and tethered lipid bilayer, in combination with quartz crystal microbalance-dissipation (QCM-D) and electrochemical impedance spectroscopy (EIS) measurements. The results showed that the cationic LTX-315 peptide selectively disrupted negatively charged phospholipid membranes to a greater extent than zwitterionic or positively charged phospholipid membranes, whereby electrostatic interactions were the main factor to influence peptide attachment and membrane curvature was a secondary factor. Of note, the EIS measurements showed that the LTX-315 peptide extensively and irreversibly permeabilized negatively charged, tethered lipid bilayers that contained high phosphatidylserine lipid levels representative of the outer leaflet of cancer cell membranes, while circular dichroism (CD) spectroscopy experiments indicated that the LTX-315 peptide was structureless and the corresponding membrane-disruptive interactions did not involve peptide conformational changes. Dynamic light scattering (DLS) measurements further verified that the LTX-315 peptide selectively caused irreversible disruption of negatively charged lipid vesicles. Together, our findings demonstrate that the LTX-315 peptide preferentially disrupts negatively charged phospholipid membranes in an irreversible manner, which reinforces its potential as an emerging cancer immunotherapy and offers a biophysical framework to guide future peptide engineering efforts.