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Polymyxin B-lipid interactions in Langmuir-Blodgett monolayers of Escherichia coli lipids: a thermodynamic and atomic
Adrià Clausell1, M Antonia Busquets, Montserrat Pujol
1Physical Chemistry Department, Faculty of Pharmacy, University of Barcelona, Avn. Joan XXIII s/n, 08028 Barcelona, Spain.
Abstract:
The dramatically increased frequency of antibiotic resistance has led to intensive efforts towards developing new families of antibiotics. Membrane-active antibiotic peptides such as polymyxin B (PxB) hold promise as the next generation of antibiotics, since they rarely spur the evolution of resistance. At low concentrations in the membrane, PxB forms vesicle-vesicle contacts and induces lipid exchange without leakage or fusion, a phenomenon that can explain its specificity towards gram-negative bacteria by contact formation between the two phospholipids interfaces in the periplasmatic space. In this work, the interaction of PxB and the nonantibiotic derivative polymyxin B nonapeptide (PxB-NP) with monolayers of Escherichia coli membrane lipids (ECL) has been studied by thermodynamic and structural methods. PxB inserts itself into ECL monolayers as a conformation that forms intermembrane contacts with vesicles injected underneath, and induces lipid exchange when the monolayer surface pressure is set at 32 mN/m (membrane equivalence pressure) or net transfer vesicle-to-monolayer at lower surface pressures. Thermodynamic analysis of the compression isotherms of mixed monolayers indicates that PxB inserts into the monolayer with an expansion of the mean molecular area, implying that peptide and lipids form nonideal mixtures. At low concentrations, corresponding to the membrane-membrane contact form of PxB, the mixed monolayers present positive excess energy values (deltaGm(Ex)), and atomic force microscopy (AFM) imaging reveals structures of approximately 120-nm diameter that protrude from the lipid surface approximately 0.7 nm. At concentrations of PxB above 4 mol %, thermodynamic analysis gives a very high deltaGm(Ex), corresponding to nonfavorable interactions, and AFM images show round structures of 20-30 nm diameter. PxB-NP behaves in a totally different way, in agreement with its inability to form vesicle-vesicle contacts and its lack of antibiotic effect. These results are discussed in the light of the mechanism of action of PxB on the membrane of gram-negative bacteria.
Insights
Polymyxin B (PxB) antibiotic peptides interact with bacterial membranes, forming contacts and exchanging lipids. This study reveals PxB
Area of Science:
- Membrane biophysics
- Antimicrobial drug discovery
- Gram-negative bacterial infections
Background:
- Rising antibiotic resistance necessitates novel therapeutics.
- Membrane-active peptides like Polymyxin B (PxB) show promise due to low resistance evolution.
- PxB's mechanism involves vesicle-vesicle contacts and lipid exchange, crucial for targeting Gram-negative bacteria.
Purpose of the Study:
- To investigate the interaction of PxB and its non-antibiotic derivative, PxB-NP, with Escherichia coli membrane lipid monolayers.
- To elucidate the thermodynamic and structural basis of PxB's membrane interaction and lipid exchange capabilities.
Main Methods:
- Thermodynamic analysis using compression isotherms of mixed monolayers.
- Structural characterization via Atomic Force Microscopy (AFM) imaging.
- Study of interactions with lipid monolayers at varying peptide concentrations and surface pressures.
Main Results:
- PxB inserts into lipid monolayers, forming intermembrane contacts and inducing lipid exchange at specific surface pressures.
- Low PxB concentrations (membrane contact form) show positive excess energy and distinct AFM structures (~120 nm).
- Higher PxB concentrations (>4 mol%) exhibit unfavorable interactions and different AFM structures (~20-30 nm); PxB-NP shows no such effects.
Conclusions:
- PxB's interaction mechanism involves conformational changes and nonideal mixing with membrane lipids.
- The observed membrane contact and lipid exchange phenomena correlate with PxB's antibiotic activity against Gram-negative bacteria.
- PxB-NP's distinct behavior confirms the structural and functional importance of specific PxB domains for membrane interaction.
