Journal Article

Liquid crystalline bacterial outer membranes are critical for antibiotic susceptibility

Skoda, Maximilian W. A., Clifton, Luke A., Lakey, Jeremy H., Paracini, Nicolò

2018

Overview

The outer membrane (OM) of Gram-negative bacteria is a robust, impermeable, asymmetric bilayer of outer lipopolysaccharides (LPSs) and inner phospholipids containing selective pore proteins which confer on it the properties of a molecular sieve. This structure severely limits the variety of antibiotic molecules effective against Gram-negative pathogens and, as antibiotic resistance has increased, so has the need to solve the OM permeability problem. Polymyxin B (PmB) represents those rare antibiotics which act directly on the OM and which offer a distinct starting point for new antibiotic development. Here we investigate PmB’s interactions with in vitro OM models and show how the physical state of the lipid matrix of the OM is a critical factor in regulating the interaction with the antimicrobial peptide. Using neutron reflectometry and infrared spectroscopy, we reveal the structural and chemical changes induced by PmB on OM models of increasing complexity. In particular, only a tightly packed model reproduced the temperature-controlled disruption of the asymmetric lipid bilayer by PmB observed in vivo. By measuring the order of outer-leaflet LPS and inner-leaflet phospholipids, we show that PmB insertion is dependent on the phase transition of LPS from the gel to the liquid crystalline state. The demonstration of a lipid phase transition in the physiological temperature range also supports the hypothesis that bacteria grown at different temperatures adapt their LPS structures to maintain a homeoviscous OM.

• Publisher: National Academy of Sciences
• Subject: Anti-Bacterial Agents - pharmacology / Antibiotic resistance / Antibiotics / Bacteria / Biological Sciences / Cell Membrane - chemistry / Cell Membrane - metabolism / Cell Membrane Permeability - drug effects / Cell Membrane Permeability - physiology / Crystal structure / Crystallinity / Drug Resistance, Bacterial / Gram-negative bacteria / Gram-Negative Bacteria - physiology / Infrared spectroscopy / Lipid Bilayers - chemistry / Lipopolysaccharides / Lipopolysaccharides - chemistry / Lipopolysaccharides - physiology / Liquid crystals / Liquid Crystals - chemistry / Membranes / Microbiology / Models, Chemical / Molecular sieves / Molecular structure / Organic chemistry / Outer membranes / Permeability / Phase Transition / Phase transitions / Phospholipids / Phospholipids - chemistry / Phospholipids - physiology / PNAS Plus / Polymyxin B / Polymyxin B - pharmacology / Proteins / Reflectometry / Spectrum Analysis / Temperature