Liquid crystalline bacterial outer membranes are critical for antibiotic susceptibility

Abstract

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.

Corrosion Motivated ROS Generation Helps Endow Titanium with Broad‐Spectrum Antibacterial Abilities

“…The OM is made up of a unique asymmetrical lipid bilayer, with the inner leaflet consisting of phospholipids and the outer leaflet composed by lipopolysaccharide. The OM can act as a barrier to prevent drugs from reaching intracellular targets, rendering the Gram‐ E. coli presents a relatively high resistance to minocycline . Nevertheless, it is reported that the OM is sensitive to ROS .…”

“…Unlike Gram+ species, Gram‐ bacteria lack the thick peptidoglycan cell wall; instead, they possess an additional outer membrane (OM). Most antibiotics can hardly pass through the hydrophobic OM, resulting in their little inhibition effect to Gram‐ bacteria . Therefore, destroying the OM is an effective way to help antibiotics kill Gram‐ bacteria and acquire broad‐spectrum antibacterial abilities .…”

Lipopolysaccharide Simulations are Sensitive to Phosphate Charge and Ion Parameterization

“…Overall, simulations demonstrate that models with protonated lipid A form more compact bilayers, which may represent a better match to the liquid-crystalline phase the outer membrane is believed to adopt physiologically. 5,67 Determination of the physiologically-relevant lipid A phosphate charge representation is a crucial problem that simulations need to address, especially in light of these differences observed between simulations with protonated or deprotonated lipid A phosphate groups.…”

Physical Properties of Bacterial Outer Membrane Models: Neutron Reflectometry & Molecular Simulation

“…OM model membranes were constructed using an inner leaflet of phospholipids and an outer leaflet of LPS without O-antigen. These were made and deposited directly onto silicon or onto lipid-coated gold surfaces (2)(3)(4)(5). The former produces membranes that are essentially in contact with the substrate, whereas the latter produces asymmetric membranes that float $15 Å above the solid support, reducing the frictional influence of the support on the membrane and making these biomimetics suitable for biophysical and structural studies using neutron reflectometry (NR).…”