Molecular interactions, elastic properties, and nanostructure of Langmuir bacterial-lipid monolayers: Towards solving the mystery in bacterial membrane asymmetry

The membrane of Gram-negative bacteria (GNB) is especially robust due to the additional, unique, highly asymmetric outer membrane, with lipopolysaccharides (LPSs) as the main component. This LPS layer serves as a protective barrier against antibiotics, host immune responses, and other environmental stresses. However, constructing model membranes containing LPS that capture the structural asymmetry for fundamental studies of the GNB cell wall remains an open challenge. In this context, we discuss how recent physicochemical studies of Langmuir monolayers incorporating LPS help us better understand the elastic properties and structural integrity of model LPS bacterial membranes. The classic Langmuir–Blodgett trough has been used to reveal different lipid phase behaviors of monolayers containing LPS mutants with different molecular architectures to mimic the outer leaflet of the GNB outer membrane, shedding light on the underpinning molecular interactions. Permeation and penetration of antimicrobial peptides are shown to alter the viscoelastic properties of LPS monolayers. The LPS-containing Langmuir monolayer can also be transferred to a substrate as the outer leaflet of an asymmetric solid-supported bilayer, and we will discuss the limitations and potential optimization of this method. Finally, we highlight how different physicochemical methods can corroborate and contribute to unravelling the structural characteristics of model bacterial membranes.