Role of O-Antigen in Response to Mechanical Stress of the E. coli Outer Membrane: Insights from Coarse-Grained MD Simulations

Lipopolysaccharide (LPS) is an important component of the outer membrane of Gram-negative bacteria, contributing to the structural integrity of the bacterial cell wall and conferring resistance to chemical attack. The rough variant of LPS contains a conserved lipid A domain and a complete core saccharide section, whereas the smooth variant additionally contains a terminal O-antigen chain. In the following, smooth LPS lipids are simulated in multicomponent membrane models using coarse-grained molecular dynamics. The simulations reveal that the lipid environment of smooth LPS lipids affects the orientation and clustering of their O antigen chains. When the outer membrane leaflets contain smooth LPS lipids alone, the O-antigen chains are packed tightly, leading to strong cohesive intermolecular interactions. When the outer leaflets incorporate interstitial phospholipids and rough LPS variants, the O-antigen chains are tilted and less tightly bound. The different packing of terminal O-antigen chains affects lipid mobility and the mechanical strength of the Gram-negative membrane models. Gram-negative membranes with outer leaflets of smooth LPS alone can withstand surface tensions (150 mN m(-1)) that cause the membrane models with rough LPS lipids and comparable phospholipid bilayers to rupture much more readily.