Modeling and Simulation of Bacterial Outer Membranes with Lipopolysaccharides and Enterobacterial Common Antigen

Enterobacterial common antigen (ECA) is a surface glycolipid shared by all members of the Enterobacteriaceae family. In addition to lipopolysaccharides (LPS), ECA is an important component in the outer membrane (OM) of Gram-negative bacteria, making the OM an effective, selective barrier against the permeation of toxic molecules. Previous modeling and simulation studies represented OMs exclusively with LPS in the outer leaflet. In this work, various ECA molecules were first modeled and incorporated into symmetric bilayers with LPS in different ratios, and all-atom molecular dynamics simulations were conducted to investigate the properties of the mixed bilayers mimicking OM outer leaflets. Dynamic and flexible conformational ensembles are sampled for each ECA/LPS system. Incorporation of ECA_LPS (an LPS core-linked form) and ECA_PG (a phosphatidylglycerol-linked form) affects lipid packing and ECA/LPS distributions on the bilayer surface. Hydrophobic thickness and chain order parameter analyses indicate that incorporation of ECA_PG makes the acyl chains of LPS more flexible and disordered and thus increases the area per lipid of LPS. The calculated area per lipid of each ECA/LPS provides a good estimate for building more realistic OMs with different ratios of ECA/LPS, which will be useful in order to characterize their interactions with outer membrane proteins in more realistic OMs.