Dynamics of the lipopolysaccharide assembly on the surface of Escherichia coli

Lipopolychacharides (LPSs) cover the surface of Gram-negative bacteria. The LPS assembly provides a penetration barrier to molecules larger than 700- 1000 Da, such as many antibiotics, and when LPS molecules are released from the surface of bacteria, they cause toxic shock in the infected patient. The surface of Escherichia coli JM 109 was visualized by atomic force microscopy, providing the highest resolution images of any bacterium to date (50-Å lateral and 5-Å vertical resolutions). These images indicated that LPS molecules are assembled in bundles of 600-3500 molecules. The LPS molecule is comprised of lipid A, then an inner and outer core, and an outermost region of O-antigen units. Analyses indicated that the O-antigen repeat units vary between 1 and 26 for E. coli JM 109, and the fraction of the LPS molecules with zero to three repeat units made up approximately 50% of the total LPS content. A matrix of 16 LPS molecules was constructed as a representative region of the surface of E. coli. The molecular dynamics simulations of this assembly indicated that the structural components closer to the milieu experienced more movement than those closer to the interior, and that the metal ions coordinated to the inner core were indispensable for the stability of the assembly. In the absence of metal coordination to the inner core, the assembly of the LPS molecules disaggregated such that simulations beyond 67 ps could not be attempted. Simulations also indicated that the metal ions allowed for assembly of the LPS molecules one next to another in a tight formation. Such a tight assembly dramatically decreases the surface accessibility to solvent, so that there is no access for even a water molecule beyond the inner core. This structural property accounts for the aforementioned penetration barrier that is characteristic of Gram-negative bacteria.