Bacteria are under significant internal turgor pressure, due to high concentration of solutes inside the cell compared to outside. To protect themselves from rupturing, they possess a cell wall, a porous mesh-like structure composed of peptidoglycan. Peptidoglycan is a polymer made up of stiff glycan strands cross-linked by short, flexible peptides. While the chemical structure of the basic repeat unit is clear, how it is organized into a macroscopic wall is not. Two main proposals distinguished by the orientation of the glycan strands have been put forth. In one, the layered model, the glycan strands run circumferentially around the cell body; in the other scaffold model, they are perpendicular to the membrane. To test these models, we have constructed and simulated at an atomic scale patches of both Gram-positive and Gram-negative bacterial cell walls in different organizations up to 50 nanometers in size.
Gram-positive cell wall simulations
In the case of Gram-positive PG, the simulations elucidate the mechanisms behind a distinct curling effect observed in three-dimensional electron cryo-tomography images. Namely, when mechanically sheared, patches of the cell wall are observed to curl up on themselves in a direction orthogonal to their original curvature around the cell. Simulations of a layered model, but not of a scaffold model, recapitulate the curling behavior, which is found to arise due to a differential tension at the inside and outside of the patch.
- Architecture and assembly of the Gram-positive cell wall.
Morgan Beeby, James C. Gumbart, Benoit Roux, and Grant Jensen. Mol. Microbiology, 88:664-672, 2013.