X-ray Diffraction Studies of Cell Walls and Peptidoglycans from Gram-positive Bacteria

THE cell walls of Gram-positive bacteria consist principally of a water-insoluble polymer and peptidoglycan (synonyms, murein, mucopeptide, glycosaminopeptide), which in some cases accounts for as much as 90% of the cell wall. After other components (teichoic acid, teichuronic acid, polysaccharide or protein) have been gently removed from the cell walls, peptidoglycan remains as a cell-shaped structure at least 100 Å thick. We report here results of X-ray diffraction observations on whole cell walls and peptidoglycans of Staphylococcus aureus, Bacillus licheniformis and Micrococcus lysodeikticus. Chemical data shows that all the muramic acid residues in the glycan chains of the peptidoglycan of S. aureus are substituted with the peptide L Ala-D GluNH₂-L Lys-D Ala and that there is extensive cross linking by pentaglycine bridges between peptides on adjacent glycan chains^1,3. Such a peptidoglycan might be expected to have an ordered crystalline structure. On the contrary, peptidoglycans of the bacilli, in which the cross linking between peptides is direct and considerably less^4,5 might be expected to have a less ordered structure. The mode of packing of the glycan and peptide moieties has been considered by Kelemen and Rogers⁶. When the glycan chains are stacked in pairs, as in the analogous polysaccharide chitin⁷, the muramic acid residues are orientated in such a way as to allow a three-dimensional structure to be built. If the bulk of the peptides are then arranged in a pseudo β configuration, calculations show that the expected dimensions of the cell wall calculated from the model are of the right order and also such a model allows for the existence of extensive stabilizing hydrogen bonds between adjacent peptide chains.