Chemical structure of the peptidoglycan, its modifiability and relation to the biological activity

The peptidoglycan is a heteropolymer composed of polysaccharide chains which are cross-linked through short peptides. The polysaccharide moiety (glycan) is made up of beta-1,4 glycosidically linked N-acetylglucosamine and N-acylmuramic acid residues. The carboxyl group of muramic acid is usually substituted by a peptide which consists of alternating L- and D-amino acids. These peptide subunits are cross-linked between the diamino acid in position 3 and the C-terminal D-alanine in position 4 of an adjacent peptide subunit either in a direct way or via an interpeptide bridge (Group A). In some coryneform bacteria the cross-linkage extends from the alpha-carboxyl group of D-glutamic acid in position 2 to D-alanine of a neighbouring peptide subunit (Group B). In the latter case a diamino acid is always found in the interpeptide bridge. A chemical modification of the peptidoglycan may occur in some bacteria due to growth in a quite unbalanced medium. The influence of glycine-rich and glycine-deficient growth medium on the chemical structure of the peptidoglycan of S. aureus will be discussed. Inhibiting concentrations of penicillin, glycine or D-amino acids can also modify the peptidoglycan. Further modification can occur through different extraction procedures which are used to obtain a clean peptidoglycan free of non-peptidoglycan cell wall material. Little is known about the molecular basis of the biological activity. The chemical composition is at least important for the antigenic determinants. The lysozyme susceptibility and the size of the preparation may be other crucial points for the biological activity of the peptidoglycan.