Peptidoglycans are components of the coat of bacterial cell walls. Figure 9.27 shows the structure of a peptidoglycan from Gram-positive bacteria. In Gram-positive bacteria, the cell wall is multi-layered, due to cross-links between the individual layers of peptidoglycans. Lipoteichoic acid projects through the lipid bilayer and intermeshes with the peptidoglycan layer. Gram-negative bacteria have only a single layer of peptidoglycans on their outer cell wall. The peptide part of the peptidoglycan has two unusual amino acids, D-alanine and D-isoglutamic acid.Cross-links between the peptides are formed by pentaglycine chains between the -amino group of the lysine on one chain and the C-terminal carboxyl group of the alanine on an adjacent chain.

Peptidoglycan synthesis in bacteria is a target of several antibiotics, such as penicillin, which block bacterial growth by interfering with formation of the peptidoglycan layer. Lysozyme is an enzyme that acts as a natural antiobiotic. It catalyzes the hydrolysis of the glycosidic links between N-acetylglucosamine and the N-acetylmuramic acid residues of the peptidoglycan.