Peptidoglycan is a heteropolymer that consists of glycan strands that are crosslinked by peptides. The glycan backbone is composed of alternating units of N-acetylglucosamine and N-acetylmuramic acid linked by β-1,4-glycosidic bonds. The peptide subunits usually contain four alternating L- and D-amino acids, and they are connected to the glycan stands via the lactyl groups of the N-acetylmuramic acid residues. Cross-linking of the glycan strands generally occurs between the carboxyl group of D-Ala at position 4 and the amino group of the diamino acid at position 3, either directly or through a short peptide bridge.