Schematic representation of the structure of peptidoglycan. This is the type of structure found in L. lactis and numerous lactobacilli. In other LAB species, the nature of the interpeptide cross-bridge (depicted as D-Asp/D-Asn in the figure) may vary, the third di-amino acid (L-Lys) may be replaced by mDAP or L-ornithine, and the D-Ala in position five of the stem peptide may be replaced by D-lactate. Possible modifications of the PG structure, such as O-acetylation (O-Ac), N-deacetylation (leading to GlcNH2), or amidation (NH2), are indicated in red. The cleavage sites of the different classes of PG hydrolases are indicated with arrows.