The different structures of the cell walls of Gram-positive and Gram-negative bacteria provide the means of distinguishing them. Gram-positive bacteria retain a dye-iodine complex, but it can be washed from Gram-negative bacteria. The difference arises because the cell walls in Gram-positive bacteria consist of a cross-linked multilayered polysaccharide-peptide complex called peptidoglycan, layered on top of a lipid bilayer membrane, whereas the cell walls in Gram-negative bacteria consist of a single layer of peptidoglycan sandwiched between outer and inner lipid bilayer membranes (Figure 9.25).

In Gram-positive bacteria, the long polysaccharide chains are strictly alternating copolymers of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). An unusual tetrapeptide, (L-ALA)-(D-GLU)-(L-LYS)-(D-ALA), is attached to the lactic acid moiety of the NAM (Figure 9.26). The presence of the D-amino acids and the unusual linkage of the glutamic acid through its -carboxyl group may help protect the peptide against protease action. Individual peptides are cross-linked together with a glycine pentapeptide joining adjacent lysines through the amino group (Figure 9.26).

In addition, lipid-oligosaccharide complexes called lipoteichoic acids protrude from the membrane through the peptidoglycan wall. (Figure 9.26).