The structure of teichoic acid differs depending on the bacterial species. Teichoic acids are polymers of ribitol phosphate, glycerol phosphate and their derivatives. In some cases, the hydroxyl groups of the poly alcohols are bonded with amino acids or sugars. Teichoic acid’ is linked to murein through a linkage unit (Figure 2.9). While murein is a structure rendering physical strength, teichoic acid has some physiological functions. Due to the phosphate in teichoic acids, the surface of Gram-positive bacteria is negatively charged, attracting cations including Mg2+ which stabilize cell wall structure. An autolytic enzyme is necessary to hydrolyze murein at the growing tip for growth of a cell. This enzyme is attached to teichoic acid, ensuring it is not released into the medium and enabling control of activity. Teichoic acid is also a receptor for certain phages. Gram-positive bacteria synthesize teichuronic acid in place of teichoic acid under phosphatelirnited conditions. Lipoteichoic acid has a similar structure to teichoic acid. A diacylglycerol-containing sugar is attached to the terminal polyalcohol phosphate. This lipid part of lipoteichoic acid is embedded in the cytoplasmic membrane, while the polyalcohol phosphate partly constitutes the cell wall. The Gram-positive bacterial cell wall contains various proteins, including the autolytic enzymes mentioned above. They are attached to the cell wall through the action of an enzyme called sortase (Section 6.9.2.4) after they are excreted through the cytoplasmic membrane. Their functions include (1) metabolism of cell surface structures, (2) invasion into the host, (3) hydrolysis of polymers such as proteins and polysaccharides and (4) adhesion to solid surfaces.