Additional polymers in bacterial walls

Abstract

In addition to peptidoglycan, the walls of most Gram-positive bacteria contain other polymers, frequently with repeating acidic groups. Often these groups are phosphodiesters, which generally occur in polymers called teichoic acids. The teichoic acids were discovered in 1958, as the result of a search for a role for the CDP-glycerol and CDP-ribitol that had been identified in Gram-positive bacteria [4]. As originally defined, they consist of polymerized polyol phosphates, often with side-chains of mono- or oligo-saccharide units, and also ester-linked D-alanine residues. These ester linkages have exceptional lability, occasioned by the presence of vicinal hydroxyl or phosphate groups [75]. In some cases, such as the membrane teichoic acids of group D streptococci, the D-alanyl groups are attached to D-glucose residues rather than to the polyol part of the molecule, and in these instances the ester linkages are appreciably more stable to alkali [86]. As more information has accumulated it has become clear that many variations on the basic structural pattern of teichoic acids exist, although the various polymers presumably serve the same function in the cell. Some characteristic structures of teichoic acids are set out in Fig. 7.1 and of related structures in Fig. 7.2. An exception to the general rule that teichoic acids occur only in Gram-positive bacteria came with the description of lipoteichoic acid in the Gram-negative rumen bacterium Butyrivibrio flbrisolvens [36]. However, ultrastructural study showed that the walls were of the Gram-positive morphological type, but exceptionally thin. The thinness was probably the cause of the apparent Gram-negativity [11].