In this article we will discuss about the polymers present in the cell wall of bacteria.
Apart from murein, several other polymers are present in the cell wall of bacteria. In Gram- positive bacteria, the thick, rigid covering of murein is surrounded by polymers, like teichoic acids and teichuronic acid. Teichoic acids are hydrophilic linear negatively charged molecules and they are linked to murein chain possibly by phosphodiester bonds with the N-acetyl muramic acid residues. Teichoic acids are synthesized from CDP-ribitol or CDP-glycerol precursors with the help of specific polymerases. Sugar side chains present in some teichoic acids are transferred from the UDP-derivatives.
Teichoic acids often contain N-acetyl glucosamine or glucose. These are added from their UDP- derivatives. In Gram-negative bacteria, the thin murein layer is surrounded by a soft covering consisting of a polymer made of lipids and polysaccharides of complex structure.
The cell-wall of Gram-positive acid-fast bacteria is characterized by an exceptionally high lipid content, a feature which is unusual for Gram-positive bacteria. The acid-fastness of these bacteria has been attributed to their high lipid content.
These lipids are mainly derivatives of mycolic acids which have a general structure as shown below, though other lipids are also present:
R1 and R2 represent long chain hydrocarbons which are highly hydrophobic in nature.
These mycolic acids are conjugated to polymers of arabinose and galactose (arabinogalactans) and are linked to the peptidoglycan polymer of the cell wall through phosphodiester bonds as shown in Fig. 8.79:
The peptidoglycan layer of the cell wall of acid-fast bacteria differs from that of other eubacteria in at least two respects. Firstly, the disaccharide repeating unit of murein consists of N-acetyl glucosamine and N-glycolyl muramic acid. Secondly, among the amino acids of the tetra peptide side- chains attached to the muramic acid residues, D-glutamic acid and m-diaminopimelic acid often contain amido (-CONH2) groups.
The structure of a repeating unit of murein is shown below (Fig. 8.80):
