DIFFERENT CELL WALL STRUCTURE

Gram-Positive Cell Walls

           

  Normally the thick, homogeneous cell wall of gram-positive bacteria is composed primarily of peptidoglycan, which often contains a peptide interbridge. However gram-positive cell walls usually also contain large amounts of teichoic acids, polymers of glycerol or ribitol joined by phosphate groups. Amino acids such as D-alanine or sugars like glucose are attached to the glycerol and ribitol groups.

The teichoic acids are connected to either the peptidoglycan itself by a covalent bond with the six hydroxyl of N-acetylmuramic acid or to plasma membrane lipids; in the latter case they are called lipoteichoic acids. Teichoic acids appear to extend to the surface of the peptidoglycan, and, because they are negatively charged, help give the gram-positive cell wall its negative charge. The functions of these molecules are still unclear, but they may be important in maintaining the structure of the wall. Teichoic acids are not present in gram negative bacteria.

 Gram-Negative Cell Walls

Gram-negative cell walls are much more complex than gram-positive walls. The thin peptidoglycan layer next to the plasma membrane may constitute not more than 5 to 10% of the wall weight. In E. coli it is about 2 nm thick and contains only one or two layers or sheets of peptidoglycan. The outer membrane lies outside the thin peptidoglycan layer. The most abundant membrane protein is Braun’s lipoprotein, a small lipoprotein covalently joined to the underlying peptidoglycan and embedded in the outer membrane by its hydrophobic end. The outer membrane and peptidoglycan are so firmly linked by this 
lipoprotein that they can be isolated as one unit.

Another structure that may strengthen the gram-negative wall and hold the outer membrane in place is the adhesion site. The outer membrane and plasma membrane appear to be in direct contact at many locations in the gram-negative wall. Possibly the most unusual constituents of the outer membrane are its lipopolysaccharides (LPSs). These large, complex molecules contain both lipid and carbohydrate, and consist of three parts: (1) lipid A, (2) the core polysaccharide, and (3) the O side chain. The LPS from Salmonella typhimurium has been studied most, and its general structure is described. The lipid A region contains two glucosamine sugar derivatives, each with three fatty acids and phosphate or pyrophosphate attached. It is buried in the outer membrane and the remainder of the LPS molecule projects from the surface. The core polysaccharide is joined to lipid A. In Salmonella it is constructed of 10 sugars, many of them unusual in structure. The O side chain or O antigen is a polysaccharide chain extending outward from the core. It has several peculiar sugars and varies in composition between bacterial strains.

            LPS has many important functions. As a major constituent of the exterior leaflet of the outer membrane, lipid A also helps in stabilizing outer membrane structure. LPS may contribute to bacterial attachment to surfaces and aids in creating a permeability barrier. LPS restrict the entry of bile salts, antibiotics and other toxic substances. Apart from protecting, LPS is a potent antigen which can elicits an immune response in host e.g. O side chain of LPS.

taken from,

Microbiology book- Prescott, Harley and Klein's Microbiology, McGraw Hill International Edition.