CELL WALL

              The cell wall is the layer, usually fairly rigid, that lies just outside the plasma membrane. Cell wall has many different functions to perform :

1. It helps to determine the shape of the cell, 2. It helps protect the cell from osmotic lysis, 3. It can protect the cell from toxic substances and is the site of action of several antibiotics, 4. In pathogen it can contribute to pathogenicity. Due to all these factors it is important to understand its structure.

             

                After Christian Gram developed the Gram stain in 1884, it soon became evident that bacteria could be divided into two major groups based on their response to the Gram-stain procedure. Gram-positive bacteria stained purple, whereas gram-negative bacteria were colored pink or red by the technique. The true structural difference between these two groups became clear with the advent of the transmission electron microscope. The gram-positive cell wall consists of a single 20 to 80 nm thick homogeneous peptidoglycan  layer lying outside the plasma membrane . Whereas the gram-negative cell wall is quite complex, it has a 2 to 7 nm peptidoglycan layer surrounded by a 7 to 8 nm thick outer membrane. Because of the thicker peptidoglycan layer, the walls of gram-positive cells are stronger than those of gram-negative bacteria. Frequently a space is seen between the plasma membrane and the outer membrane in electron micrographs of gram negative bacteria, and sometimes a similar but smaller gap may be observed between the plasma membrane and wall in gram positive bacteria. This space is called the periplasmic space. The substance that occupies the periplasmic space is the periplasm. The nature of the periplasm space and periplasm differs in gram positive and gram negative bacteria.

Peptidoglycan Structure

 Peptidoglycan or murein is an enormous polymer composed of many identical subunits. The polymer contains two sugar derivatives, N-acetylglucosamine and N-acetylmuramic acid and several different amino acids,—D-glutamic acid, D-alanine, L-alanine and meso-diaminopimelic acid. Three from these amino acids are not found in proteins. The presence of D-amino acids protects against attack by most peptidases. The backbone of this polymer is composed of alternating N-acetylglucosamine and N-acetylmuramic acid residues. A peptide chain of four alternating D- and L-amino acids is connected to the carboxyl group of N-acetylmuramic acid. Many bacteria substitute another diaminoacid, usually L-lysine, in the third position for meso-diaminopimelic acid.  Chains of linked peptidoglycan subunits are joined by crosslinks between the peptides. Often the carboxyl group of the terminal D-alanine is connected directly to the amino group of diaminopimelic acid, but a peptide interbridge may be used instead. This cross-linking results in an enormous peptidoglycan sac that is actually one dense-interconnected network. These sacs have been isolated from gram-positive bacteria and are strong enough to retain their shape and integrity, yet they are elastic and somewhat stretchable, unlike cellulose. They also must be porous, as molecules can penetrate them.