The gram-positive cell wall
Gram-positive cell walls are
thick and the peptidoglycan ( also known as murein) layer constitutes almost
95% of the cell wall in some gram-positive bacteria and as little as 5-10% of
the cell wall in gram-negative bacteria. The gram-positive bacteria take up the
crystal violet dye and are stained purple. The cell wall of some gram-positive
bacteria can be completely dissolved by lysozymes which attacks the bonds
between GA and MA. In other gram-positive bacteria, such as Staphylococcus
aureus, the walls are resistant to the action of lysozymes. They have O-acetyl
groups on carbon-6 of some MA residues. The matrix substances in the walls of
gram-positive bacteria may be polysaccharides or teichoic acids. The latter are
very widespread, but have been found only in gram-positive bacteria. There are
two main types of teichoic acid: ribitol teichoic acids and glycerol teichoic
acids. The latter one is more widespread. These acids are polymers of ribitol
phosphate and glycerol phosphate, respectively, and only located on the surface
of many gram-positive bacteria. However, the exact function of teichoic acid is
debated and not fully understood. A major component of the gram-positive cell
wall is lipoteichoic acid. One of its purposes is providing an antigenic
function. The lipid element is to be found in the membrane where its adhesive
properties assist in its anchoring to the membrane.
The gram-negative cell wall
Gram-negative cell walls are
thin and unlike the gram-positive cell walls, they contain a thin peptidoglycan
layer adjacent to the cytoplasmic membrane. Gram-negative bacteria is stained
as pink colour. The chemical structure of the outer membrane's lipopolysaccharides
is often unique to specific bacterial sub-species and is responsible for many
of the antigenic properties of these strains. Lipopolysaccharides, also called
endotoxins, are composed of polysaccharides and lipid A which are responsible
for much of the toxicity of gram-negative bacteria. It consists of
characteristic lipopolysaccarides embedded in the membrane.
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