Cell Wall

Gram positive bacteria stain violet due to the presence of a thick layer of peptidoglycan in their cell walls, which retains the crystal violet. Gram negative bacteria has a thinner peptidoglycan wall, which does not retain the crystal violet during the decolouring process.

This is the function of capsules, which enable many organisms to resist phagocytic engulfment called phagocytosis.

Peptidoglycans (mucopeptides, glycopeptides, mureins) are the structural elements of almost all bacterial cell walls. They constitute almost 95% of the cell wall in some gram-positive bacteria and as little as 5 to 10% of the cell wall in gram-negative bacteria. Peptidoglycans are made up of a polysaccharide backbone consisting of alternating N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) residues in equal amounts.

Teichoic acids are bacterial polysaccharides of glycerol phosphate or ribitol phosphate linked to muramic acid of peptidoglycan via phosphodiester bonds. Teichoic acids are found within the cell wall of gram positive bacteria, such as species in the genera Staphylococcus, Streptococcus, Bacillus, Clostridium, Corynebacterium and Listeria ,and appear to extend to the surface of the peptidoglycan layer. Teichoic acids are not found in gram- negative bacteria.

If peptidoglycan is digested away from the cell, gram-positive cells lose their cell walls and become protoplasts while the gram negative cells become spheroplasts. A spheroplast is a cell from which the cell wall has been almost completely removed as by the action of penicillin. The name stems from the fact that after a microbe's cell wall is digested, membrane tension causes the cell to acquire a characteristic spherical shape.

Some antibacterial drugs, such as penicillin interferes with the production of peptidoglycan by binding to bacterial enzymes known as penicillin binding proteins or transpeptidases. Penicillins inhibit the transpeptidation reaction and block cross-linking of the cell wall. This results in lysis of the cell wall due to high internal osmotic pressure.

In the primary (growing) plant cell wall, the major carbohydrates are cellulose, hemicellulose and pectin. The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix.

Each N-acetylmuramic acid is attached to a short amino acid chain containing L-alanine, D- glutamine, L-lysine and D-alanine with a pentaglycine interbridge between tetrapeptides in the case of Staphylococcus aureus, a gram positive bacterium. Meso-diaminopimelic acid serves as the final precursor in the biosynthesis of L-lysine in both bacteria and higher plants, and is also an essential component of the peptidoglycan of most pathogenic bacteria, nearly all gram-negative and most gram-positive bacteria with the notable exception of Staphylococcus aureus where it is replaced by lysine.

The human body's own antibiotic lysozymes found in tears work by breaking the β(1, 4) glycosidic bonds in peptidoglycan, and thereby destroying many bacterial cells.

It is a major constituent of the outer membrane and is often toxic. The outer leaflet of the membrane comprises of a complex lipopolysaccharide whose lipid portion acts as an endotoxin. If the endotoxin enters the circulatory system, it causes a toxic reaction resulting in high temperature, high respiration rate and low blood pressure. This may lead to endotoxic shock which may be fatal.

The cell wall is a tough, usually flexible but sometimes fairly rigid layer that surrounds different types of cells. Cell walls are found in plants, bacteria, fungi, algae and some archaea. Animals and protozoa do not have cell walls.

Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria forming the cell wall. Peptidoglycan serves a structural role in the bacterial cell wall by giving structural strength as well as counteracting the osmotic pressure of the cytoplasm.

Fungi possess cell walls made of the glucosamine polymer chitin. It is a polymer of nitrogen-containing polysaccharide rendering a tough protective covering or structural support in certain organisms.