BACTERIAL CELL WALL

GRAM POSITIVE BACTERIAL CELL WALL

Gram positive bacterial cell wall (Source: open.oregonstate.education)

A model of the constitution of Gram positive bacterial cell wall (Source: scienceprofonline.com)

·       Cell wall is present outside the Cell membrane.

·       Cell wall acts as a backbone of cell and gives shape and rigidity to the cell.

·  Bacterial cell wall confers Osmotic protection from toxic substances and Gram staining characteristics.

·   The cell walls of almost all the bacteria is made up of Peptidoglycan, also called murein or mucopeptide cell wall. It is found only in Prokaryotes.

·    The Peptidoglycan consists of two components viz., (i) Peptide portion which is composed of Amino acids connected by Peptide linkages and (ii) Glycan or Sugar portion.

·      The Glycan portion, which forms the backbone of Peptidoglycan, is composed alternating units of amino sugars N-acetyl-glucosamine (NAG) and N-acetyl-muramic acid (NAM) joined together by β-1,4 glycosidic linkages.

·   The peptidoglycan chains are laterally liked by short chains of four amino acids which are attached to NAM residues. The four amino acids of this Tetrapeptide are D-alanine, L-alanine, D- glutamic acid and L-lysine.

·       The Tetrapeptide chains are also interlinked by a Peptide bridge (Polypeptide linkage chains).

·    Teichoic acid, an acidic polymer consisting of a Carbohydrate (e.g., glucose), Phosphate and Alcohol is found in cell walls of Gram +ve bacteria. There are two classes of Teichoic acids. They are (i) Wall teichoic acid and (ii) Lipoteichoic acid.

·     Teichoic acid has several functions such as binding with metals, act as receptor sites for some viruses and maintaining cells at low pH to prevent degradation of cell walls by self-produced enzymes.

GRAM NEGATIVE BACTERIAL CELL WALL

Gram negative bacterial cell wall (Source: open.oregonstate.education)

·       The cell walls of Gram negative bacteria are much more complex.

·      The peptidoglycan layer of Gram negative bacteria is very thin making up only 10 % or less of the cell wall. 

·      The Outer membrane is a thin, bilayered structure consisting chiefly of Phospholipids, Proteins and Lipopolysaccharides (LPS).

·    The Outer membrane serves as a barrier to prevent the escape of important enzymes from the Periplasmic space.

·    The permeability of Outer membrane to nutrients is provided by proteins called Porins which form channels in the membrane thought which substances of hydrophilic nature and low molecular weight can diffuse.

·     The space between the Cytoplasmic membrane and the Outer membrane is called Periplasmic space.

·       The Periplasmic space prevents the entry of various chemicals that could damage the cell. It also acts as main surface antigen in cell wall.

ACID FAST BACTERIAL CELL WALL

·    The cell wall of Acid fast bacteria (Mycobacterium sp. and Nocardia sp.) is thick, like that of Gram positive bacteria.

·       It is approximately 60 % lipid (Hydrophobic waxy lipid - Mycolic acids) and contains much less Peptidoglycan. The Mycolic acid and Peptidoglycan are held together by Polysaccharides.

·    The lipids make Acid fast organisms impermeable to most other stains and protect them from acids and alkalis.

·     The organisms grow slowly because the lipids impede entry of nutrients into cells, and the cells must expend large quantities of energy to synthesize lipids.

·       If Acid fast cells can be stained by the Gram stain method, they stain as Gram positive.

CELL WALL DEFICIENT BACTERIA (Mycoplasma sp.)

·       Bacteria that belong to the genus Mycoplasma have no cell walls.

·       Mycoplasma are protected from osmotic swelling and bursting by a strengthened cell membrane that contains sterols.

·     Without a rigid cell wall, they vary widely in shape, often forming slender, branched filaments and exhibiting extreme “Pleomorphism”.

·     Mycoplasma sp. were often mistaken for viruses because of their small size and lack of walls. However, they do have other features of Prokaryotic cells, such as prokaryotic Ribosomes and ability to grow in an Artificial medium (Non-obligate parasites).

L - FORMS

·    Some bacteria that ordinarily have a cell wall can lose it during part of their life cycle. These wall-deficient forms are referred to as L-forms or L-phase variants (for the Lister Institute, where they were discovered).

·   L-forms arise naturally from a mutation in the wall-forming genes, or they can be induced artificially by treatment with a chemical such as Lysozyme or Penicillin that disrupts the cell wall.

·     When a Gram positive cell is exposed to either of these two chemicals, it will lose the cell wall completely and become a “Protoplast”.

·     A Gram negative cell exposed to these same substances loses its peptidoglycan but retains at least part of its outer membrane, leaving a less fragile but nevertheless weakened “Spheroplast”.

·     Protoplasts and Spheroplasts burst in pure water or very dilute salt or sugar solutions surrounding fluid rapidly move into and enlarge the cell, which has a much lower internal concentration of water. This rupturing is called Osmotic lysis.

·     Treatment with Antibiotics that affect cell wall synthesis will kill most of the bacteria in some infections, but it leaves a few alive as L-forms. When treatment is discontinued, the L-forms can revert to walled forms and regrow an infecting population.

Formation of Protoplast and Spheroplast