1. Define the eubacterial cell wall.
A eubacterial cell wall is a strong form but flexible external structure that surround most yu bacterial cell and established the shape it is mainly composed of peptidoglycan teichoic acid and lipopolysaccharide (LPS)
2. Name the methods of cell wall staining
- Ringer’s method
- Dyar’s method
3. Give the principle of cell wall staining by Dyar’s method.
Cetylpyridinium chloride dissociates in an aqueous solution to form positively charged cetylpyridinium ions and negatively charged chloride ions. Cetylpyridinium ions binds to the negatively charged cell surface.
When anionic congo red is applied, it is precipitated at the point of contact with the outer surface of the cell. A red ring of precipitate is observed. The cytoplasm stains blue on applying methylene blue as a counterstain.
4. What is peptidoglycan?
Peptidoglycan (murein or mucopeptide) is one of the major constituents of the cell wall of almost every bacterial cell. It is a heteropolysaccharide composed of alternately repeating units of the amino sugars N-acetyl glucosamine (NAG) and N-acetylmuramic acid (NAM) linked to each other by B (1 → 4) glycosidic, bonds.
A tetrapeptide chain is usually attached to most of the NAM subunit, which is further cross-linked by a peptide bridge. Activ Go to S Q.
5. What is lysozyme?
Lysozyme is an enzyme that can digest peptidoglycan by breaking the B (1- 4)
6. Distinguish between Gram-positive and Gram-negative bacterial cell walls.
|Peptidoglycan||Occurs as a thick layer||Occurs as a thin layer|
|Peptidoglycan tetrapeptide||Most contain lysine||All contain diamino- pimelic acid.|
|Cross-linkages||Generally giycine pentapeptide||Direct linkag: between DAP and D-tdanine|
7. What types of cell walls present archaea ?
The archaea do not contain peptidoglycan in their cell walls. Some methanogenic and extremely halophilic archaea possess cell walls with pseudo-peptidoglycan (contain N-acetylalosaminuronic acid instead of N-acetylmuramic acid and L-amino acids instead of D-amino acids).
Some archaea possess cell walls composed of proteins, or polysaccharides, or other macro-molecules composition, e.g., glycoproteins.
8. Give the chemical composition of algal, fungal, and protozoal cell walls.
|Organism||Cell wall composition|
|Algae||Cellulose or other polysaccharides. Calcium or silicon may be present, sometimes called the test or frustule.|
|Fungi (including Yeasts)||Mainly chitin*, a nitrogen-containing polysaccharide, it is (including yeasts) relatively resistant to microbial decomposition.|
|Protozoa||Usually do not have a true cell wall|
* also found in the hard shells of crabs and exoskeletons of arthropods.
9. Name a few bacteria which do not possess a cell wall.
Mycoplasmas, Thermoplasma (archaea), etc.
10. What are L- forms?
The L- forms (named after the Lister Institute in London where they were discovered) also lack cell walls. The loss may be complete or partial (some have a defective cell wall), and the parent strain may be either Gram-positive or Gram-negative.
L- forms may be stable that is they may continue to grow and reproduce after the penicillin treatment is ceased and other L – forms may synthesize a cell wall again.