Bacterial Cell wall: Structure, Composition and Types
- Cell wall is an important structure of a bacteria. It give shape,rigidity and support to the cell.
- On the basis of cell wall composition, bacteria are classified into two major group ie. Gram Positive and gram negative.
![](https://www.onlinebiologynotes.com/wp-content/uploads/2017/07/cell-wall.png)
Types of cell wall
1. Gram positive cell wall
Cell wall composition of gram positive bacteria.
- Peptidoglycan
- Lipid
- Teichoic acid
2. Gram negative cell wall
Cell wall composition of gram negative bacteria
- Peptidoglycan
- Outermembrane:
- Lipid
- Protein
- Lipopolysaccharide (LPS)
Composition of cell wall:
1. Peptidoglycan:
- Peptidoglycan is porous cross linked polymer which is responsible for strength of cell wall.
- Peptidoglycan is composed of three components.
- Glycan backbone
- Tetra-peptide side chain ( chain of 4 amino acids) linked to NAM
- Peptide cross linkage
- Glycan backbone is the repeated unit of N-acetyl muramic acid (NAM) and N-acetyl glycosamine (NAG) linked by β-glycosidic bond.
- The glycan backbone are cross linked by tetra-peptide linkage. The tetra-peptide are only found in NAM.
- More than 100 peptidoglycan are known with the diversity focused on the chemistry of peptide cross linkage and interbridge.
- Although the peptidoglycan chemistry vary from organism to organism the glycan backbone ie NAG-NAM is same in all species of bacteria.
![](https://www.onlinebiologynotes.com/wp-content/uploads/2017/07/NAG-NAM.png)
The aminoacids found in tetra-peptide are-
- L-alanine: 1st position in both gm+ve and gm-ve bacteria
- D-glutamic acid: 2nd position
- D-aminopimelic acid/ L-lysine: 3rd position (variation occurs)
- D-alanine: 4th position
Peptide cross linkage in Gram positive and Gram negative bacteria
![](https://www.onlinebiologynotes.com/wp-content/uploads/2017/07/peptidoglycan.jpg)
- In gram negative bacteria, peptide cross linkage occur between Diaminopamilic acid (3rd position) of one glycan back bone and D-alanine of adjacent glycan back bone.
- In gram positive bacteria, peptide cross linkage occur by peptide interbridge. The type and number of aminoacids in interbridge vary among bacterial species.
2. Teichoic acid:
- Teichoic acid is water soluble polymer of glycerol or ribitol phosphate.
- It is present in gram positive bacteria.
- It constitutes about 50% of dry weight of cell wall.
- It is the major surface antigen of gram positive bacteria.
3. Outer membrane
![](https://www.onlinebiologynotes.com/wp-content/uploads/2017/07/Gram-Negative-Bacteria-Cell-Wall-fixed-v1-1024x741.png)
Figure: Outer membrane in cell wall of gram negative bacteria
- It is an additional layer present in gram negative bacteria.
- It is composed of lipid bilayer, protein and lipopolysaccharide (LPS) layer
Function of outer membrane:
- Structure component of gram-ve cell wall
- LPS is an endotoxin produced by gram –ve bacteria
- Lipid-A is antigenic
4. LPS
- LPS is attached to outer membrane by hydrophobic bond. LPS is synthesized in cytoplasmic membrane and transported to outer membrane.
- LPS is composed of lipid-A and polysaccharide.
- Lipid-A: it is phosphorylated glucosamine disaccharide.
- Polysaccharide: it consists of core-polysaccharide and O-polysaccharide.
Gram positive bacteria
Micrococcus
Staphylococcus
Streptococcus
Leuconostoc
Gram negative bacteria
E. coli
Salmonella
Klebsiella
Shigella
Pseudomonas
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