Bacterial Capsule: structure, function and examples of
Capsulated bacteria
Capsule
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Capsule is 0.2µm thick viscus layer firmly attached to the
cell wall of some capsulated bacteria.
If capsule is too thick it is known as slime.
Slime layer are loosely attached to cell wall and can be
lost on vigorous washing and on sub culture.
Composition of capsule: 98% water and 2%
polysaccharide or glycoprotein/ polypeptide or both.
In case of Acetic acid bacteria, capsule is composed of
homopolysaccharide (hemicellulose)
leuconostoc: capsule is composed of cellulose, consisting
of glucose or fructose.
Klebsiella pneumoniae: capsule is made up of glucose,
galactose,rhamnose etc.
In Bacillus anthracis: capsule is made up of Polypeptide
(Polymer of D-glutamic acid) and in Streptococci, it is Laminoacids.
Capsule is very delicate structure. It can be removed by
vigorous washing.
Capsule is most important virulence factor of bacteria.
Capsule in visualized by Negative staining technique
Types of capsules:
1. Macro-capsule: thickness of 0.2µm or more, visible
under light microscope
2. Microcapsule: thickness less than 0.2µm, visible under
Electron microscope
Function of capsule:
1. Prevent the cell from desiccation and drying: capsular
polysaccharide bind significant amount of water making
cell resistant to drying
2. Protection: it protect from mechanical injury,
temperature, drying etc
3. Attachment: capsule helps in attachment on the surface.
Eg. Streptococcus mutants that cause dental carries attach
on teeth surface by its capsule.
4. Anti-phagocytic : Capsule resist phagocytosis by WBCs
5. Capsule prevent attachment of bacteriophage on cell
surface
6. Source of nutrition: capsule is source of nutrition when
nutrient supply is low in cell.
7. Repulsion: same charge capsulated bacteria repel each
other.
Examples of Capsulated bacteria:
Bacillus subtilis
Bacillus anthracis ( contains polypeptide capsule)
Streptococcus pneumoniae
Klebsiella pneumoniae
Haemplhilus influenza
Clostridium perfingens
Neisseria meningitidis
Pseudmonas aeruginosa
Acenetobacter calcoaceticus
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