Bacterial Capsule: structure, function and examples of Capsulated bacteria

Capsule

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 L-aminoacids.
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

There are two types of capsule.

Prevent the cell from desiccation and drying: capsular polysaccharide bind significant amount of water making cell resistant to drying
Protection: it protect from mechanical injury, temperature, drying etc
Attachment: capsule helps in attachment on the surface. Eg. Streptococcus mutants that cause dental carries attach on teeth surface by its capsule.
Anti-phagocytic : Capsule resist phagocytosis by WBCs
Capsule prevent attachment of bacteriophage on cell surface
Source of nutrition: capsule is source of nutrition when nutrient supply is low in cell.
Repulsion: same charge capsulated bacteria repel each other.

Bacillus subtilis

Bacillus anthracis ( contains polypeptide capsule)

Streptococcus pneumoniae

Klebsiella pneumoniae

Haemplhilus influenza

Clostridium perfingens

Neisseria meningitidis

Pseudmonas aeruginosa

Acenetobacter calcoaceticus