Bacterial Capsule: structure, function and examples of Capsulated bacteria




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.

  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

Bacterial Capsule: structure, function and examples of Capsulated bacteria