The bacterial capsule is a large structure of many bacteria. It is a layer of polysaccharide that is found outside the cell envelope and is therefore considered part of the outer envelope of a bacterial cell. It is a well organized coat, it does not wash easily, and it can be the cause of various diseases.
The capsule has a 0.2 µm thick viscous layer firmly attached to the cell wall of some encapsulated bacteria.
If the capsule is too thick it is known as slime.
The slime layer adheres weakly to the cell wall and can be lost with vigorous washing and subculture.
Capsule composition: 98% water and 2% polysaccharide or glycoprotein / polypeptide or both.
In the case of acetic acid bacteria, the capsule is composed of homopolysaccharide (hemicellulose)
leuconostoc: capsule is composed of cellulose, composed of glucose or fructose.
Klebsiella pneumoniae: the capsule is made up of glucose, galactose, rhamnose, etc.
In Bacillus anthracis: the capsule is made up of a polypeptide (D-glutamic acid polymer) and in Streptococci, it is L-amino acids.
The capsule has a very delicate structure. It can be removed by vigorous washing.
The capsule is the most important virulence factor of bacteria.
Capsule in visualized by negative staining technique
Types of Capcules:
Macro-capsule: thickness of 0.2µm or more, visible under light microscope
Microcapsule: thickness less than 0.2µm, visible under Electron microscope
Function 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.
Examples of Capsulated bacteria/yeasts:
Mneomonics to remember capsulated bacteria– Some Killers Have Pretty Nice Capsule
Capsule compositions of encapsulated organisms.
Streptococcus pneumoniae (Pneumococcus)
Neisseria meningitidis (Meningococcus)
Streptococcus pyogenes (some strain)
Importance of Bacterial Capsule
Virulence determinants: Capsules are anti-phagocytic. They limit the ability of phagocytes to engulf the bacteria. The smooth nature and negative charge of the capsule prevents the phagocyte from adhering to and engulfing the bacterial cell. If a pathogenic bacteria lose capsule (by mutation), they wont be able to cause disease (i.e. loses disease causing capacity).
Saving engulfed bacteria from the action of neutrophil: Bacterial capsule prevents the direct access of lysosome contents with the bacterial cell, preventing their killing.
Prevention of complement-mediated bacterial cell lysis.
Protection of anaerobes from oxygen toxicity.
Identification of bacteria:
Using specific antiserum against capsular polysaccharide. E.g. Quellung reaction
Colony characteristics in culture media: Bacteria with capsules form smooth (S) colonies while those without capsules form rough (R) colonies. A given bacterial species may undergo a phenomenon called S-R variation whereby the cell loses the ability to form a capsule. Some capsules are very large and absorb water; bacteria with this type of capsule (e.g., Klebsiella pneumoniae) form mucoid (M) colonies.
Development of Vaccines: Capsular polysaccharides are used as the antigens in certain vaccines. For examples:
Polyvalent (23 serotypes) polysaccharide vaccine of Streptococcus pneumoniae capsule.
Polyvalent (4 serotypes) vaccine of Neisseria meningitidis capsule.
A monovalent vaccine made up of capsular material from Haemophilus influenzae.
Initiation of infection: Capsules helps the organism to adhere to host cells.The capsule also facilitates and maintains bacterial colonization of biologic (e.g. teeth) and inanimate (e.g. prosthetic heart valves) surfaces through formation of biofilms.