Capsule ( capsule glycocalyx) is an extra cellular polymeric substance that envelopes the bacterial cell. If the gelatinous substance is tightly attached to the cell wall, it is called as capsule. If it is too thin to be visible in the electron microscope, it is termed as microcapsule. If this material is loosely bound to the cell wall, it's is termed as slime layer.
Capsule can be seen by negative staining.
It may be composed of polysaccharide (eg. Klebsiella pneumoniae) or polypeptide ( eg . Bacillus anthracis).
The polysaccharide may be a homopolysaccharide ( derived of single type of sugar) eg. S. mutants or heteropolysaccharide ( derived of different sugars) eg. Klebsiella pneumoniae.
Bacillus anthracis produces a capsule composed of D-glutamic acid.
Functions of Capsule:
- It prevents the bacterial cell from dessication due to the water holding capacity of the capsule.
- It prevents the attachment of bacteriophages.
- It prevents the phagocytosis by WBC hence contributes to the virulence of bacteria. eg. Capsulated strains of Streptococcus pneumoniae is more virulent as compared to uncapsulated strain of Streptococcus pneumoniae.
- It encourages the attachment of bacterial cell on various surfaces. eg. Capsule of Streptococcus mutans is composed of glucan which is insoluble in water and attached to smooth surface of the teeth leading to dental caries.
- It act as an energy source. S. mutans breaks down the sugars of its capsule when stored energy is in low amount.
- If capsules are composed of compounds having electrical charge such as sugar uronic acids, they may promote stability of the bacterial suspension as similarly charged surfaces tend to repel each other.
