In this article we will discuss about the meaning and functions of capsule.
Meaning of Capsule:
Some of the bacterial cells are surrounded by the extracellular polymeric substances (EPS) which are commonly called capsule or glycocalyx. It forms an envelope around the cell wall and can be observed under light microscope after special staining technique (Fig.4.2). The presence of capsule may be detected by negative staining also such as India ink method.
The capsule is gelatinous polymer made up of either polysaccharide (Klebsiella pneumoniae) or polypeptide (S. anthracis) or both. The polysaccharides may be of a single type of sugars (homopolysaccharide) or several types of sugars (heteropolysaccharides). The heteropolysaccharide is synthesized by sugar precursors within the cell.
Homopolysaccharide constitutes the capsule of Acetobacter xylinum, and heteropolysaccharide (consisting of D-glucose, D-galactose, D-mannose, D-gluconic acid and D-rhamnose) is secreted by Pseudomonas aeruginosa. The capsule of pneumococci is made up of hexoses, uronic acids and amino sugars and that of streptococci consists of L-amino acids.
The bacterial capsule is species specific and, therefore, can be used for immunological differentiation of related species. Amount of these polymers vary with bacterial species. It is sticky in nature and secreted from the inner side of cell which gets firmly attached to the surface of cell wall.
If the substances are unorganised and loosely attached to cell wall, the capsule is called slime layer. The fresh water and marine bacteria form trichomes which are enclosed inside the gelatinous matrix called sheath. Sheath is also found in cyanobacteria and other algae.
Functions of Capsule:
Capsule may have a number of functions according to bacterial species. Costerton (1981) have reviewed the function of bacterial glycocalyx.
Some of the functions are discussed below:
(i) The capsule may prevent the attachment of bacteriophages.
(ii) It protects the bacterial cells against desiccation as it is hygroscopic and contains water molecules.
(iii) It may survive in natural environment due to its sticky property. After attachment they can grow on diverse surfaces e.g. plant root surfaces, human teeth and tissues (dental carries, respiratory tract), rocks in fast flowing streams, etc.
(iv) They may inhibit the engulfment by WBCs (anti-phagocytic feature) and, therefore, contribute to virulence. Capsule protects from phagocytosis for example the capsulated strains of Streptococcus pneumoniae causes pneumonia and un-capsulated strain is phagocytized.
(v) S. mutans uses its capsules as a source of energy. It breaks down the sugars of capsule when stored energy is in low amount.
(iv) Capsule protects the cell from desiccation, maintains the viscosity and inhibits the movement of nutrients from the bacterial cell.