VMC 211: General Veterinary Microbiology (1+1)

CAPSULE

• Some bacteria are covered by a viscous substance. It is called glycocalyx. This forms an envelope around the cell wall. It may be in the form of a capsule or a slime layer. Slime layer is loosely associated with the bacterial cell. It can be easily washed off.  Capule is tightly attached to the cell and has definite boundaries. Capsule can be classified in to two types. If it is seen by special staining methods it is called a capsule. If it is too thin to be seen by light microscopy then it is called as microcapsule.
• Most capsules are made of polysaccharides. Capsules made of single type of sugar are called homopolysaccharides. They are synthesized outside the cell from disaccharides by extracellular enzymes.
• Capsules made of many types of sugars are called heteropolysaccharides. These are synthesized from sugar precursors that are activated within the cell, attached to a lipid carrier, transported outside and polymerized.
• A few capsules are made of polypeptides. Eg: Capsule of Bacillus anthracis. It is made of a polymer of glutamic acid.

 Blood smear - Bacillus anthracis - Pink stained capsule

 

Functions

• Provide protection against temporary drying by binding water molecules.
• Block attachment of bacteriophages.
• Capsule is antiphagocytic. It inhibits engulfment by leuckocytes. So it contributes the virulence.
• Promote attachment of bacteria to surfaces.
• When the capsule is having compounds with an electric charge the stability of the bacteria suspension is increased. This is by prevention of cells from aggregating and settling out because cells bearing similar charges repel each other.