Introductory Microbiology

Capsules and slime

• Many prokaryotic organisms synthesise loose amorphous organic exopolymers which are deposited outside the cell wall called the capsules or the slime. The term capsule refers to the layer tightly attached to the cell wall while the slime layer is the loose structure that often diffuses into the growth medium. These structures are not essential for growth and development of bacterial cells but their presence confers certain advantages to the bacterial cells that possess these structures.
• In fact, many bacteria do not produce neither a capsule nor a slime and those which can produce can lose the ability to synthesize these compounds without any ill effects. Primary interest in these exopolymers was for determining their role in pathogenicity since most pathogenic organisms are found to produce a capsule or slime.
• Composition of exopolymers varies with bacteria which contains amino acids (D- Glutamic acid L- Glutamic acid) or sugars (glucose, rhamnose, galactose, fructose and fructose and sugar derivatives).
Functions of capsule:
1. Capsule may prevent the attachment of bacteriophages.
2. It protects the bacterial cell against desiccation as it is hygroscopic and contains water molecules.
3. It may survive in natural environment due to its sticky property. After attachment they can grow on diverse surfaces. E.g., root surfaces, of human teeth and tissues, rocks in fast growing stream etc.
4. They may inhibit the engulfment by white blood cells (WBC’s) and therefore contribute to virulence.
5. Protects the cell from desiccation, maintaines the viscosity and inhibit the movement of nutrient from the bacterial cell.
6. Acts as sources of energy for some bacterial species.