Capsules

Capsules

	Capsules - Many bacterial species are surrounded by a capsule. This may consist of a loose aggregate of material which may not be strictly a part of the organism. It may also be present in the form of a clearly recognizable capsule. The capsule is not essential for .the survival of the organisms under favourable growth conditions. It, however, provides protection in unfavourable environments. The presence of a, capsule is of importance in the disease-causing ability of some bacteria.
For example, the non-capsulated laboratory strains of pneumococcus (organism causing& pneumonia) are destroyed by the host phagocytes, and are therefore non-infective. The encapsulated strains of the bacterium are protected from phagocytosis, and hence take part in virulence. Capsules may be divided into two categories, macro capsules and microcapsules. Macrocapsules are atleast 0.2 µm thick, and can be seen under the light microscope. Microcapsules cannot be seen under the light microscope but can be demonstrated immunologically. In some aerobic bacteria the capsules form a raft in which are found the actively growing cells. This permits the bacteria to float near the surface, where aerobic conditions are greater. Acetobacter xylinum, which is used to produce wine vinegar and converts ethyl alcohol into acetic acid, synthesizes a loose pellicle of cellulose which permits the organisms to float at the surface of the fermenting vessel.
The capsule may also function in the storage of food substances, and as a storage site for the disposal of waste material. The capsule is secreted by the bacterial cell and is usually composed of polysaccharides or disaccharides, but is sometimes made up of polypeptides. In Acetobacter xylinum the polysaccharide (cellulose) is a homopolysaccharide made up of glncose units linked fH-4. Most bacterial polysaccharides contain more than one type of sugar, i. e. they are heteropolysaccharides. Pseudomonas aeruginosa secretes a heteropolysaccharide consisting of D- glucose, D-galactose, D-mannose, D-glucuronic acid and L-rhamnose residues.
In the pneumococci the capsule is made up of hexoses, uronic acids and amino sugars. In the streptococci the protein capsule consists of L-amino acids. In Bacillus anthracis the polypeptides in the capsule contain D-glutamic acid. Bacterial capsules are species-specific, and can therefore be used for immunological distinction of closely related species. The dextrans produced by Leuconostoc mesenteroides are used as plasma expanders in hospitals. They are also used as laboratory gels for, filtration.

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