Bacterial capsule

A cell capsule (or glycocalyx) is a very large organelle in some prokaryotic cells, such as cells of bacteria. It is a layer that lies outside the cell wall of bacteria. This layer is well organized and not easily washed off. It is usually composed of polysaccharides, but could be composed of other materials (e.g., polypeptide in B. anthracis). Because the capsule helps to protect bacteria against phagocytosis, the capsule is considered a virulence factor. A capsule-specific antibody may be required for phagocytosis to occur. Capsules also contain water which protects bacteria against desiccation. They also exclude bacterial viruses and most hydrophobic toxic materials such as detergents. The capsule is found most commonly among gram-negative bacteria, such as E.coli. However, some gram-positive bacteria may also have a capsule. B.megaterium for example, synthesizes a capsule composed of polypeptide and polysaccharides and S. pyogenes synthesizes a hyaluronic acid capsule.

The cell capsule is also known as "Outer Membrane" or slime layer or exopolysaccharide layer. It is formed when excess carbon is available with the cell. The layer can be homopolysaccharide or heteropolysaccharide. Vaccination using capsular material is effective against some organisms (e.g., H. influenzae type b and S. pneumoniae).