Capsule are the outmost structures of bacterial and fungal. The capsules protect microbial cells from immune and killing during infection of mammalian hosts.
Except for the poly-γ-glutamate (PGA) capsule of Bacillus anthracis, other known capsule are all composed of polysaccharides. Certain bacteria (e.g. B. anthracis and Streptococcus pyogenes produce only one capsule structure, whereas many other bacteria express capsules with great biochemical, structural and immunological diversity within the same species.
This diversity is driven by immune selection from the mammalian hosts. The genes for capsule synthesis are typically clustered in a single locus of bacterial chromosome.