Bacterial Capsules

Abstract

Capsules are the outmost structures of bacterial and fungal cells. The capsules protect microbial cells from immune recognition and killing during infection of mammalian hosts. Except for the poly-γ-glutamate (PGA) capsule of Bacillus anthracis, other known capsules 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. The number of genes associated with capsule synthesis ranges from one in serotype 37 Streptococcus pneumoniae to >20 in serotype 38 S. pneumoniae. Different bacterial species can share similar genes or mechanisms for capsule synthesis. The expression of the capsule genes is often regulated by environmental conditions. The capsular polysaccharides are the antigens of the current polysaccharide-based vaccines for S. pneumoniae, Neisseria meningitidis, Haemophilus influenzae and Salmonella enterica serovar Typhi. Certain capsule polymers also have important industrial applications.