Genetics of capsular polysaccharide production in bacteria

Many pathogenic bacteria produce an extracellular capsule, composed of any one of a large range of polysaccharides, which plays a crucial role in the interaction with the host. Molecular techniques have been exploited to study the biosynthesis, export and cell surface assembly of these polymers. In the case of Escherichia coli a cassette of biosynthesis genes, unique and specific for a given polysaccharide and found only in bacteria producing this polysaccharide, is flanked on both sides by a cluster of genes which encode functions common to the production of chemically distinct polymers. On one side the DNA encodes products which function at some stage following the polymerisation of the sugar components of the polysaccharide. The nature of the reactions mediated by these products is unclear. To the other side of the biosynthesis cassette the DNA encodes five proteins for export of polysaccharide from the periplasm to the cell surface and its organisation into a capsule.