Many bacteria can express polysaccharide capsules on their cell surface, which may be important virulence factors. Escherichia coli is able to express more than 70 distinct capsular polysaccharides, or K antigens, which are organised into four groups on the basis of biochemical, genetic and regulatory properties. The K5 antigen belongs to group 2 and is expressed by extra-intestinal isolates of E. coli associated with urinary tract infections and septicaemia.