Generally, extraintestinal E. coli are encapsulated. The capsules are important virulence determinants, which enable the pathogenic bacteria to evade or counteract the unspecific host defense during the early (preimmune) phase of infection. They interfere with the action of complement and phagocytes.

Do all E. coli have a capsule?

Many strains of Escherichia coli produce extracellular polysaccharide capsules (for a review see reference 23). Some capsules have been correlated with pathogenic strains of E. coli, and those that produce K1 antigen (polysialic acid) are often associated with septicemia, urinary tract infections, and meningitis (16).

What types of bacteria are encapsulated?

The term 'encapsulated bacteria' refers to bacteria covered with a polysaccharide capsule. Examples of such bacteria include Streptococcus pneumoniae, Klebsiella, Haemophilus influenzae, Neisseria meningitidis, and Pseudomonas aeruginosa.