Polysaccharide antigens of Escherichia coli

The major surface antigens of Escherichia coli are the cell wall lipopolysaccharides (LPS; O antigens) and the capsular polysaccharides (PS; K antigens). These polysaccharides are synthesized at the cytoplasmic membrane of the bacteria; the LPS are transported to the outer membrane, where they reside, whereas the PS are secreted into capsules. The LPS consist of lipid A covalently linked to the core oligosaccharide, which itself is covalently linked to the O-specific polysaccharide. The latter, which determines the O specificity of the bacteria may be neutral or contain negative charges (carboxyl groups or phosphate). The relatedness of E. coli to other genera (e.g., Klebsiella or Shigella) frequently is borne out by structural identity. This intergeneric relation is paralleled by similar pathogenic properties of the bacteria in question. The capsular antigens of E. coli are acidic polysaccharides, which can be divided into groups (I and II) on the basis of molecular size, nature of the acidic component, coexpression with O antigens, and temperature regulation of their biosynthesis. The major acidic components are hexuronic acid (mainly of Klebsiella-like group I) as well as 2-keto-3-deoxy-D-mannooctulonic acid, N-acetylneuraminic acid, or phosphate (mainly in Neisseria- and Haemophilus-like group II). Relatedness of encapsulated E. coli to encapsulated bacteria of other genera (Neisseria, Haemophilus, Klebsiella) is based on structural identity or similarity of the respective capsules. Identity not only refers to structure and serology of these capsules but also to the pathogenicity of the respective bacteria (e.g., E. coli K1 and Neisseria meningitidis b). Bacterial pathogenicity may be caused by the host's inability to raise an immune response to bacterial capsules (E. coli K1 and K5) because of the identity of the capsular polysaccharides and the host carbohydrates. This can be described as camouflage used by the bacteria as a strategem for bacterial virulence.