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Polysialic acid (PSA) is a capsular polysaccharide of Escherichia coli composed of a linear polymer of N-acetylneuraminic acid (Neu5Ac) with αt-(2,8) linkages.The polysialic acid (PSA) capsule is an essential virulence determinant enabling E.coli to evade normal host defense mechanisms as molecular mimicry.Purified PSA is a poor immunogen in humans and other mammals.It does not stimulate synthesis of antibodies required for phagocytic removal of the organism.