The K1 capsular polysaccharide of Escherichia coli

Epidemiologic, immunologic, and genetic evidence indicate that the K1 capsular polysaccharide confers invasiveness to Escherichia coli. The capsule, an alpha-2----8-linked homopolymer of sialic acid (NeuNAc), provides the bacterium with a physical antiphagocytic barrier. Structural similarities between K1 and human tissue components suggest that immune tolerance may also be a factor in pathogenesis of K1 disease. The molecular and genetic events involved in the synthesis and export of the K1 polysaccharide were examined. The cloned K1 genes encode at least 12 proteins involved in capsule biosynthesis. These genes appear to be coordinately regulated and functionally clustered. One cluster is associated with the synthesis and activation of NeuNAc and includes the gene encoding CMP-NeuNAc synthetase. This enzyme catalyzes the activation of NeuNAc to CMP-NeuNAc. A second region, encoding five proteins, is associated with translocation of polysaccharide to the bacterial surface. The K1 polysaccharide is a poor immunogen in humans, and an understanding of the key reactions involved in K1 synthesis may help in providing an alternative to anticapsular immunity.