The K1 capsular polysaccharide of Escherichia coli

Epidemiologic, immunologic, and genetic evidence indicate that the Kl capsular polysac-charide confers invasiveness to Escherichia coli. The capsule, an a-2—>8-linked homopolymer of sialic acid (NeuNAc), provides the bacterium with a physical antiphago-cytic barrier. Structural similarities between Kl and human tissue components suggest that immune tolerance may also be a factor in pathogenesis of Kl disease. The molecular and genetic events involved in the synthesis and export of the Kl polysaccharide were examined. The cloned K1 genes encode at least 12 proteins involved in capsule biosynthe-sis. These genes appear to be coordinately regulated and functionally clustered. One clus-ter is associated with the synthesis and activation of NeuNAc and includes the gene en-coding 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 Kl polysaccharide is a poor immunogen in humans, and an understanding of the key reactions involved in Kl synthesis may help in providing an alternative to anticapsular immunity. Capsular polysaccharides are important virulenc