Structure and Assembly of Escherichia coli Capsules

Abstract

The capsule is a cell surface structure composed of long-chain polysaccharides that envelops many isolates of Escherichia coli . It protects the cell against host defenses or physical environmental stresses, such as desiccation. The component capsular polysaccharides (CPSs) are major surface antigens in E. coli . They are named K antigens (after the German word K apsel ). Due to variations in CPS structures, more than 80 serologically unique K antigens exist in E. coli . Despite the hypervariability in CPS structures, only two capsule-assembly strategies exist in E. coli . These have led to the assignment of group 1 and group 2 capsules, and many of the key elements of the corresponding assembly pathways have been resolved. Structural features, as well as genetic and regulatory variations, give rise to additional groups 3 and 4. These employ the same biosynthesis processes described in groups 2 and 1, respectively. Each isolate possesses a distinctive set of cytosolic and inner-membrane enzymes, which generate a precise CPS structure, defining a given K serotype. Once synthesized, a multiprotein complex is needed to translocate the nascent CPS across the Gram-negative cell envelope to the outer surface of the outer membrane, where the capsule structure is assembled. While the translocation machineries for group 1 and group 2 CPSs are fundamentally different from one another, they possess no specificity for a given CPS structure. Each is conserved in all isolates producing capsules belonging to a particular group.