Structure and biology of Shigella flexneri O antigens.

The species Shigellajiexneri comprises a serologically heterogeneous group of dysentery bacilli whose 0 antigens are polysaccharide-lipid Apolypeptide-lipid B complexes strictly comparable in their gross structural and biological properties to the analogous antigens present in all gram-negative bacilli of the family Enterobacteriaceae (Simmons, 1971a). As shown in fig. 1 , the lipopolysaccharide (LPS) component of these antigens comprises two distinct regions-a common basal structure shared by all S . JIexneri serotypes (except serotype 6) and 0-specific side-chains that determine the serological specificity and cross-reactivity of the whole antigen (Simmons, 1966). The common basal sugars 2-keto-3-deoxy-octonate, L-glycero-D-manno-heptose phosphate, D-glucose, D-galactose and Nacetyl-D-glucosamine are incorporated into the growing basal structure in that order and defects in the genes controlling this biosynthetic pathway result in an incomplete basal structure of rough (R) specificity (Johnston et al., 1967). The 0-specific side-chains (each consisting of six to eight repeating pentasaccharide units) may be further subdivided into two regions-a primary unbranched chain of L-rhamnose and N-acetyl-D-glucosamine identical to the variant Y antigen, and secondary side-chains of a-D-glucosyl and 0-acetyl residues which are substituted on the primary chain in a position that is unique for each serotype (Simmons, 1969). The specificity of the linkage of the glucose secondary side-chain is governed by specific UDP-glucose transferases that are phage-dependent and map near the lac locus (Manson et al., 1970). Loss of the phage attachment site in recombination experiments with Escherichia coli HfrC (lac+) strains usually leads to loss of the glucose secondary sidechains and, thus, to the production of lactosefermenting hybrids of variant Y-type specificity (Manson et al., 1970). However, hybrids with other specificities have also been described (Romanowska and Lachowicz, 1970 ; Katzenellenbogen et al., 1973; Lugowski et al., 1975). In an earlier review (Simmons, 197 la), structural and genetic aspects of the biosynthesis of the S. JIexneri 0 antigens were considered in detail. The conclusions drawn at that time about the structural changes involved in smooth to rough (S-+R) mutation, in the production of X and Y variants, and in the modification of type-specificity by lysogenic conversion remain valid today. However, some of the structures themselves require to be revised, largely as a result of methylation studies performed during the past decade by Lindberg and his colleagues (Lindberg et al., 1973; Kenne et al., 1977a and b; Kenne et al., 1978). This review summarises the evidence for these currently accepted structures and indicates how these studies have elucidated the biology of the S. J-lexneri 0 antigens.