Escherichia coli is a clonal species. The best-understood components of its clonal variation are the flagellar (H) and polysaccharide (O) antigens, both well documented since the mid-1930s because of their use in serotyping. Flagellin is the protein subunit of the flagellum that carries H-antigen specificity. We show that 43 of the 54 H-antigen specificities of E. coli map to the flagellin gene at fliC and sequenced all 43 forms and confirmed specificity of each by cloning and expression. This is, to our knowledge, the first time that all known forms of such a highly polymorphic gene have been fully sequenced and characterized for any species. The established distinction between a highly variable central region and more conserved flanking regions is upheld. The sequences fall into two groups, one of which may be derived from the fliC gene of the E. coli/Salmonella enterica common ancestor, the other perhaps obtained by lateral transfer since species divergence. Comparison of sequences revealed that both horizontal DNA transfer and fixation of mutations under diversifying selection pressure contributed to polymorphism in this locus.The O polysaccharide and flagellin are the two major antigens of gram-negative bacteria, also known respectivly as the O and H antigens. Both are highly polymorphic, and Escherichia coli, if one includes the Shigella strains, has 187 O and 53 H forms defined by serology (4,6,15,21). In this study, we show that 43 of the 53 H forms map to the fliC locus and have sequenced all 43 alleles. In some strains the H-antigen phenotype maps to alternative loci, so we cloned, sequenced, and expressed the fliC gene from type strains to relate definitively H-antigen specificity and sequence. These data supplement the genome sequence data of E. coli K-12 and O157:H7 to give more comprehensive genetic information on the species and, in conjunction with the recently published structure (31) of one flagellin form, will allow analysis of the structural basis of the antigenic variation and development of a molecular typing scheme for the H antigen.The bacterial flagellum projects well beyond the surface of the cell and is rotated to provide motive power. The flagellar filament is composed of a single protein, flagellin. The flagellin proteins of E. coli and several other species are conserved in their terminal regions, while the central region is variable and carries H-serotype-specific epitopes (9,17,22,39,40). The structure of the Salmonella enterica LT2 flagellum is known from electron microscopy, X-ray fiber diffraction, and X-ray crystallography. Three domains are recognized (Fig. 1). The conserved terminal segments form the D1 domain located in the center of the flagellum, while the central region of the protein forms two domains (D2 and D3) exposed on the surface (31). The boundaries between D1 and D2 correspond quite well to the boundaries between the central and terminal regions of the protein as determined by alignment of sequences of different forms. However, because we are dealing mostly with...

Escherichia coli O55 is an important antigen which is often associated with enteropathogenic E. coli clones. We sequenced the genes responsible for its synthesis and identified genes for O-antigen polymerase, O-antigen flippase, four enzymes involved in GDP-colitose synthesis, and three glycosyltransferases, all by comparison with known genes. Upstream of the normal O-antigen region there is a gne gene, which encodes a UDP-GlcNAc epimerase for converting UDP-GlcNAc to UDP-GalNAc and is essential for O55 antigen synthesis. The O55 gne product has only 20 and 26% identity to the gne genes of Pseudomonas aeruginosa and E. coli O113, respectively. We also found evidence for the O55 gene cluster's having evolved from another gene cluster by gain and loss of genes. Only three of the GDP-colitose pathway genes are in the usual location, the other two being separated, although nearby. It is thought that the E. coli O157:H7 clone evolved from the O55:H7 clone in part by transfer of the O157 gene cluster into an O55 lineage. Comparison of genes flanking the O-antigen gene clusters of the O55:H7 and O157:H7 clones revealed one recombination site within the galF gene and located the other between the hisG and amn genes. Genes outside the recombination sites are 99.6 to 100% identical in the two clones, while most genes thought to have transferred with the O157 gene cluster are 95 to 98% identical.Escherichia coli is a clonal species, with clones normally identified by their combination of O and H (and sometimes K) antigens. E. coli O55:H7 and O157:H7 are important pathogenic clones causing serious diseases in humans (20, 33). Multilocus enzyme electrophoresis of a large number of E. coli strains has shown that E. coli O157:H7 isolates belong to a distinct group which also contains E. coli O55:H7 (9,26,43,44). The H7 fliC genes of O55:H7 and O157:H7 strains are almost identical but differ from those of strains with other O antigens (29, 41). Thus, it has been proposed that transfer of O157 O-antigen genes into an O55:H7 strain was one of the events resulting in the origin of the O157:H7 enterohemorrhagic E. coli clone (34).The O antigen contributes major antigenic variability to the cell surface, and on the basis of this antigenic variation, 166 O forms have been recognized in E. coli (not including Shigella strains). The surface O antigen is subject to intense selection by the host immune system, which may account for maintenance of the many different O-antigen forms within species such as E. coli. The genes specific to O-antigen synthesis in E. coli are commonly clustered adjacent to the gnd gene between the colanic acid (CA) and his operons (28).Lateral transfer of large DNA segments is thought to have played an important role in the evolution of bacterial pathogens. The evidence is usually the presence of genes with atypical GC content and/or the distribution of pathogenicity islands or other gene clusters. We, among others, have undertaken extensive studies on O-antigen genes by sequencing and identifying the O-antigen ...

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Flagellar (H) antigens are mostly encoded by genes at thefliC locus in E. coli. We have sequenced 11 H7fliC genes from Escherichia coli strains that belong to seven O serotypes. These sequences, together with those of nine other H7 fliC genes (from strains of three different O serotypes) sequenced recently (S. D. Reid, R. K. Selander, and T. S. Whittam, J. Bacteriol. 181:153–160, 1999), include 10 different sequences. The differences between these 10 sequences range from 0.06 to 3.12%. By comparison with other E. coliflagellin genes, we have identified primer length sequences specific for H7 genes in general and others specific for H7 genes of O157 and O55 strains: the specificity was confirmed by PCR testing the type strains for all 53 E. coli H types. We have previously identified genes specific for the E. coli O157 antigen, and use of the combination of O157- and H7-specific primers allows the sensitive and rapid detection of O157:H7 E. coli strains, which cause the majority of hemorrhagic colitis cases.