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The occurrence of unilateral flagellar phase variation was previously demonstrated in Escherichia coli strains carrying the non-fliC flagellin-specifying locus flk. In this study, we investigated the mechanism involved in this process. By using sequencing and sequence analysis, the flk region between the chromosomal genes yhaC and rnpB was characterized in all described flk-positive E. coli strains, including the H35 strain identified in this study (the other strains used are H3, H36, H47, and H53 strains), and this region was found to contain a putative integrase gene and flanking direct repeats in addition to the flk flagellin-specifying gene flkA and a fliC repressor gene, flkB, indicating that there is a typical genomic islet (GI), which was designated the flk GI. The horizontal transfer potential of the flk GI was indicated by detection of the excised extrachromosomal circular form of the flk GI. By generating fliC-expressing variants of H3 and H47 strains, unilateral flagellar phase variation in flk-positive strains was shown to be mediated by excision of the flk GI. The function of the proposed integrase gene was confirmed by deletion and a complementation test. The potential integration sites of the flk GI were identified. A general model for flagellar phase variation in flk-positive E. coli strains can be expressed as fliC off ؉ flkA on 3 fliC on ؉ flkA none . This is the first time that a molecular mechanism for flagellar phase variation has been reported for E. coli.Flagella are best known for conferring motility to bacteria, which allows the bacteria to swim toward attractants and away from repellents (5). Flagella also play a variety of other roles in many bacterial lifestyles, including bacterial pathogenesis and biofilm formation (27,45). Pathogenic bacteria specifically produce flagella to promote colonization and invasion of mucosa (27). Flagellar antigen, also known as H antigen, is one of the major antigens in gram-negative bacteria. The serological variety of flagella is important for intrageneric differentiation of bacteria (12). Flagellin is the protein subunit of the flagellar filament and determines the specificity of the flagellar antigen.Phase variation of antigenic expression, especially expression of surface structures, such as flagella, fimbria, capsular polysaccharide, and lipopolysaccharide, is a common strategy used by many bacteria for adaptation to particular environments (38). Phase variation of flagellar antigens was first described in Salmonella enterica serovar Typhimurium, in which two different flagellin genes, fljB and fliC, are alternatively expressed, giving rise to two different H phases (18, 50). Flagellar phase variation is thought to be related to bacterial survival in the presence of host defense systems and therefore is linked to bacterial virulence (19). The molecular mechanism for flagellar phase variation in S. enterica has been well studied (1,20,22,40,47,51). Alternating expression of fljB and fliC is controlled by site-specific inversion of an approximately ...

Data for further development of the flagellar antigen genetics of the species Escherichia coli are reported. Two new flagellin genes named fllA and flmA were found in E. coli 781-55, E2987-73, and E223-69, the test strains for E. coli flagellar antigens H44, H55, and H54, respectively (collection of the International Escherichiaand Klebsiella Centre of the World Health Organization, Copenhagen, Denmark). Two alleles of fllAwere identified that encode flagellar antigens H44 (fllA 44) and H55 (fllA 55), and the only flmA allele found (flmA 54) encodes antigen H54. The sites of their integration in the E. coli K-12 chromosome after P1-mediated transduction were approximately determined and found to be separate from each other and from the known regions of flagellar genes of E. coli and salmonellae. The region offlm 54 was found to repress the expression of some alleles of the flagellin gene fliC. In addition, cryptic genes encoding antigens H4 and H38 were found in phenotypically monophasic test strains 781-55 and E2987-73, respectively.

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We have now studied 13 STEC O157:H7 strains and 1 O55:H7 strain that were epidemiologically unrelated, that originated from six countries on two continents, and that had different profiles when analyzed by multilocus enzyme electrophoresis, pulsed-field gel electrophoresis, and PCR for stx and eae. They were all found to possess the H7a,c flagellar antigen. Serum cross-absorption assays confirmed that their H antigens were indistinguishable from each other and from that of E. coli O55:H7a,c but differed from the standard H7a,b antigen of E. coli H test strain U5/41. It was shown by phage-mediated transduction that the flagellin genes for these two H-antigen subserotypes were alleles of the E. coli fliC locus. On the basis of the serological data obtained in this study and the molecular characteristics of E. coli fliC H7 alleles recently published, it is inferred that H7a,c and H7a,b are the main serological subtypes of the group of E. coli H7 flagellins.Escherichia coli flagella, called the H antigen in classical bacteriology, are fine long tubular structures, the walls of which consist of flagellin subunits. The serological specificities of the H antigens are determined by epitopes that are displayed exclusively on the outer flagellar surface and that are recognizable by the classical method of agglutination of flagellated whole bacterial cells.Absorbed monovalent H-antigen antisera are highly specific in H-antigen identification (4,25,39,43). The diversity and genetics of H antigens are complex in the natural population of the E. coli species. Numerous different flagellar antigens and at least five different flagellin-specifying loci, fliC (fliCЈ and fliCЉ), flkA, fllA, and flmA, are distributed at distantly located positions on the chromosomes (23,26,27) among the strains of the species. About 15% of the H-antigen test strains possess nonfliC or double fliC (one strain) flagellin-encoding genes (26). In all, 53 E. coli H antigens have been officially registered and named by successive numbers in order of their description: H1 to H12, H14 to H21, H23 to H49, and H51 to H56 (4, 20). However, the variety of flagellar antigens in the E. coli natural population is much greater: there are other H antigens serologically unrelated to the 53 reference antigens (24,27,28,31), and fine serological distinctions exist between antigens of the same name (number) in different strains. This concerns many H antigens (for instance, H1, H2, H3, H4, H6, H7, H8, H10, H12, H18, and H34), and special designations such as H7a,7b and H7a,7c (briefly, H7a,b and H7a,c, respectively) have been proposed as designations for the subtypes (2,17,18,19,21,29,30,32). Thus, some E. coli O:H serotypes may consist of several distinct H subserotypes that differ in the fine structures of their H antigens, e.g., O55:H2a,b and O55: H2a,c (30, 32).E. coli O55:H7 is considered an ancestor of E. coli O157:H7 (5, 41, 42). Two variants of the former serotype have been reported; one (O55:H7a,c) possesses an H antigen distinct from the standard H7 antigen ...