ROLE OF THE ESCHERICHIA-COLI O157-H7 O-SIDE-CHAIN IN ADHERENCE AND ANALYSIS OF AN RFB LOCUS

Shiga-toxigenic Escherichia coli strains belonging to serotype O157 ar e important human pathogens, but the genetic basis of expression of th e O157 antigen and the role played by the lipopolysaccharide O side ch ain in the adherence of this organism to epithelial cells are not unde rstood, We performed TnphoA mutagenesis on E. coli O157:H7 strain 86-2 4 to identify a mutant (strain F12) deficient in O-antigen expression, Nucleotide sequence analysis demonstrated that the transposon inserte d within an open reading frame with significant homology to rfbE of Vi brio cholerae O1 (U, H, Stroeher, L. E, Karageorgos, R Morona, and P. A. Manning, Proc, Natl, Acad, Sci, USA 89:2566-2570, 1992), which is p ostulated to encode perosamine synthetase. This open reading frame was designated rfbE(EcO157:H7). The guanine-plus-cytosine fraction (0.35) suggests that rfbE(EcO157:H7) may have originated in a species other than E. coli. rfbE(EcO157:H7) is conserved in nontoxigenic E, coli O15 7 strains expressing a variety of other flagellar antigens but is not found in E. coli O55:H7 strains, which are more closely related to E, coli O157:H7, Strain F12 was significantly more adherent to HeLa cells in a quantitative adherence assay than was its E, coli O157:H7 parent , but they did not differ in other phenotypes, Restoration of the expr ession of the O side chain by complementation of the TnphoA mutation i n strain F12 by a plasmid expressing intact rfbE(EcO157:H7) reduced th e adherence of the hyperadherent strain F12. We conclude that rfbE(EcO 157:H7) is necessary for the expression of the O157 antigen, that acqu isition of E. coli rfb genes occurred independently in E, coli O157:H7 and unrelated O157 strains, and that the O side chain of E, coli O157 :H7 lipopolysaccharide interferes with the adherence of E. coli O157:H 7 to epithelial cells.