. of serotype O157: H7 are Vero cytotoxin-producing enteric pathogens that have recently been associated with outbreaks of haemorrhagic colitis, sporadic cases of haemorrhagic colitis and with the haemolytic uraemic syndrome. The organisms demonstrate attaching and effacing binding to the caecum and colon of orally infected gnotobiotic piglets, chickens and infant rabbits. O157:H7 cells adhere to the surface but do not invade the cytoplasm of human epithelial cell lines in tissue culture. Since outer membranes, lipopolysaccharides and flagella have been identified as bacterial adhesins on other enteric pathogens, we evaluated their roles in the binding of non-fimbriated O157:H7 to HEp-2 cells. Hyperimmune rabbit antisera were prepared to whole cells, outer membranes and flagella of O157:H7. The presence of antibody to homologous antigen was confirmed by dot blot immunoassays. Both antisera and purified outer membrane and flagellar antigens were co-incubated with bacteria and HEp-2 cells to quantitate inhibition of bacterial attachment. Adherence of O157:H7 to tissue culture cells was inhibited by rabbit antisera raised to whole cells (76.0 ± 5.6% inhibition compared with bacterial adherence in the presence of pre-immune rabbit serum) and outer membranes (69.2 ± 3.4% inhibition). In contrast, inhibition of bacterial attachment to tissue-culture cells was significantly less when two antisera to H7 flagella were co-incubated with O157:H7 and HEp-2 cells (12.4 ± 7.6%; 6.0 ± 3.5% inhibition). Outer-membrane extracts inhibited adherence to O157:H7 to HEp-2 cells in a concentration dependent manner whereas isolated flagella and lipopolysaccharide antigens did not inhibit bacterial attachment. These data suggest that constituents in the outer membranes of O157:H7 mediate binding of the organisms to epithelial cells


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