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Theafa gene clusters, which encode proteins involved in adhesion to epithelial cells, fromEscherichia coli strains associated with urinary and intestinal infections in humans have been characterized. Pathogenic isolates of bovine and porcine origin that possessafa -related sequences have recently been described. We report in this work the cloning and characterization of theafa-7 andafa-8 gene clusters from bovine isolates. Hybridization and sequencing experiments revealed that despite similarity in genetic organization, theafa-7 andafa-8 genes, and the well-characterizedafa-3 operon expressed by human-pathogenic isolates, correspond to three different members of theafa family of gene clusters. However, like theafa-3 gene cluster, both theafa-7 andafa-8 gene clusters were found to encode an afimbrial adhesin (AfaE) and an invasin (AfaD). The AfaD peptides encoded by the three gene clusters were only 45% identical, but functional complementation experiments indicated that they belong to the same family of invasins. Hemagglutination and adhesion assays demonstrated that the AfaE-VII and AfaE-VIII adhesins bind to different receptors and that these receptors are not the human decay-accelerating factor recognized to be the receptor of all previously described AfaE adhesins. The AfaE-VIII adhesin is very similar to the M agglutinin of human-uropathogenic strains. We used PCR assays to screen 25 bovine strains forafaD andafaE genes of either theafa-7 orafa-8 gene cluster. Theafa-8 gene cluster was highly prevalent in bovine isolates previously reported to carryafa -related sequences (23 of 24 strains), particularly in strains producing cytotoxic necrotizing factors (16 of 16 strains). The location of theafa-8 gene cluster on the plasmids or chromosome of these isolates suggests that it could be carried by a mobile element, facilitating its dissemination among bovine-pathogenicE. coli strains.

infection and immunity

Molecular Cloning and Characterization of the afa-7 and afa-8 Gene Clusters Encoding Afimbrial Adhesins in Escherichia coli Strains Associated with Diarrhea or Septicemia in Calves