Pathogenicity of a bovine attaching effacing Escherichia coli isolate lacking Shiga-like toxins.

Escherichia coli isolate 7996-90, obtained from a calf with diarrhea, had negative results of tests for K-88, K-99, 987P, F41, CS31A, F1845, F165 or E. coli adherence factor adhesins and had negative results of tests for the toxins heat-labile, heat-stable A, heat-stable B, Shiga-like toxin (SLT)-I or SLT-II. Strain 7996-90 had localized adherence to HEp-2 cells, caused actin rearrangement in host cells to which it adhered, hybridized with the eaeA probe, and produced the 94-kd outer membrane protein associated with attaching effacing lesions. This isolate caused attaching effacing lesions in Caco-2 cell polar monolayers, rabbit intestinal loops, and the intestines of gnotobiotic pigs. The isolate belongs to serotype O26: NM and is considered a class-II attaching effacing enteropathogenic E coli. Until recent addition of more sensitive assays at veterinary diagnostic laboratories, isolates such as 7996-90 were not readily recognized as pathogens because they failed to fit into the enterohemorrhagic E coli group, members of which, be definition, produce SLT. The assays described can facilitate diagnosis of attaching effacing E coli infection when histologic evaluation is hampered by autolysis.