Scanning and transmission electron microscopic study of the small intestine of colostrum-fed calves infected with selected strains of Escherichia coli.

Colostrum-fed calves were fed milk replacer containing 10(11) enteropathogenic Escherichia coli (EEC) or nonenteropathogenic E coli (NEEC). The NEEC failed to colonize the small intestine or to be associated with the intestinal wall. The EEC colonized the middle and caudal portions of the small intestine, and in these areas, 80% of the organisms were associated with the intestinal wall. Light, immunofluorescence, scanning, and transmission electron microscopic studies demonstrated a layer of EEC adherent to the mucosal surface of the jejunum and ileum of infected calves. Transmission electron microscopy revealed that each EEC was surrounded by an electron-lucent zone and that some organisms had dense fuzzy surface structures which resembled pili. In the staining with ruthenium red, the electron-lucent areas were occupied by thick capsular material which seemed to be in contact with the microvilli. In some areas of contact with EEC, the microvilli were elongated and projected into the intestinal lumen. Light microscopy demonstrated stunted, thickened, and fused villi with the lamina propria expanded by inflammatory infiltrate. Changes in the villous surface topography, such as denudation of the tips of the villi and exposure of the lamina propria into the lumen, were observed by light and scanning microscopies in samples taken after death of the calves, but not in adjacent samples removed from calves under general anesthesia.