Experimental infection of newborn pigs with an attaching and effacing Escherichia coli O45:K"E65" strain.

The ability of a nonenterotoxigenic, K88-negative porcine Escherichia coli strain of serogroup O45:K"E65" to induce attaching-effacing lesions was investigated in newborn pigs. Typical attaching-effacing lesions, characterized by intimate adherence of bacteria to mature enterocyte brush borders with effacement of the microvilli, were observed on light and electron microscopy. Bacteria were also seen in intracytoplasmic vacuoles of mature enterocytes and, in areas of heavier colonization, in the lamina propria of the intestinal mucosa. A moderate inflammatory response with mild focal ulceration of the intestinal mucosa was observed. In a sequential study, we observed that the attaching-effacing lesions were well established in the duodenum, jejunum, and ileum at 12 h postinoculation but did not develop in the cecum and colon until 24 to 48 h postinoculation, although bacteria had colonized the latter areas as early as 12 h postinoculation. Initially, bacteria were very intimately attached, with an irregular arrangement on the enterocyte apical cell membrane, and subsequently reoriented to form a typical palisade arrangement with a narrow regular gap between the bacterial cell wall and the enterocyte apical cell membrane. This phenomenon of early intimate attachment of irregularly disposed bacteria has not been reported for human enteropathogenic attaching and effacing E. coli and could represent a new and different mechanism of attachment and effacement to intestinal epithelial cells.