BackgroundEscherichia coliK12 is a laboratory strain considered nonpathogenic. The purpose of this study was to examine the effect ofE. coliK12 infection on colonic structure and function.MethodsSuckling rabbits were infected at 10 days of age with 6 × 109CFUE. coliby intragastric inoculation and were examined 4 to 5 days later. Segments of ileum and proximal and distal colon were removed for light and electron microscopy, and NaCl transport was examined in vitro under short-circuited conditions in Ussing chambers.ResultsInfection did not cause weight loss or diarrhea. Colonic mucosa was inflamed with infiltration by polymorpho-nuclear neutrophils mainly in the lamina propria. The proximal and distal colon exhibited reduced Na+absorption. The proximal colon also showed increased Cl-secretion; the ileum was unaffected.ConclusionsInfection withE. coliK12 disrupts the epithelium and alters ion transport in the colon, probably as a result of mucosal inflammation. The changes indicate that nonpathogenicE. colihave the potential to cause intestinal disease.