Location of bacteria in the mid-colon of the rat

The distribution of microorganisms in the mid-colon of the rat was studied by light and scanning electron microscopy. An antiserum against rat colon mucus was used to stabilize the mucus in situ. In samples not incubated with antiserum, the mucus disintegrated and contracted into patchy strands only partly covering the luminal surface of the colon. Bacteria were seen within fecal pellets, tangled among the strands of mucus, and scattered on the epithelial surface. However, when incubated with antiserum, mucus almost completely filled the lumen and coated the fecal pellets. Bacteria in these stabilized preparations were limited mainly to the fecal pellets, and there were small numbers scattered in the luminal mucus, but none were observed on the epithelial surface or within the crypts. Latex particles introduced into the lumen with the antiserum or with phosphate-buffered saline showed the same distribution as the bacteria. These findings are at variance with previous reports that organisms occur in abundance in the mucous layer, adjacent to cell surfaces, and inside crypts. Our results suggest that conventional preparation for microscopy without prior stabilization of the mucus in situ may lead to artifactual redistribution of microorganisms and emphasize the importance of mucus in maintaining mucosal-floral homeostasis in the colon.