An avirulent, streptomycin-resistant Salmonella typhimurium strain, SL5319, and its lipopolysaccharide (LPS)-deficient mutant strain, SL5325, differ in their ability to colonize the large intestines of streptomycin-treated mice. When fed to mice independently, the strains colonize equally well, but when fed together, the LPS-deficient mutant is outcompeted by the wild-type strain during establishment in the gut (J VSports手机版. J. Nevola, B. A. D. Stocker, D. C. Laux, and P. S. Cohen, Infect. Immun. 50:152-159, 1985). In the present study, the spatial distribution in the intestinal mucosal layer of the two strains was visualized by specific hybridization to bacterial rRNA in histological sections of mouse colon and cecum. The first day after infection, 9. 8% of the smooth SL5319 cells observed in mucus were found to be associated with the mouse epithelial cells, but three days after infection, the corresponding fraction of adhering bacteria was reduced to 2. 1%. The LPS-deficient S. typhimurium strain was confined to the part of the mucosal layer closest to the colonic lumen and was not observed to adhere to the epithelium either at day 1 or 3 after infection. Quantitative determinations of the distance from the S. typhimurium cells to the epithelial wall confirmed that the average distance for the rough S. typhimurium SL5325 was much larger than for its smooth counterpart, S. typhimurium SL5319. Quantification of the hybridization signal from bacteria isolated from the cecal mucus revealed that the two strains had the same ribosome concentration, indicating that they have the same potential for growth in the intestinal environment. On the basis of these observations, we suggest that the better colonization ability of the strain carrying wild-type LPS is due to the better abilities to penetrate the intestinal mucosal layer and to subsequently bind to the epithelial cells in vivo. .