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The effect of streptomycin on the ability of Shigella sonnei 3SR and enterotoxigenic Escherichia coli 2SR to associate with cecal tissue of mice in vivo and in vitro was examined. After orogastric challenge, both pathogens associated in significantly greater numbers (P < 0.05) with the cecal tissue of streptomycin-treated mice than with the tissue of untreated mice. The population levels of the pathogens were also significantly greater (P < 0.05) in the cecal contents of streptomycin-treated mice than in untreated mice. When excised cecal tissues from the two groups of mice were exposed to the pathogens in vitro, the extent of the association of the pathogens was markedly greater with tissues from streptomycin-treated mice than with tissues from untreated mice. There was also a positive correlation between the numbers of the pathogens in the suspending fluid and the extent of the tissue associations. The population size of fusiform organisms, which are the major components of the mucus layer of the ceca of mice, was reduced 100-fold by streptomycin administration. This was determined by microscopic count. Sections of cecal tissue obtained from the mice and stained with hematoxylin and eosin demonstrated that streptomycin administration greatly decreased the number of fusiform bacteria present in the mucosal microbial layer. We speculate that the partial elimination of fusiform organisms from this layer by streptomycin administration provides available association sites for pathogens so that they can successfully colonize the mouse cecum.