In order to examine the ability of R plasmid-bearing Escherichia coli strains to colonize in the mouse alimentary tract, an R plasmid-positive (R(+)) E. coli strain and its R plasmid-negative (R(-)) counterpart were together inoculated into the streptomycin-treated mouse alimentary tract, and the numbers of fecal E. coli strains were enumerated. The numbers of R(+) strains were always at the level similar to or lower than those of their counterparts and rapidly decreased in the fecal population. However, when R plasmids, which were originated from a cryptic plasmid of the host E. coli strain, were utilized, an R(+) strain dominated over its R(-) counterpart during the experimental period. These experimental results indicated that the relationship between the host strain and R plasmids affected the ability of the host strain to colonize in the alimentary tract.
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