Antagonisms among isogenic strains of Escherichia coli in the digestive tracts of gnotobiotic mice.

We have observed that antagonisms occur between isogenic strains of Escherichia coli associated with gnotobiotic mice. The strains differed in the carriage of plasmids or in chromosomal mutations. The plasmid-free strains, in general, inhibited the establishment of plasmid-bearing strains in the gastrointestinal tract of mice. The outcome of the interactions between isogenic pairs, however, depended on the order in which the strains were introduced into the mice. Maintaining the bacterial strains in monoassociation with gnotobiotic mice resulted in the "adaptation" of the bacteria to their host. Thus, in all cases, "adapted" strains became the dominant population in the feces of mice, regardless of whether the adapted strains was introduced into mice before or after its isogenic partner which had been cultured in vitro. The ecological advantage disappeared when the adapted strain was cultured in broth. Ultrastructural differences in cell morphology were observed between strains maintained in vivo and in vitro.