Interaction between Escherichia coli and Shigella flexneri in the digestive tract of "gnotobiotic" mice.

When a strain of Shigella flexneri and then a strain of Escherichia coli were implanted at an interval of 1 day in the digestive tract of germfree mice, the S. flexneri was eliminated in 8 days. However, when S. flexneri was mono-associated with germfree mice in the absence of E. coli, a population of S. flexneri appeared which was resistant to the antagonistic effects of E. coli in vivo. The emergence of this population appeared to be a stepwise process, extending over a period of 3 months. The interaction between S. flexneri and E. coli in vivo and in vitro was significantly different in that the Shigella capable of coexisting with E. coli in the digestive tract was suppressed by that organism in vitro.