The period of highest susceptibility was rather short, extending from the day of birth to approximately 2 weeks of age. Adult NCS and NCS-D mice failed to become experimentally colonized with E. coli, even when large doses were administered per os on 3 consecutive days.

The extent of colonization of the various parts of the gastrointestinal tract was related to the size of the infective dose. Many of the young mice died within 2 to 3 days following per os infection with large doses of enteropathogenic E. coli. However, practically all the animals which survived cleared their intestinal infection at approximately the same age. For example, in mice infected with 23 x 10⁶ bacteria, colonization of the intestinal tract usually came to an abrupt end when the animals were 24 to 28 days old, irrespective of the age at which they had been infected.

There is suggestive evidence that the acquisition of resistance with age, and the ability of adult animals to control the intestinal infection, are related to the development in the gastrointestinal tract of a microbiota which is antagonistic to E. coli.