Germfree mice were given food contaminated with pure cultures of various bacterial species isolated from ordinary healthy mice. The cultures were given singly, or in association, or consecutively at weekly intervals.

Whatever the technique of administration, the lactobacilli and anaerobic streptococci immediately established themselves throughout the gastrointestinal tract, and became closely associated with the walls of the organs. In contrast, the organisms of the bacteroides group were found in large numbers only in the large intestine.

Within a week after exposure, the populations of these three bacterial species reached levels similar to those found in ordinary mice. They remained at these characteristic levels throughout the period of observation (several months). Their presence resulted in a progressive decrease in the size of the cecum which eventually became normal in gross appearance.

Coliform bacilli multiplied extensively and persisted at high levels in all parts of the gastrointestinal tract of germfree mice, even after these had become colonized with lactobacilli, anaerobic streptococci and bacteroides. However, the coliform population fell precipitously within a few days after the animals were fed the intestinal contents of healthy pathogen-free mice.

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