The bacterial flora of the gastrointestinal tract differs qualitatively and quantitatively from one colony of mice to another. Certain components of this flora, however, are always present in large and approximately constant numbers in healthy adult mice, irrespective of the colony from which the animals are derived.

Lactobacilli and anaerobic streptococci are extremely numerous in the stomach, the small intestine, and the large intestine. In contrast, organisms of the bacteroides group proliferate only in the large intestine. These three bacterial species persist at approximately constant levels in their characteristic localization throughout the life span of healthy animals. They are closely associated with the walls of the digestive organs, and are probably concentrated in the mucous layer.

A few experiments carried out with rats and young swine indicate that lactobacilli are also present in large numbers in the stomach of these animal species.

It is suggested that some of the components of the gastrointestinal flora have become symbiotic with their hosts in the course of evolutionary development and thus constitute a true autochthonous flora. The other components of the indigenous flora are acquired early in life either through accidental contact or because they are ubiquitous in the environment. The "normal" flora is that which is always present in the environment of the animal colony under consideration.

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