The digestive system

The gastrointestinal tract contains an immensely complex ecosystem of micro-organisms. A typical person hosts more than 500 different species of bacteria, representing dozens of different growth patterns and capabilities. The composition and distribution of this menagerie varies with age, state of health and diet.

The number and type of bacteria in the gastrointestinal tract vary dramatically by region. In healthy individuals the stomach and proximal small intestine contain few micro-organisms, largely a result of the bacteriocidal activity of gastric acid; those that are present, are aerobes and facultative anaerobes. One interesting testimony to the ability of gastric acid to suppress bacterial populations is seen in patients with achlorhydria, a genetic condition which prevents secretion of gastric acid. Such patients, which are otherwise healthy, may have as many as 10,000 to 100,000,000 micro-organisms per ml of stomach contents.

In sharp contrast to the stomach and small intestine, the contents of the colon literally teem with bacteria, predominantly strict anaerobes (bacteria that survive only in environments virtually devoid of oxygen). Between these two extremes is a transitional zone, usually in the ileum, where moderate numbers of both aerobic and anaerobic bacteria are found.

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