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| The Human Body as an Ecosystem |
The human body is inhabited by billions of bacteria. In fact, we normally carry ten times more prokaryotic than eukaryotic cells. Our mouths alone are host to four hundred identified - and probably hundreds more unidentified - species of bacteria. Most bacteria are benign to their host, some even provide valuable services. For example, bacteria in the gut aid digestion and generate vitamins used by their human hosts.
The bacteria we possess are an ecological community; thus, the principles of community ecology and evolution are vital in understanding how these bacteria (both the benign and the potentially harmful) live within us. Each bacterial species is adapted to the habitat and ecological niche it fills, existing in somewhat of an ecological balance. This balance helps thwart the invasion of pathogens, which must compete with resident bacteria for nutrients and space. Resident bacteria also produce antimicrobial proteins called bacteriocins, which inhibit the growth of related species.
When the normal flora are disrupted, it shifts the mix of microbiota and can lead to disease. For example, the use of some broad-spectrum antibiotics can dramatically decrease the numbers of bacteria in the colon. In this situation, Clostridium difficile, normally present only in low numbers, can overgrow. This bacterium produces toxins that cause potentially fatal damage to the lining of the colon. In the few individuals that normally harbor the microbe, normal levels of other bacteria keep C. difficile numbers low. It is only when the balance is disrupted that such a "superinfection" occurs.