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An article in Live Science reports on a study that used a DNA testing technique called “pyrosequencing” to assess the number of species of bacteria present in a sample. According to the article,

In sheer numbers, the mammalian colon harbors one of the densest microbial communities found on Earth. For every human cell in your body, there are roughly 10 single-celled microbes, most of which live in your digestive tract.

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Pyrosequencing generates extremely large numbers of small DNA “tags” copied from the genes of organisms being examined. Species can be sorted out from each other by looking at variations in DNA sequences that code for a molecule universal among all living cells.

Having a lot of bacteria in our digestive systems does not mean it’s all bad. Intestinal microbes perform functions crucial to the human body, including keeping the immune system balanced and producing essential vitamins. If the intestinal microbe population is thrown off-balance, such as in the case of taking antibiotics, other harmful bacteria may take their place—hence many scientists’ concerns about over-prescribing antibiotics.