A healthy adult carries a huge population of bacteria, mostly in the gut. About how many individual bacteria is that?
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Answer
About 38 trillion bacteria
Despite their numbers, all of those microbes together weigh only about 200 grams.
How to estimate it
Nearly all of them live in the colon, which holds about 400 grams of material. Say one tenth of that, 40 grams, is bacterial cells. A bacterium is about one micrometre across, so it weighs roughly a trillionth of a gram. 40 grams / 0.000000000001 grams = 40 trillion. The current scientific estimate is about 38 trillion bacteria.
Source: Sender, Fuchs and Milo 2016, PLOS Biology
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