What proportion of the human body is bacteria (and how do we measure it)?

Asked by: Richard Asselin, Ottawa, Canada

For decades, scientists’ best guess was that the body contains 10 times as many bacteria as human cells. However, a team from Israel and Canada revisited the calculations in 2016 to estimate that we have a roughly equal number of bacteria and human cells.

They based their calculations on the fact that most of our bacteria are found in the colon. Using information from MRI scans, they calculated the colon volume of a ‘reference man’. Since there are about 90 billion bacteria per gram of wet stool, they estimated that there are around 38 trillion bacteria in the body, compared with 30 trillion human cells.

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