Do the microbes on your body really outnumber your own cells ten to one?

You carry about as many microbes as you have human cells, not ten times as many. For decades textbooks and headlines said microbes outnumber your own cells ten to one, so that you are mostly bacteria. When scientists finally rechecked the maths in 2016, they found that bacteria in the body are of the same order of magnitude as human cells, at roughly 38 trillion bacterial cells against 30 to 40 trillion human ones.

The famous ratio had a surprisingly thin origin. Its main source dates back to the 1970s and was only a sentence-long calculation: it assumed the gut held about a litre and multiplied that by how many bacteria sit in a gram of gut contents. Then the figure was repeated so often that it became, in the checkers' words, an established common knowledge fact, even though values for the number of cells in the body were rarely backed by real measurements.

Almost all of those microbes live in one place, the colon. The stomach and the small intestine hold very few, and the mouth, though crowded in spots, adds up to less than a hundredth of the colon's total. Weighed instead of counted, the whole community comes to about 0.2 kilograms, around 0.3 percent of body weight rather than the 1 to 3 percent once claimed.

The old number was not pulled from nowhere. Count only the human cells that have a nucleus and you do get about ten to one, because red blood cells, which have no nucleus and make up most of your cells, were left out of the old sum.

So the correction is about honesty with numbers, not about microbes being unimportant. They still matter: bacterial genes outnumber human genes about a hundred to one.