Not a fact anymore

Bacterial cells in the human body outnumber human cells by about ten to one.

What we know now

Bacterial and human cell numbers are roughly comparable. One widely cited estimate for a reference adult male is about 38 trillion bacterial cells and 30 trillion human cells, a ratio near 1.3 to 1.

Why it changed

Researchers revisited the old estimates, replacing rough assumptions about gut volume and human cell counts with updated measurements and calculations.

Status: Corrected
Category: Biology
Accepted for: ≈41 years
Accepted approximately: 1970s–2010s
Changed approximately: 2016

The ratio varies with body size, age, sex, health, and bowel contents. Even a bowel movement can change the estimate noticeably.

This correction concerns cell numbers only. It says nothing simple about the importance of the microbiome, the number of microbial genes, or its metabolic activity.

Evidence

Sources and what they establish

Previous belief

Estimating Gene Numbers inside a Human BodyPLOS ONE
Provides an example of the formerly common claim that bacterial cells outnumbered human cells by roughly ten to one.

Primary research

Revised Estimates for the Number of Human and Bacteria Cells in the BodyPLOS Biology
Recalculates human and bacterial cell numbers and revises the widely cited ten-to-one ratio to roughly the same order of magnitude.

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