The human body

The human microbiome

The body carries about 38 trillion bacteria, roughly one for every human cell, and the great majority live in the colon.

16 facts

At a glance

38 trillion

Bacteria in a 70 kg reference man, 2016 estimate

30 trillion

Human cells in the same reference man, 2016 estimate

1.3 to 1

Revised ratio of bacterial to human cells

0.2 kg

Total mass of bacteria in the body

over 95 percent

Share of the body's bacteria held in the colon

For decades it was said that bacteria in the body outnumber human cells ten to one. The figure behind it was about 100 trillion bacteria, and that value dates back to the 1970s.
The 1970s estimate assumed about 100 billion bacteria per gram of gut content in an alimentary tract of 1 litre. It was a single back-of-the-envelope calculation in a 1972 paper.
In 2016 researchers re-estimated the numbers for a 70 kilogram reference man: about 38 trillion (3.8 x 10^13) bacteria and about 3.0 x 10^13 human cells. That is a ratio of about 1.3 bacterial cells to each human cell.
The bacterial count in the reference man could range from 30 trillion to 50 trillion, and the ratio varies by up to 2-fold across population groups, from 1.3 to 2.3.

The colon holds more than 95 percent of the body's bacteria. The estimate rests on a density of about 10^11 bacteria per millilitre in roughly 400 millilitres of contents.
Bacterial density reaches about 10^11 colony-forming units per gram in the colon, against 10^3 to 10^6 per gram in the small intestine.
The stomach holds only about 10^2 to 10^4 colony-forming units per gram.
Dental plaque holds about 10^12 bacteria. The ileum, saliva and skin hold about 10^11 each, while the stomach, duodenum and jejunum hold only about 10^7 each.
On skin, moist areas hold the most bacteria but the least diversity, while dry areas hold fewer bacteria with more diversity.

All the bacteria in the body together weigh about 0.2 kilograms, around 0.3 percent of body mass for a 70 kilogram adult.
One analysis counted 3.3 million nonredundant microbial genes in human faecal samples, about 99 percent of them from bacteria.
The National Institutes of Health estimates the pool of unique microbial genes in healthy people at about 8 million, against about 23,000 human genes.
The aerodigestive tract harbours 775 different bacterial species, and the human gut holds over 200 fungal species.

The US National Institutes of Health launched the Human Microbiome Project in 2007. Its first phase surveyed five areas: the digestive tract, mouth, skin, nasal cavity and vagina.
The reference microbiome came from 300 healthy people, who were 26 years old on average.
Bacteria in the large intestine help break down remaining nutrients and make vitamin K.