Do Bacteria Really Outnumber Your Own Cells 10 to 1?
― Recounted, the numbers are about equal. And the ratio flips each time you use the toilet
"Bacteria in your body outnumber your own cells ten to one." You hear it on TV and in books. But when scientists recounted in 2016, the answer was about 1 to 1. There weren't too many bacteria. Two assumptions in the old count had been off.
The yogurt package says it "balances your gut environment." Many bacteria are said to live in your gut.
"Bacteria in your body outnumber your own cells ten to one. So you are more than half bacteria." Have you ever heard this and felt a little uneasy?
But who counted them, and how? Bacteria are invisible. And nobody has counted the cells of the body one by one, either.
The "10 times" was off for just two reasons
Most bacteria live in the large intestine. The old calculation is said to have assumed about 1 kilogram of gut contents, with bacteria packed equally densely throughout. The real contents of the colon were less than half that.
People used to put the number of human cells at about 10 trillion. Recounted, it came to about 30 trillion. Most of them are tiny red blood cells. They are so small that they are easy to overlook if you think in terms of weight.
One number was too big and the other too small. Together, the two errors produced the "10 times" figure.
Bacteria gather in the large intestine, not all over the body
You can estimate the number of bacteria by multiplying the size of their home by how crowded it is. It is the same idea as working out a town's population from its area times its population density.
The stomach is very acidic, so it holds few bacteria. The small intestine also has fewer, because its contents flow through quickly. In the large intestine, though, the contents move slowly. That is where the bacteria's "big city" is.
In 2016, researchers in Israel re-measured the volume of the colon's contents. In an adult it was about 400 millilitres. That is less than one plastic bottle. The bacteria there are put at about 90 billion per millilitre. Multiply, and you get about 36 trillion.
Use the slider in Figure 1 to change the volume of the colon's contents. The number of bacteria rises and falls in direct proportion to the volume.
Eight in ten of your cells are tiny red blood cells
When human cells were recounted, about 25 trillion of the roughly 30 trillion were red blood cells. A red blood cell has no nucleus and is very small. There are overwhelmingly many of them, yet they make up only a few per cent of body weight.
The old estimate is said to have worked by dividing body weight by the weight of an "average cell." That method undercounts small, numerous cells like red blood cells. So human cells came out at about 10 trillion and bacteria at about 100 trillion. The ratio of those two figures is the "10 times."
Put the other way round, ordinary cells other than red blood cells number about 5 trillion. Compared with that, bacteria outnumber them by about seven to one. What you compare with changes the answer a great deal.
A single bowel movement is said to carry out about a third of the bacteria in the colon. The researchers who did the recount wrote in their paper that for a while afterwards, human cells may well outnumber bacteria. Whether bacteria "win" is a difference small enough to swap within a single day.
Each bacterium is tiny, so all of them together weigh only about 200 grams. For a 70 kg person, that is about 0.3%. "Half of you is bacteria" was an overstatement, both by number and by weight.
Summary
The claim that "bacteria outnumber human cells ten to one" came from an old estimate that overrated the bacteria's home and underrated the number of human cells. Recounted, there are about 38 trillion bacteria and about 30 trillion human cells, close to 1 to 1. Even so, a colon holding less than a plastic bottle's worth of contents is home to about as many living things as you have cells. The gut is an ecosystem inside the body.
Not ten times as many, but about the same number.
Even so, the colon is a "big city" that rivals the cells of a whole person.
For how body cells are counted, see How long would all the DNA in your body be if you joined it end to end?. For examples of bacteria breaking down food, see the story of the cow's stomach and why milk makes your belly rumble.
- Pour 400 mL of water into a plastic bottle (500 mL). That is roughly the volume of an adult's colon contents. You will probably find it smaller than you expected.
- Next, get 1 kilogram (about 1000 mL) of water and compare the two amounts. The old calculation assumed this 1 kilogram.
- Finally, work out the ratio of the two volumes (1000 ÷ 400). The estimate of the bacteria's home alone changes their number by more than a factor of two.
An estimated number depends on "what was multiplied by what." Next time you hear that "X is N times Y," ask what was multiplied to get that number. You may find something interesting.
For those who want to know more ― terms, formulas, and links to textbooksEach part is labelled by level, from middle school science to university specialist courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school biology (Basic Biology / Biology)
- High school+Advanced high school material, or covered in textbook sidebars
- UniversityNot covered in high school; university specialist courses (microbiology, ecology)
- ResearchNot taught even at university as settled fact; being studied by researchers right now
Middle schoolTerms: this phenomenon has names
- Gut bacteria: bacteria living in the intestines of people and animals, especially the large intestine.
- Gut microbiota: the name for gut bacteria seen as one community. Also called "gut flora."
- Red blood cell: a cell in the blood that carries oxygen. Human red blood cells have no nucleus and are about 8 micrometres across.
Middle schoolHigh schoolCheck with a formula: how many bacteria are in the colon?
Think of the colon as the "home" and the crowding of bacteria as the "density," and estimate the number. The units are: volume in millilitres, density in "number per millilitre."
| In symbols | N ＝ V × n |
| In words | Number of bacteria ＝ volume of colon contents × bacteria per millilitre |
| Where it comes from | The basic form of an estimate, "number ＝ volume × density." Like working out a population from area times population density, it treats the bacteria as packed roughly evenly through the colon |
| N | Number of bacteria in the colon |
| V | Volume of colon contents (millilitres) |
| n | Bacteria per millilitre of contents (per millilitre) |
| Volume of an adult's colon contents | said to be about 400 millilitres |
| Bacteria per millilitre | said to be about 90 billion (0.09 trillion) |
| Number of human cells | said to be about 30 trillion, of which about 25 trillion are red blood cells |
| Old estimate | Bacteria about 100 trillion, human cells about 10 trillion |
| Bacteria in the colon (trillions) | 400 × 0.09 ＝ 36 |
| Ratio to human cells | 36 ÷ 30 ＝ 1.2 |
| Ratio in the old estimate | 100 ÷ 10 ＝ 10 |
| Human cells other than red blood cells (trillions) | 30 − 25 ＝ 5 |
| Ratio to non-red-cell cells | 36 ÷ 5 ≒ 7.2 |
| Bacteria lost in one bowel movement (trillions, taken as one third) | 36 ÷ 3 ＝ 12 |
| Bacteria remaining afterwards (trillions) | 36 − 12 ＝ 24 |
The colon alone holds about 36 trillion; adding other sites, the paper's value is about 38 trillion. That is almost the same as the 30 trillion human cells, and after a bowel movement it falls to 24 trillion, below the human cells. The "10 times" came from estimating the bacteria at about 2.6 times too many and human cells at about one third too few.
High schoolHigh school+Why do bacteria gather only in the large intestine?
High schoolIn the stomach, gastric acid makes things strongly acidic, and most bacteria cannot grow. In the small intestine, digestive juices are released and the contents pass through in a few hours, so bacteria have too little time to multiply. In the large intestine, contents stay for a day or more and there is almost no oxygen. So bacteria that dislike oxygen grow to very high density.
High school+Bacteria in the large intestine break down dietary fibre that humans cannot digest and make short-chain fatty acids (acetic acid, butyric acid and others). People absorb these and use them as part of their energy. The inside of the gut is a tube connected to the "outside" of the body, so strictly speaking the bacteria are not inside body tissue, but a community of organisms living in the environment of that tube.
UniversityHow the recount was done: order-of-magnitude estimates and the "reference man"
The 2016 recount (a paper by Sender, Fuchs and Milo) assumed a "reference man" of 70 kg aged 20 to 30, and added up the numbers cell type by cell type. Red blood cells were found from blood volume and the number per microlitre, and bacteria from the volume of colon contents and the bacterial density. This kind of rough calculation that gets the order of magnitude right is also called a "Fermi estimate." Bacterial density is measured by direct counting under a microscope, or by quantifying genes as a clue.
📖 Further reading: Gut bacteria (Wikipedia, Japanese) ／ The original recount paper (English)
ResearchWhat we still don't know
- How much do people differ? The volume of colon contents and the density of bacteria change with diet, body size and bowel habits. The ratio is thought to swing widely between people and between days.
- How many non-bacterial residents are there? The gut also holds viruses that infect bacteria, and fungi. Viruses may even outnumber bacteria, it has been suggested, but the exact number is not settled.
- Which bacteria do what? The make-up of gut bacteria and its links to health and disease are being studied intensively. But in many cases, what is cause and what is effect is still unknown.
So what this article says is also "an explanation of what is known so far." Note, too, that the recount's figures are rough estimates for an average person.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "Digestion and absorption," "The work of microorganisms" | The large intestine and bacteria |
| High school | Basic Biology: "The internal environment," "Components of blood" | Number of red blood cells; different conditions in stomach, small and large intestine |
| High school+ | Biology: "Symbiosis" | Breakdown of dietary fibre, and the gut as an environment |
| University | Microbiology, ecology, methods of estimation | The reference-man assumption, and adding up cell counts |
| Research | Research on the gut microbiota | Differences between people, virus numbers, working out functions |
| ― | Link to daily life | The habit of asking, when you hear "N times," what is being compared with what |
- Sender R, Fuchs S, Milo R. Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLoS Biology 14(8): e1002533 (2016)
- Wikipedia, Japanese edition, "腸内細菌" (Gut bacteria)
- Luckey TD. Introduction to intestinal microecology. American Journal of Clinical Nutrition 25 (1972) (said to be the source of the old estimate of "about 100 trillion bacteria")
- Savage DC. Microbial ecology of the gastrointestinal tract. Annual Review of Microbiology 31 (1977)
※This article is a general-audience science explainer. The figures given are rough guides to help you understand the mechanism. For decisions about gut problems or your health, consult a doctor or other professional.