An article in the NY times about microbes and their interaction with our bodies contained the following statement:

We have over 10 times more microbes than human cells in our bodies

I'm a bit skeptical about that statement. It compares microbes to human cells, but since the article describes DNA analysis methods to describe our microbiome, is the statement really about genetic material and not cell number? Where can I find additional material on this subject?

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    I have no reason to doubt it, but I can't find any article as a source (Wikipedia does not seem to have very good references)
    – Sklivvz
    Apr 22, 2011 at 9:54
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    @sklivvz: that's exactly why I've asked the question here :) Apr 22, 2011 at 10:00
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    how about this ScienceDaily article? It sources the 'American Society for Microbiology'
    – Oliver_C
    Apr 22, 2011 at 10:17
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    “is the statement really about genetic material and not cell number?” There is (almost) no difference: there is generally a 1:1 relation between the number of genomes and cells because every cell hosts exactly one copy of our DNA (with the exception of erythrocytes (= red blood cells), and mitochondrial DNA). Apr 22, 2011 at 10:35
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    I imagine the diversity figures are even more extreme. After all, there is no diversity in all the genetically-identical human cells. (Gametes and pregnancies notwithstanding.)
    – Oddthinking
    Nov 27, 2011 at 23:02

2 Answers 2


This is from Elizabeth Pennisi (Science Magazine, 2010)

This past decade has seen a shift in how we see the microbes and viruses in and on our bodies. There is increasing acceptance that they are us, and for good reason. Nine in 10 of the cells in the body are microbial. In the gut alone, as many as 1000 species bring to the body 100 times as many genes as our own DNA carries.

Their genes and ours make up a metagenome that keeps the body functioning. This past decade we’ve begun to see how microbial genes affect how much energy we absorb from our foods and how microbes and viruses help to prime the immune system.

The ideas of a microbiome and a virome didn’t even exist a decade ago. But now researchers have reason to hope they may one day manipulate the body’s viral and microbial inhabitants to improve health and fight sickness.

From Discover Magazine (2011)

There are 20 times as many of these microbes as there are cells in the body, up to 200 trillion in an adult, and each of us hosts at least 1,000 different species.

... a person is not so much an individual human body as a superorganism made up of diverse ecosystems, each teeming with microscopic creatures that are essential to our well-being.

Two of the largest efforts [to use genetic sequencing to explore how the ­diversity of the microbiome impacts our health] are the Human Microbiome Project, funded by the National Institutes of Health, and the European Union’s Metagenomics of the Human Intestinal Tract.

Although these groups have only just begun to publish their findings, it is already clear that the micro­biome is much more complex and very likely more critical to human health than anyone suspected.

To clarify a possible point of confusion, microbial biomass is only a small portion of a human body's mass, due to the small size of bacterial cells relative to human cells.

From San Francisco Chronicle (2012):

The human body carries more than 100 trillion bacteria - up to five pounds of the tiny >single-celled organisms.

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    I'd question whether bugs in our gut constitute in our body. We have an epithelial layer between these bugs and what we consider self. And of course we can eradicate most of them with gut sterilization techniques.
    – HappySpoon
    Jun 5, 2014 at 8:26
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    Wikipedia says they account for 1-3% of a person's mass, while outnumbering human cells 10 to 1. Jan 27, 2015 at 12:08

No, we do not.

Although this figure is widely cited (as per Oliver_C's answer), it is based on rather old data (paywalled, unfortunately), and these estimates are now thought to be inaccurate. A recent estimate by Sender, Fuchs, and Milo puts it at a close to one-to-one ratio of human to microbial cells:

Our analysis updates the widely-cited 10:1 ratio, showing that the number of bacteria in our bodies is actually of the same order as the number of human cells. Indeed, the numbers are similar enough that each defecation event may flip the ratio to favor human cells over bacteria.

Less dense articles reporting these findings can be read here and here (Nature).

  • Your Nature article says the paper is still in review. Has it finished review yet? Nov 15, 2016 at 20:24
  • Yes, see the updated link to the Sender, Fuchs and MIlo paper above. Nov 15, 2016 at 20:26

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