Microbe Cells Don’t Outnumber Your Own

For years people have cited the ten-to-one ratio, with microbes dominating human cells, but that number is probably wrong, according to recent research

microbe hand
Pasieka/Science Photo Library/Corbis

Good news for the germaphobes. A frequently cited statistic—that the microbes living in and on your body outnumber human cells ten-to-one—is most likely entirely made up, reports Ed Yong for The Atlantic. Instead, you probably have equal numbers microbial cells and cells you can truly call your own. Doesn’t that make you feel better?

Sure, that still sounds like an incredible number of teeming creepy crawlies to which you play host. But the microbial cells are vital parts of a functioning body. They make up the microbiome and without them, the truly harmful bacteria could take over.

In recent years, scientists have increasingly realized that the human microbiome is remarkably responsible for maintaining human health as well as responsive to the things people do—microbes can get jet lagged and change if we travel to space. These minuscule creatures are even responsible for making humans stink.

Scientists and writers alike, in communicating all this new research, like to throw around the ten-to-one ratio to impress exactly how important the microbiome is.

But, Yong reports, that number was really just a “a back-of-the-envelope calculation that became enshrined as hard fact based on little more than its catchy nature and its sounds-about-right-ness."

In 1970, microbiologist Thomas D. Luckey estimated that there are 100 billion microbes in a gram of intestinal fluid or feces and that every adult had about 1000 grams of these substances. So humans, on average, must play host to 100 trillion microbes, write Elio Schaechter and Stanley Maloy in a 2010 blog post for Small Things ConsideredAnother microbiologist, Dwayne Savage, compared that number to the 10 trillion human cells our bodies carry in a 1977 paper. Thus the ratio was born. 

The problem was that Luckey didn’t really have much basis for his numbers. In a 2014 letter to Microbe Magazine, Judah L. Rosner of the National Institutes of Health, points out that more recent estimates for the number of human cells in the average body range from 15 to 724 trillion. That very wide range called into question the ten-to-one citation. 

More recently, a team of researchers led by Ron Milo of the Weizmann Institute of Science decided to come up with a better, more accurate ratio. Along with Ron Sender and Shai Fuchs, Milo’s team scoured the literature for actual measurements of the microbes contained in feces samples and the measure of human cells in different types of tissue.

Their estimate, published online in the preprint server bioRxiv, puts those numbers at about 39 trillion microbes to 30 trillion human cells. That ratio, 1.3-to-1, is pretty close to equal—though the researchers caution that their numbers are still very rough estimates.

However, our microbes’ genes still easily outnumber our human genes, Yong writes. So humans’ dominance over their own bodies is still up for debate (if it matters at all—the microbes aren’t coordinating to overthrow us).

The new ratio comes with the added comfort that it’s close enough to influence easily, all you need to do is visit the toilet. The microbes lost in each “defecation event” could be enough to flip the ratio in favor of human cells, the researchers write.

Or as Yong puts it: “You gain temporary dominance over your own body with every flush.”

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