Photograph by Ian Cuming, Alamy
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You may have heard that there are ten microbial cells in your body for every one of yours—and it turns out you might have heard wrong.

Photograph by Ian Cuming, Alamy

How Many Cells Are in the Human Body—And How Many Microbes?

New counts revise a popular factoid about how many bacteria live inside us.

Your body is a microbial melting pot, home to trillions of bacteria that help keep you healthy and regular. And for decades, scientists have shown their importance with this alluring factoid: The microbes in your body outnumber your own cells ten to one.

But a new estimate indicates that, as far as cell counts are concerned, it may well be a tie.

In a new study posted ahead of publication on the bioRxiv website, three scientists led by the Weizmann Institute of Science’s Ron Milo find that the average human male is made of 30 trillion cells and contains about 40 trillion bacteria, most of which reside in his digestive tract.

In other words, your body’s cell count is regularly poised to overtake the bacteria—and sometimes it does, depending on how recently you rid yourself of a few trillion via a bowel movement.

The ten-to-one ratio is cited everywhere, in no small part because it easily rolls off the tongue. Think pieces name-check it. Researchers dutifully cite it. And news outlets including National Geographic often use the factoid to introduce the magnitude of the microbiome, the world of microbes that call us home.

But where did this number come from?

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In 2010, researchers Moselio Schaechter and Stanley Maloy dove into the thicket of academic citations behind the stat, ultimately finding that many pointed to an influential 1977 review by preeminent microbiologist Dwayne Savage. Savage’s reputation as a stickler spoke for itself, leaving few to question what Savage himself said only “may” be true.

Savage derived it by combining a good-faith guesstimate by researcher and noted eccentric T.D. Luckey with a textbook’s reasonable, but unsupported human cell count. Since the 1970s, scientists have been gazing upon Savage’s sketch as if it were a true-to-life portrait.

“The story of the ten-to-one ratio has all the characteristics of an academic urban legend,” says Ole Bjørn Rekdal of Bergen University College, who has studied how knee-jerk citations can propagate plausible but unsubstantiated academic claims.

“A very large number of authors have disregarded the principle of consulting primary sources,” he adds, “and of checking the reliability of the knowledge they pass on to their readers.”

Best Guesstimate

To rebuild Luckey’s shaky bacterial estimate, Milo and colleagues lavished particular attention on the volume of the average male colon—409 milliliters, to be exact—and how many bacteria reside in a gram of feces. And the team’s human cell count, which more or less jibes with one of the best-ever estimates, cleverly takes advantage of the fact that, by count, we’re mostly red blood cells.

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The story of the ten-to-one ratio has all the characteristics of an academic urban legend.
Social scientist Ole Bjørn Rekdal

But as you might imagine, this study, not even yet reviewed by other scientists, hardly closes the book on the ratio. It’s notoriously tricky to count human cells. What’s more, the estimate rests on average bacterial counts that are bound to fluctuate from person to person.

“Once you’re in the billions [of bacteria], does it really matter?” says Carolyn Hovde Bohach, a microbiologist at the University of Idaho. “I don’t know.”

In fact, there’s an argument to be made that the ratio ought to be dethroned by more useful substitutes. What about, say, comparing the number of unique genes in our microbiome to humans’ ~20,000 genes? (By the way: global estimates of this ratio have ranged from about 150-to-1 to nearly 500-to-1, though geneticist Julien Tap pegs it at a more modest 30-to-1, based on the microbiome’s most dominant species.)

Regardless of the exact numbers, the microbiome remains a vital, underrated contributor to human health. It’s not as if our bacterial tenants are now one-tenth of their former importance.

“It’s really easy to forget that we live in a microbial world,” says Bohach. “They’re emperors of our planet.”

Follow Michael Greshko on Twitter.