Magazine article Science News

Human Body Not Overrun by Bacteria: New Calculation Suggests People's Cell Counts Are about 50-50

Magazine article Science News

Human Body Not Overrun by Bacteria: New Calculation Suggests People's Cell Counts Are about 50-50

Article excerpt

Human bodies don't contain 10 times as many bacterial as human cells, new calculations suggest.

A "standard man" weighing 70 kilograms has roughly the same number of bacteria and human cells, researchers report online January 6 at bioRxiv.org. This average guy would be composed of about 40 trillion bacteria and 30 trillion human cells, calculate researchers at the Weizmann Institute of Science in Rehovot, Israel, and the Hospital for Sick Children in Toronto. That's a ratio of 1.3 bacteria to every one human cell.

That estimate could be off by as much as 25 percent, with the average number of bacteria ranging from 30 trillion to 50 trillion. Among individuals, the bacterial count could vary as much as 52 percent, say Ron Sender, Shai Fuchs and Ron Milo. With a fudge factor of 10 trillion to 20 trillion bacteria, the number of microbes may pretty well match the number of human cells in the body, which also varies somewhat. "Indeed, the numbers are similar enough that each defecation event may flip the ratio to favor human cells over bacteria," the researchers write.

Scientists who study the microbiome, the collection of microorganisms that live in and on the human body, have peppered research papers with an estimate that bacteria outnumber human cells 10-to-1 (SN: 6/18/11, p. 26) or even 100-to-1. In recent years, those estimates have come into question, with the American Academy of Microbiology suggesting in 2013 that the real figure is probably closer to three bacterial cells for each human cell.

Judah Rosner, a molecular biologist at the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Md., called the 10-to-l ratio a "fake fact" in a 2014 issue of Microbe. It probably wormed its way into scientific literature because it sounds good, he says. "Everybody likes a nice, round number. And it had such impact. It was good PR." But Rosner and others wondered where the number had come from.

Sender and Milo, of the Weizmann Institute, and Fuchs, now at the Hospital for Sick Children, traced the figure to a single back-of-the-envelope calculation in a 1972 paper. The researchers then combed the scientific literature to come up with their own estimates.

Plenty of cocktail party fodder is buried in the results. For instance, the team finds that red blood cells are the most numerous cells in the body, accounting for 84 percent of cells. By weight, muscle and fat are the heavy hitters, making up 75 percent of cell mass. But those cells tend to be big and represent only about 0.2 percent of the human body cell number. As expected, most of the bacteria --about 39 trillion--live in the colon. …

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