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Part Human, Part Bacteria

The human cells of your body are not actually outnumbered by the bacterial cells, researchers from Israel and Canada say.

In a paper posted at bioRXiv, the trio of researchers from the Weizmann Institute of Science and the Hospital for Sick Children describe how they tested the 'common' knowledge that bacterial cells outnumber human cells by some 10 to one. The report that a 'reference man' — who is between 20 years and 30 years old, 5-and-a-half feet tall, and weighs about 155 pounds — contains an average 30 trillion human cells and 39 trillion bacteria.

"The numbers are similar enough that each defecation event may flip the ratio to favor human cells over bacteria," the threesome writes.

They based their estimation on updated experimental data from the literature about both cell number and organ volume. For instance, they report that the colon typically houses a bacterial concentration of 1011 per milliliter and has a volume of about 420 milliliters, leading to some 1014 bacteria. They then applied such numbers to determine what their 'reference man' might harbor.

"It is good that we all now have a better estimate to quote," European Molecular Biology Laboratory's Peer Bork tells Nature News. "But I don't think it will actually have any biological significance."