Do bacteria really outnumber the cells in your body?

Slightly, but not by the margin often claimed. The popular figure of ten bacteria for every human cell has been replaced by an estimate of about 1.3 bacteria per human cell for a 70 kilogram reference man. That is the same order of magnitude, so the two populations are close to equal. Because the numbers are estimates, the answer is best read as roughly one to one. [1] [2]

In 1972, biochemist Thomas Luckey estimated that each gram of feces held 100 billion bacteria and that the digestive system contained about one kilogram, giving about 100 trillion bacteria. In 1977, Savage's team set this against an estimated 10 trillion human cells, producing the 1:10 ratio. It spread because it was simple and vivid, and it is still taught. In 2014 microbiologist Judah Rosner pointed out that the 10 trillion human-cell figure first appeared in a 1970 book without proper citation. [2]

Ron Sender, Shai Fuchs and Ron Milo revisited the numbers in PLoS Biology. They used the colon, where most bacteria live, and found about 0.4 litres of content at roughly 0.9 times 10 to the 11th bacteria per gram, giving about 3.8 times 10 to the 13th bacteria. For human cells they drew on organ-by-organ data and found about 3.0 times 10 to the 13th, with red blood cells making up around 90 percent of the count. Their ratio was 1.3, with an uncertainty of 25 percent. For a reference woman the same table gives about 2.2 bacteria per human cell, because she has fewer human cells and a somewhat larger estimated colon content. [1]

The authors say the bacterial figure may be an upper limit, since bacterial concentration rises along the colon and stool density may overstate the average. Their total bacterial mass is about 0.2 kilograms. [1]

The ratio depends partly on definition. Sender and colleagues note that if only nucleated human cells are counted, leaving out red blood cells, the ratio comes out near 10:1. They treat red blood cells as genuine cells but acknowledge that others regard them as bags of hemoglobin, calling it a question of definition. Platelets are also debatable but matter little numerically. [1]

Sources also differ on the wider microbiome. An NCBI FAQ cites about 37 trillion human cells and about three times more bacterial cells than human ones, and says there is general agreement that viruses outnumber bacteria, perhaps by as much as 5 to 1, with fungi about ten-fold fewer than bacteria. Sorbonne Université reports that 2021 studies revised the virus estimate downward to a number comparable to bacteria. Both sources agree that, counting viruses and bacteria together, microbes outnumber human cells. [3] [2]

The balance also shifts over time. Sorbonne Université notes that after a bowel movement human cells are temporarily more numerous. [2]