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In the "Iliad", the Greek poet Homer already described a common phenomenon: "...while we hearing rumor know nothing ourselves for sure." According to such a persistent rumor, the human body contains ten times as many bacteria as human cells. However, this claim was not supported by any conclusive evidence. And surprisingly, hardly anybody questioned this daring thesis – until now. In 2016, researchers showed that this estimate from 1972 belongs in the realm of legends.
Ron Milo and Ron Sender of the Weizmann Institute of Science in Rehovot, Israel, and Shai Fuchs of the Hospital for Sick Children in Toronto, Canada, took a closer look at the 45-year-old 10:1 claim. At the time, the microbiologist Thomas Luckey estimated that there were some 1011 (100 billion) bacteria in each gram of feces. Luckey estimated the volume of the human digestive tract from the mouth to the anus to equal around one liter. On this basis, he extrapolated that some 1014 (100 trillion) bacteria live in the intestine.
According to Milo and his colleagues, this was an "elegant" estimate. Nevertheless, they believe that it was certainly never intended to be widely quoted decades later. The authors actually found that far fewer bacteria colonize the human body than was estimated by Luckey.
Many roommates in the large intestine
For their investigation, they analyzed numerous studies on the number of bacteria found in the intestine, on the skin, in saliva, in dental plaque, and in the stomach. These are the areas where the majority of our "roommates" live, with the large intestine containing most of them – 0.9x1011. In a 1.70 m tall man weighing 70 kg, the large intestine itself has a volume of only 409 ml. The researchers calculated the number of bacteria in the large intestine to be 3.9x1013. All other organs of the human body contain at most another 1012 bacteria. This means that in total, some 3.9x1013 (39 trillion) bacteria frolic in and on the human body – which is far fewer than previously thought.
Bacteria slightly outnumber human cells
To calculate the number of human cells, the team of researchers again used an average man weighing 70 kg. With reference to a recent study from 2013, the researchers calculated that he contains about 3.0 × 1013 (30 trillion) cells.
Hence the number of bacteria only slightly exceeds the number of human cells. Since on average, microorganisms are much smaller than human cells, they make up only a fraction of the body's volume. Overall, in the match between human cells and microorganisms, we therefore have at least a tie.
Going to the bathroom can turn the game around
The researchers state that the bacterial count is only an estimate and can deviate by some 52 percent. What sounds like high uncertainty actually means the following: Another human being may be host to only half as many bacteria – or twice the number. However, the bacterial count will never be ten times higher.
Nevertheless, Milo and his colleagues explain that merely going to the bathroom can already change the score in favor of human cells.
Ron Sender, Shai Fuchs, Ron Milo: Are We Really Vastly Outnumbered? Revisiting the Ratio of Bacterial to Host Cells in Humans, in: Cell 164, January 28, 2016
Luckey, T.D.: Am. J. Clin. Nutr. 25, 1292–1294 (1972).