What do you think of this fact?
You’ve probably heard it: for every human cell in your body there are ten bacteria, so technically you’re mostly microbe. Great party line. Too bad it’s off by a lot.
Where the 10 to 1 came from
The famous ratio goes back to a 1972 estimate by microbiologist Thomas Luckey. According to Scientific American, he assumed about 100 billion bacteria per gram of feces and scaled that up to the roughly one liter volume of the digestive tract. The result: about 100 trillion gut bacteria, and a ratio that got repeated for decades. The authors of the recount say it “was probably never meant to be widely quoted decades later.”
The catch: most gut bacteria live in the colon, which holds only about 0.4 liters, and stool measurements show fewer bacteria than Luckey assumed.
The recount
In early 2016, Ron Milo and Ron Sender at the Weizmann Institute of Science in Israel, together with Shai Fuchs from the Hospital for Sick Children in Toronto, ran the numbers again. For a reference man (70 kilograms, 20 to 30 years old, 1.7 meters tall) they arrived at about 30 trillion human cells and about 39 trillion bacteria. That’s roughly 1.3 bacteria per human cell, with wide uncertainty. So the ratio is much closer to 1 to 1.
Part of the old mistake was size. The earlier estimate partly assumed that the average bacterium is about 1,000 times smaller than the average human cell. But the vast majority of your human cells are red blood cells, and those are at least 100 times smaller than fat or muscle cells.
Bonus fact: Because the two counts are so close, the authors suggest that each trip to the toilet may flip the ratio in favor of human cells.
Not every scientist was impressed. Peer Bork of the European Molecular Biology Laboratory said the better number is useful to quote but probably has no biological significance. Still, you’re not a bacteria colony in a human costume. It’s more of a shared flat where the microbes have a slim majority.