If you’ve ever read anything about the colonies of bacteria that live on and inside you, you’ll no doubt have come across the neat little ‘fact’ that microbial cells outnumber human cells in your body by a ratio of around 10:1. You’ll find it in scientific papers, magazine articles, TED talks, and popular science books, and while it does a good job at illustrating just how crucial bacteria are to the existence of human beings, it isn’t actually even remotely true.
A review of more than four decades of research into the human microbiome has found that there is zero scientific evidence to back this oft-cited factoid up. Instead, the ratio looks to be about 1.3-to-1, with the average human playing host to around 100 trillion microbes, give or take. But even that isn’t the whole story.
A team of biologists led by Ron Milo from the Weizmann Institute of Science set out to review all the available literature on the microbe populations that live inside us, and found that for a man between 20 and 30 years old, with a weight of about 70 kg (154 pounds) and a height of 170 cm (about 5’7) – they call him the ‘reference man’ – there would be about 39 trillion bacterial cells living among 30 trillion human cells. Which gives us a ratio of about 1.3:1 – almost equal parts human to microbe. Source: Despite what you’ve read, our bodies don’t contain 10 times more bacteria cells than human cells