We've been taught for decades that the microbes inside us outnumber our own cells. And we've often been told it's by a ratio of 10:1. That number was first introduced in 1972 as more of a vague estimate, without much significant factual basis, and has been perpetuated ever since. Well, sort of. In 2014, a researcher from the National Institutes of Health called this very issue into question, and now, Ron Milo and Ron Sender from the Weizmann Institute of Science in Israel, and Shai Fuchs from the Hospital for Sick Children in Canada have offered up a new estimate.
To determine the new ratio, the researchers looked at the available literature about microbe population numbers as they relate to the "reference man." The reference man is between 20 and 30 years old, weighs about 154 pounds (70 kg) and is about 5'7" (170 cm). The researchers combed through the original bacteria calculations and found that one of the larger overestimations was for microbes found in the colon. Known to house one of the largest populations of bacteria, the gut is indeed full of microbes. But, when previous studies made their estimates, they used the density of bacteria per gram of "wet content" of the colon, times the volume of the entire alimentary canal. But, these researchers argue, the bacteria density of the colon is much higher than the rest of the tract, so assuming that the entire alimentary canal is as bacteria-filled as the colon is would be overkill. (If you want to get really philosophical, you could question whether the so-called wet content's bacteria is even part of our body, since it cycles through us daily.)
The new calculation came down to about 39 trillion bacteria to about 30 trillion human cells, a roughly 1:1.3 ratio. It's important to note though, that this ratio is still an estimation, not an undisputed fact. As Ed Yong writes in The Atlantic, "my preference would be to avoid mentioning any ratio at all—you don't need to it convey the importance of the microbiome and scientifically, it's not all that interesting."