It’s often said that the bacteria and other microbes in our body outnumber our own cells by about ten to one. But that’s a myth that should be forgotten according to a new paper – the ratio between resident microbes and human cells is more likely to be one-to-one. The myth arises from a rough estimate made in 1972 which has persisted ever since.
Of course, this only applies to cells – the number of viruses in your body vastly outnumber all the cells.
Revised estimates for the number of human and bacteria cells in the body. (2016) bioRxiv http://dx.doi.org/10.1101/036103
We critically revisit the ″common knowledge″ that bacteria outnumber human cells by a ratio of at least 10:1 in the human body. We found the total number of bacteria in the ″reference man″ to be 3.9·1013, with an uncertainty (SEM) of 25%, and a variation over the population (CV) of 52%. For human cells we identify the dominant role of the hematopoietic lineage to the total count of body cells (≈90%), and revise past estimates to reach a total of 3.0·1013 human cells in the 70 kg ″reference man″ with 2% uncertainty and 14% CV. Our analysis updates the widely-cited 10:1 ratio, showing that the number of bacteria in our bodies is actually of the same order as the number of human cells. Indeed, the numbers are similar enough that each defecation event may flip the ratio to favor human cells over bacteria.