Humans may not coexist with as many bacterial cells as experts previously thought. The common statistic cited (including by us) hypothesizes that there are 10 bacterial cells for every human cell in the body. However, a new review by researchers at the Weizmann Institute of Science in Israel finds that’s a vastly overblown statistic; the numbers might be closer to equal.

The 10-to-1 ratio comes from crude assessments of both the number of microbial cells in the body and the number of human cells, based specifically on the body of an average human male. “We performed a thorough review of the literature and found a long chain of citations originating from one ‘back of the envelope’ estimate. This estimate, though illuminating, was never meant to serve as the cornerstone of an entire field,” the researchers write in a paper posted on bioRxiv [PDF], a place for biologists to post drafts of their work before publishing in a peer-reviewed journal. The new estimate is based on counts of bacterial and human cells in different body organs, especially the colon (a great portion of the human microbiome resides in the digestive system).

If the numbers are as close as these researchers hypothesize, each bathroom trip might alter the delicate balance of microbes to human cells in favor of the human. "Indeed, the numbers are similar enough that each defecation event may flip the ratio to favor human cells over bacteria,” the researchers write.

However, these numbers might not be all that exact either. Lead author Ron Milo and his colleagues thoroughly reviewed the scientific literature for the best estimates of the numbers of human cells in different kinds of tissue and the numbers of microbes in stool samples, coming up with 30 trillion human cells compared to 39 trillion microbes. Yet this is still a very rough calculation.

It's important to note, as Ed Yong does in The Atlantic, that these numbers don’t change anything about how the microbiome functions. Whatever their numbers, the bacteria on and inside our bodies protect our skin, digest our food, and even affect our mood. Still, the study does highlight how much we still don’t know about our bodies, down to the number of cells we possess.

[h/t: The Atlantic]