The estimation that bacterial cells in and on the body outnumber human cells by a ratio of ten to one has been widely cited in both popular media articles and scientific literature. Recently, three scientists from Israel and Canada took it upon themselves to critically examine where this estimate came from and whether it holds true.
In a paper published in BioRxiv, Sender, et al. showed both the numerator and the denominator of the 10:1 ratio appeared to be based on crude assessments from decades ago. They traced the origin of the numbers back to a 1972 journal article by Thomas D. Luckey, who performed what they called a “back of the envelope estimate” that was never intended to be quoted widely.
Usual estimations of bacterial cells in the body range from 1014 or 1015, assuming a “reference man” of between 20 and 30 years of age who is 170 cm tall and weighs 70 kg; measured concentrations of bacteria (that is, bacteria per gram) are then multiplied by the volume of each organ.
The main problem with the estimation method was that it assumed a constant bacterial density throughout the digestive tract. Now we know most of the microbes in the digestive tract are concentrated in the colon. By incorporating this and other more recent information, authors came up with 3.9×1013 as the new estimate of bacteria in a standard adult male.
A recent estimate of the number of cells in human body was 3.7×1013. Authors carried out their own calculation, systematically counting cells by type, and concluded that the actual number was approximately 3×1013.
Authors used their revised estimates in a calculation that divided the number of bacterial cells by human cells; the new figure was around 1.3, with an uncertainty of 25% and a variation of 53% over the population of standard males. They hypothesized this ratio may vary slightly—but probably not to a huge extent—in women, children, and older adults. With a ratio this even, they noted, the excretion of bacterial cells that happens during defecation may temporarily change the ratio in favour of human cells.
Updating the ratio from 10:1 to around 1:1, the authors said, need not take away from the “biological importance” of the human microbiota. Moreover, Sender, et al. do not purport to have had the final say on this topic; They explain, “We view this manuscript as a call to revitalize efforts in the direction of quantifying absolute cell content of human tissues and their commensal bacteria.”
Bianconi E, et al. (2013). An estimation of the number of cells in the human body. Ann Hum Biol 40, 463–471 doi: 10.3109/03014460.2013.807878
Luckey T. (1972). Introduction to intestinal microecology. Am J Clin Nutr 25, 1292–1294
Sender R, Fuchs S, & Milo R. (2016) Revised estimates for the number of human and bacteria cells in the body. bioRxiv doi: http://dx.doi.org/10.1101/036103