How many bacteria are there in the body?
Jacob Schor, ND
March 10, 2016
There is a long-standing belief that microbes in the body far outnumber our own cells by a factor of ten to one. It turns out this is a myth; the actual ratio is closer to one to one.
We have been guilty of perpetuating this myth over the years and are obligated to set the record straight.
An average guy, what researchers call a 'reference man' (70 kilograms, 20–30 years old and 1.7 meters tall), contains on average about 30 trillion human cells and 39 trillion bacteria, according to a recent paper by Ron Milo and Ron Sender at the Weizmann Institute of Science in Rehovot, Israel, and Shai Fuchs at the Hospital for Sick Children in Toronto, Canada. 
Those numbers are approximate — another person might have half as many or twice as many bacteria, for example — but this is a long way from the 10:1 ratio that has been quoted for years.
The 10 to 1 ratio stems from an estimate made by microbiologist Thomas Luckey in 1972, what was at the time an educated guess calculated on the back of an envelope. Something about that ratio proved attractive and people and scientists kept repeating it.
Milo, Sender and Fuchs estimated the number by reviewing more recent experimental data including DNA analyses to calculate cell number and magnetic-resonance imaging to calculate organ volume. The vast majority of human cells are red blood cells. 
Luckey’s early guesses may be thrown off by his estimates of the number of bacteria in the gut. Milo and colleagues suggests that there are actually fewer gut bacteria than Luckey thought. More likely the ratio is 1.3 to 1.
“The numbers are similar enough that each defecation event may flip the ratio to favor human cells over bacteria,” 
2. Sender, R., Fuchs, S. & Milo, R. Preprint on bioRxiv http://dx.doi.org/10.1101/036103 (2015).