A research group from Israel and Canada will make you rethink the popular notion that bacteria and other microbes in our body far outnumber our own cells (10:1). According to their latest research, the ratio between resident microbes and human cells is more likely to be 1:1!
A ‘reference man’ (one who is 70 kilograms, 20–30 years old and 1.7 metres tall) contains on an average about 30 trillion human cells and 39 trillion bacteria, say 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.
Even though these numbers are an approximation and there maybe some who may have half or twice as many bacteria as this reference man, it is still far from the 10:1 ratio of bacteria:cells that is commonly assumed.
Their recently posted manuscript delicately concludes that, “The numbers are similar enough that each defecation event may flip the ratio to favour human cells over bacteria.”
The myth that the bacteria:cells ratio is 10:1 persisted from an estimate made in 1972 by microbiologist Thomas Luckey which was “elegantly performed, yet was probably never meant to be widely quoted decades later.”
In 2014, a commentary written by molecular biologist Judah Rosner at the US National Institutes of Health expressed concerns over the “frequent assertion that the number of cells in the human microbiota is ten times as numerous as the number of cells in the human body.”
Milo, Sender and Fuchs decided to re-estimate the number by reviewing a wide range of recent experimental data in the literature, 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, they note.
A particular overestimate in Luckey’s work relates to the proportion of bacteria in our guts, Milo and colleagues say. Luckey estimated that guts contain around 1014 bacteria, by assuming that there were 1011 bacteria in a gram of faeces, and scaling that up by the one-litre volume of the alimentary canal, which stretches from the mouth to the anus.
But most bacteria reside only in the colon (which has a volume of 0.4 litres), Milo and colleagues point out – and measurements suggest that there are fewer bacteria in stool samples than Luckey thought.
Putting these calculations together, the researchers produce a ratio for microbial to human cells for the average man of 1.3:1, with a wide uncertainty.
“It is good that we all now have a better estimate to quote,” says Peer Bork, a bioinformatician at the European Molecular Biology Laboratory in Heidelberg, Germany, who works on the human and other complex microbiomes. “But I don’t think it will actually have any biological significance.