Our bodies don't harbour more bacteria than human cells

Toronto/London: If you were convinced till now that your body has more bacteria (good and bad) and other microbes than human cells, it is time to revisit the decades-old assumption, says a team of researchers from Israel and Canada.

Turning the conventional knowledge on its head that bacteria and other microbes in our body outnumber our own cells by about 10 to one, the team has calculated that the ratio between resident microbes and human cells is more likely to be one-to-one, the prestigious scientific journal Nature reported.

“A 'reference man' (one who is 70 kg, 20-30 years old and 1.7 metres tall) contains on average about 30 trillion human cells and 39 trillion bacteria,” said Ron Milo and Ron Sender from the Weizmann Institute of Science in Rehovot in Israel and Shai Fuchs at the Hospital for Sick Children in Toronto.

“Those numbers are approximate. Another person might have half as many or twice as many bacteria, for example, but far from the 10:1 ratio commonly assumed,” they noted in their paper.

“The numbers are similar enough that each defecation event may flip the ratio to favour human cells over bacteria,” they added.

The 10:1 myth persisted from a 1972 estimate by microbiologist Thomas Luckey, which was “elegantly performed, yet was probably never meant to be widely quoted decades later”, the authors added.

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.

In 2014, molecular biologist Judah Rosner at the US National Institutes of Health at Bethesda, had expressed his doubts about the 10:1 claim, noting that there were very few good estimates for the numbers of human and microbial cells in the body.