Which bacteria have humans developed?

Do humans consist of more bacteria than body cells?

According to a popular rumor, we harbor around ten times more bacteria and unicellular microbes than our body has cells. However, this wisdom should be quickly forgotten by anyone who has ever heard of it - at least that's what researchers from Israel and Canada think, who have calculated particularly carefully. They came up with a body cell-to-microbe ratio of around 1 to 1.

The basis for this number was a 70 kilo, 1.70 meter tall and around 20 to 30 year old "reference man": on average, he would consist of around 30 trillion human cells and be home to 39 trillion bacteria, calculate Ron Milo and Ron Sender from the Weizmann Institute in Rehovot, Israel, and her colleague Shai Fuchs from the Hospital for Sick Children in Toronto. These numbers are estimates; and another person could well be populated by only half or twice as many bacteria - but in no case by around ten times more, i.e. the number that is particularly common among interested laypeople. The amount of germs and body cells is likely to be so similar that even a trip to the toilet could change whether you have more of one or the other - according to the detailed statement of the researchers in their pre-publication on the bioRxiv document server

The much-cited 10: 1 ratio probably goes back to a 1972 publication by the microbiologist Thomas Luckey, which "was elegantly carried out, although it should never have been intended to be widely cited decades later," say the authors of the new study. In 2014, the molecular biologist Judah Rosner of the US National Institutes of Health in Bethesda expressed his doubts about the 10: 1 thesis: There are simply too few really reliable figures on the amount of human and bacterial cells in the body.

Milo, Sender and Fuchs decided to clarify the data situation and compiled a comprehensive overview of older experimental data that came from all kinds of publications - such as DNA analyzes to estimate the number of cells or magnetic resonance imaging studies in which organ sizes were measured.

By far the largest fraction of all body cells are the red blood cells. According to Milo and colleagues, Luckey probably misjudged the number of bacteria in the digestive tract the most. Luckey had assumed that there were around 1014 Bacteria live because it increases the number of germs in one gram of fecal matter to 1011 had estimated and then simply extrapolated to a volume of one liter, which he applied for the entire digestive tract from the mouth opening to the anus outlet.

However, the vast majority of bacteria live in the large intestine, which holds around 0.4 liters, as Milo's team notes - and there are also fewer bacteria in stool samples than Luckey assumed.