How many bacteria cells really exist in a human body: 1.3:1 to replace 10:1?

In a paper published in 2016, Sender and colleagues¹ explain that the numbers used for the 10:1 ratio were from crude measures of assessment.  

 

For the total number of human cells, 10¹² – 10¹⁴ was often quoted, and this was based on a 100kg man consisting of mammalian cells that have a volume of 1,000-10,000µm3.  However, the researchers describe an alternative method that does not use an average cell count, but instead calculates based on cell type, resulting in a revised number in the order of 3 x 10¹³ (30 trillion) human cells in a 70kg ‘reference adult man’.  

 

The figure, 10¹⁴ - 10¹⁵, that has often been quoted for total number of bacteria in humans, dates back to the 1970s when it was assumed the alimentary tract was 1 litre, with a constant bacterial density of 10¹¹ bacteria/g of wet gut contents.  However, such a high concentration of bacteria is only found in the colon, which has a volume of 0.4 litres.  A revised estimate for the total number of bacterial cells in the gut of a 70kg reference adult man is 3.8 x 10¹³… that’s 38 trillion bacteria!

 

Using these updated figures of 3.8 x 10¹³ bacteria in the body and 3 x 10¹³ human cells, for a ‘reference adult man’, the ratio of bacteria to human cells is around 1.3:1.

 

These revised calculations suggest that there are at least as many bacterial cells as there are human cells in our bodies, as opposed to there being 10-fold more bacteria. This does not take away from how important these microorganisms are and how many exist - if one considers that a typical bacterium is 1 micrometre (i.e. one millionth of a metre) in length, then 30 trillion bacteria linked one after another would make up a chain almost long enough to encircle the earth! 

 

¹ Sender, Fuchs & Milo (2016) Revised Estimates for the Number of Human and Bacteria Cells in the Body PLoS Biol 14(8): e1002533

 

13/04/18