Despite what you’ve read, our bodies don’t contain 10 times more bacteria cells than human cells

If you’ve ever read anything about the colonies of bacteria that live on and inside you, you’ll no doubt have come across the neat little ‘fact’ that microbial cells outnumber human cells in your body by a ratio of around 10:1. You’ll find it in scientific papers, magazine articles, TED talks, and popular science books, and while it does a good job at illustrating just how crucial bacteria are to the existence of human beings, it isn’t actually even remotely true.

A review of more than four decades of research into the human microbiome has found that there is zero scientific evidence to back this oft-cited factoid up. Instead, the ratio looks to be about 1.3-to-1, with the average human playing host to around 100 trillion microbes, give or take. But even that isn’t the whole story.

A team of biologists led by Ron Milo from the Weizmann Institute of Science set out to review all the available literature on the microbe populations that live inside us, and found that for a man between 20 and 30 years old, with a weight of about 70 kg (154 pounds) and a height of 170 cm (about 5’7) – they call him the ‘reference man’ – there would be about 39 trillion bacterial cells living among 30 trillion human cells. Which gives us a ratio of about 1.3:1 – almost equal parts human to microbe. Source: Despite what you’ve read, our bodies don’t contain 10 times more bacteria cells than human cells

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2 Responses to Despite what you’ve read, our bodies don’t contain 10 times more bacteria cells than human cells

  1. Phil Krause says:

    The Science Alert website that this information is from may not be as reliable as all the others referenced within it. I saw another headline from the same website stating that there were more bacterial cells than we thought on and within our bodies. It’s not an easy estimation to make reliably but I think that we can be pretty sure that there are more bacterial cells living on and within us than the number of cells that make us up. Remember that bacterial cells are, on average, about 10 times smaller than our own cells in each direction so their volume and mass will be around 10,000 times less than our own. Thus they don’t take much room up compared to our cells. There are probably more like 50 times more bacterial cells than our own.
    Our skin contains bacteria from around 1000 species from 19 phyla totaling something like 10 billion in total. Around 500 to 650 species of bacteria live in our mouths totaling around 20 billion. But if you have plaque around your teeth many, many more as these are made from bacterial mats. But it is our gut where we have the most which many people estimate to number more than 100 trillion. Our faeces (normally) contain about 75% water and 25% solid material. Of this solid material about 30% is (mainly) dead bacteria, 30% indigestible fibre, 10-20% fats, 10-20% inorganic substances and 1-2% protein. You can just imagine how many bacteria make up 30% of your faeces as they are so very small. The brown colour of our faeces comes from the action of bacteria on our bilirubin and the smell is largely due to the action of bacteria.
    Imagine how many virus particles we contain compares to our own and bacteria. 94% of all the DNA in oceans is viral DNA. They are around 10 times smaller in each direction than bacterial cells so they take up so little space its beyond belief. There are over 100,000 known fragments of viral DNA in our genome making up around 8% of our total. This is more than the % of DNA that code for our own proteins. We pee out countless trillions of viral particles several times a day and nobody knows where they reproduce inside us yet, but they must cause constant inflammation. Don’t forget to put that toilet seat down!

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