The Consensus: Our Bodies Contain More Bacteria Than Human Cells (But Not 10 Times More)

Where are the bacteria in our body and what are they doing there?

Although we commonly associate bacteria with infection and disease, most bacteria that live in and on our bodies are essential for our survival. These ‘good’ bacteria are called ‘symbiotic’, meaning that both the bacteria and the host human benefit. For example, the Prevotellafamily of bacteria, which are commonly found in our guts, help our digestive system break down the complex carbohydrates that we eat such as fibre. In fact, most of the bacteria in our bodies are found in our stomachs, small and large intestines and these bacteria are essential for digestion (there is so much bacteria in our guts that over 50% of human faeces is made of bacteria!).

Bacteria don’t just live in our guts – they can be found all over our bodies including on our skin and in our lungs. Other microbes such as fungi, protists and viruses also live alongside bacteria. All of the microbes that live in or on the human body are collectively called the ‘human microbiome’. We know that our microbiome plays an important role in our health and that if the balance of this microbial ecosystem is disrupted we can develop diseases, but we still know very little about the thousands of different species that live in our bodies and what they all do.

How accurate is this ratio?

Dr Hanne Tytgat, a microbiologist from Wageningen University in the Netherlands, attributed the revised 1:1 ratio to a paper published in 2016 which estimated the total number of human cells in a 70kg ‘reference man’ to be 3.0 x 10¹³ and the number of bacterial cells in this hypothetical man to be 3.8 x 10¹³. It is impossible to actually count each of the individual 30 trillion cells or bacteria. To estimate the total number of cells, human samples were analysed to count how many cells they contained and then multiplied up to account for the whole tissue. For example, it was already known that 97% of all human cells are red blood cells. To find the total red blood cell count in a human, the number of blood cells were counted in a small volume of blood and then this number was multiplied up to account for the 4.9 litres of blood that pump around an average body. Similarly, by looking at how many bacteria cells were found in stool samples, the authors of the 2016 paper revisited the previous estimations of how many bacteria are found in the colon, and added this to estimates of how many reside in the rest of our guts and elsewhere in the body. More accurate bacterial counting methods, along with acknowledging that the colon is the most bacterial-rich organ in the body, explain why the new ratio is much lower than the original 10 to 1.

Although this new ratio is likely to be more accurate than the original 10:1, it is still based on numbers which are estimates. These estimates are based on various assumptions and come with lots of uncertainties. They will also vary from person to person – we are not all the hypothetical ‘reference man’. Despite these caveats, these revised estimates are an interesting delve into the exciting world of the microbial ecosystems that we carry around in our bodies.

The takeaway: We are only about 50% human, the other half of us is made of trillions of microbes – called the microbiome.