On the blog, All about Scientist in Microword: Microbiology, I read the post We are 90% bacteria, actually, which says that humans are 90% bacterial cells.

If this is the case, then why don't we behave like them and why isn't the bacteria genetic material more dominant?

As bacteria helps us in making many enzymes for us, can we say that we are symbiotic creatures?

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    $\begingroup$ Can you please cite the source you are talking about? Your point is not completely wrong, since we harbour indeed a lot of bacteria in our gut and on our skin and their numbers (depending on the source) outnumber the number of our cells by about one magnitude. Still, they are not part of us. $\endgroup$
    – Chris
    Commented Dec 13, 2015 at 12:56
  • $\begingroup$ The bacteria within and on our bodies do engage in a symbiotic relationship – defined as an interaction between two different organisms living in close physical association. It can even be said that the relationship between certain bacteria and humans is mutualistic. For example, the bacteria residing on our skins and within our intestines benefit nutritionally, while we benefit from the bacterial "barrier effect". $\endgroup$
    – Sentient
    Commented Dec 13, 2015 at 21:00
  • $\begingroup$ One could consider energy: If you count human somatic cells as zero percent bacterial (you cheater!) then you ask what fraction of heat comes from those as opposed to bacteria. I think it is ~8% bacterial, almost all form the gut, but am not sure. $\endgroup$ Commented Oct 18, 2020 at 19:03

2 Answers 2


I am not sure what you read as you have not supplied any references, but humans are not 90% bacterial cells. (OP subsequently provided; see Edit 2)

Humans are 100% human cells, however for every one human cell, approximately 10 single celled organisms (Bacteria or Fungi) live in (colloquially) or on the human body. This is referred to as the microbiome. You can find more information in the link I provided or in the answers to the following questions on Biology SE.

Think of it this way; just because the human body provides these organisms with a home does not mean that it is composed of them, that would be like saying an apartment building is made up of x percent of humans or pets or mice or insects just because that is where they live.

You may also find some interesting information at The Secret World Inside You from the American Museum of Natural History.


As bacteria helps us in making many enzymes for us, can we say that we are symbiotic creatures?

The definition of symbiosis is:

The interaction between two different organisms living in close physical association, typically to the advantage of both.

Given that definition, and given the fact that there are some bacteria in our microbiome that are able to produce metabolites that our cells cannot, and we provide them with a "home" with the necessary nutrients for their propagation, then you can say that this is in fact a symbiotic relationship.

The more technical description that is often used for the microbiome is that these organisms that populate the human body are commensal.

Commensalism is defined as:

An association between two organisms in which one benefits and the other derives neither benefit nor harm.

In this respect, the majority of our microbiome are commensals, they live on us, but give us nothing in return, and for the most part do us no harm.

Some of the organisms can be opportunistic pathogens in the sense that if they are able to gain access to normally sterile areas of our body, such as in peritonitis arising when the intestines are perforated, then they can be harmful, but in their general niche, they are just there.

As we learn more about the microbiome, we are beginning to see that there is a far more complex relationship between host and microbe than we anticipated, so over time, the views on the microbiome are likely to change from the understanding that we have today.

Edit 2

To address the source that you added All about Scientist in Microword: Microbiology, I would say that the blogger is guilty of hyperbole. While Steven Gill, the molecular biologist that the blogger quoted, may not have chosen his words carefully, he was not implying that humans are 90% bacteria, as the blogger then posits.

Gill makes the mistake of using the word "in" when he states "There are some estimates that say 90% of the cells in our body are actually bacteria...." The microbiome exists on the surfaces of our body not in it.

We forget that the gastrointestinal tract, urogenital tract, and respiratory tract are lined with epithelial cells and are thus external surfaces, just as our skin is an external surface. So the microbiome lives on our body, and not in it or as a part of it.

No place in our body do we have organs or tissue that is a homogenate of human cells and bacterial cells functioning together normally. And while there are bacteria that are able to infiltrate cells and live within vesicles or the cytoplasm, this is a pathogenic condition and not what we would normally expect to see in a healthy human.

We know that this is the case, because developmentally all of our body cells and germ cells arise for the fertilized egg. It isn't until much later in development or at the moment of birth (there are some reports that microbiome begins to cross the placenta prior to birth) that we acquire our microbiome. If it were the case that humans, or any eukaryotic organism for that matter, were 90% bacteria, then we would not develop in the sterile environment of the amniotic sac. The lack of bacteria during development would be fatal.

  • $\begingroup$ microbioresearch.blogspot.in/2008/06/… $\endgroup$
    – user101184
    Commented Dec 14, 2015 at 13:47
  • $\begingroup$ this is the link where you will find about the fact I discussed. $\endgroup$
    – user101184
    Commented Dec 14, 2015 at 13:50
  • $\begingroup$ @user101184 Please edit your question to include the reference. Also, you should quote what in the site brought you to your conclusion. $\endgroup$
    – AMR
    Commented Dec 14, 2015 at 13:53
  • $\begingroup$ But we can say that 90% of human weight is actually bacterial. "and for the most part do us no harm." - we weigh more, so we are less enduring. We run slower and jump lower. Isn't it so? Then don't they cause us a disadvantage? $\endgroup$
    – rus9384
    Commented Jan 24, 2019 at 2:58

I just wanted to tackle the interesting numbers here. So there are roughly 10^13 human cells in our body, and 10^14 bacterial. That is where your 90% bacteria number comes from.

But remember that bacterial cells are much, much smaller than human cells. E. coli ranges from 0.5um to 2um. Human cells are ~100X larger at 0.1mm or 100um. That translates to:

Surface area - E. coli is ~3.14um^2, human cells are ~32,000um^2.

Volume - E.coli is ~0.52 um^3, human cells are ~524,000um^3.

From an information content point, each of your cells contains 3 billion base pairs of genetic information, an e. Coli cell is ~4 million base pairs (bp). So your cells contain 3^23 bp, while the total bacterial content is closer to 4^21 bp. There is almost 100 times as much of your information as bacterial information. Also yours is all the same, vs the different types, colonies, and progeny of bacteria in your body. To put in similar terms as your question, while bacterial numbers account for 90% of your total cell numbers, their genetic information accounts for ~1% of the total.

  • $\begingroup$ Nice analysis and good insight. $\endgroup$
    – AMR
    Commented Dec 15, 2015 at 21:42

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