You Are Outnumbered in Your Own Body

Half the cells you carry around are not you. The figure is less dramatic than the famous ten-to-one folklore — and somehow stranger for being true.

I want to start with a number that gets repeated so often it has become a kind of folklore: that the microbes living in and on your body outnumber your own cells ten to one. It is a striking claim, and it is also wrong, or at least badly out of date. When researchers actually sat down in 2016 and counted carefully, weighing the bacteria in the gut against the cells of a reference human body, the ratio came out closer to one to one. Roughly as many bacterial cells as human cells, give or take, with most of the bacteria packed into your large intestine. That is less dramatic than ten to one, but honestly it is strange enough. About half the cells you carry around are not you.

The collection of microbes is called the microbiome, and the more meaningful count is not cells but genes. Your own genome holds something like twenty thousand genes. The combined genomes of your gut microbes hold millions. So in terms of genetic capability, the metabolic tricks available to the community living inside you, you are vastly outgunned by your tenants. And those tricks are not idle. The bacteria in your gut do chemistry your own cells cannot. They break down kinds of dietary fiber that human enzymes simply cannot digest, and in doing so they produce short-chain fatty acids that feed the cells lining your gut and influence your metabolism. They synthesize certain vitamins. They occupy space and consume resources that would otherwise be available to harmful microbes, a kind of crowd control that makes it harder for pathogens to get a foothold.

The relationship with the immune system is the part I find most interesting, because it overturns the intuition that microbes are things to be kept out. Your immune system does not just tolerate the gut bacteria; it is educated by them. Animals raised in completely germ-free conditions, with no microbiome at all, develop abnormal immune systems, underdeveloped in specific ways, as if a teacher had failed to show up. The resident bacteria help calibrate the immune response, teaching it the difference between a harmless passenger and a genuine threat. A body that grew up without microbes does not know how to react properly to anything.

Most of what we know has come quickly, because the tools are recent. For most of microbiology’s history, studying gut bacteria meant trying to grow them in a dish, and the great majority of them refuse to grow outside the gut, so they were invisible. The breakthrough was genetic sequencing. By reading the DNA in a stool sample directly, without culturing anything, researchers could finally take a census of who is actually there, identifying species by their genetic signatures. A large coordinated effort, the Human Microbiome Project, spent the years around 2007 to 2012 cataloguing the microbes of healthy people, and the field has been moving fast ever since.

Now I have to put up a caution sign, because this is exactly the kind of science that gets oversold, and I would rather you hear the skeptical version from someone who finds the field exciting. Researchers have found differences in the microbiome associated with an enormous range of conditions, obesity, inflammatory bowel disease, diabetes, allergies, even mood and behavior through what gets called the gut-brain axis. The word doing a lot of quiet work in that sentence is associated. Finding that sick people have different gut bacteria than healthy people does not tell you which way the arrow points. The altered microbiome might be a cause of the illness, or it might be a consequence of it, or both might stem from some third factor like diet. Disentangling that is genuinely hard, and a lot of confident headlines have run far ahead of the evidence. The supplement aisle is full of products promising to fix your microbiome, and most of those promises are not backed by much.

There is one therapy where the evidence is strong enough to be beyond serious doubt, and it doubles as a proof of concept that the microbiome matters causally. Some patients suffer recurrent, dangerous infections of the gut by a bacterium called Clostridioides difficile, often after antibiotics have wiped out their normal flora and let the pathogen take over. Transplanting stool from a healthy donor, reintroducing a functioning microbial community, cures these infections at a rate that conventional antibiotics cannot match. It is a strange and somewhat undignified treatment, but it works, and it works because restoring the community restores the colonization resistance that was lost. That is the clearest demonstration that the microbiome is not just along for the ride.

So where does that leave us. With a real and important system that we are only beginning to understand, surrounded by a cloud of hype that runs well ahead of the data. The honest summary is that you are a walking ecosystem, that the ecosystem does real and necessary work, and that we know much less than the confident marketing implies about how to deliberately improve it. I think that is a more interesting place to be than the false certainty on either side, the old view that bacteria are simply germs to be eliminated, and the new view that a yogurt drink will optimize your life. The truth is that half of you is microbial, it matters, and we are still learning to read it.

You are a walking ecosystem that does real and necessary work — wrapped in marketing that runs well ahead of the data. The honest place to stand is between the old germ-phobia and the new yogurt-drink optimism.