ONE WILD WORLD — NEWSLETTER

Issue #014  |  Your Body Is Wild  |  April 2026

There are more bacterial cells inside your body right now than there are human cells. You are, by raw count, a slight majority bacterial. And those bacteria aren’t just passengers — they digest your food, train your immune system, produce vitamins you can’t make yourself, and send chemical signals to your brain that influence your mood.

BY THE NUMBERS

For decades, the textbook answer was that bacteria outnumbered human cells in your body by ten to one. You were, supposedly, a walking colony — 90 percent bacteria, 10 percent you. That number got repeated everywhere. In magazines. In TED talks. In every single wellness podcast on Earth.

In 2016, three scientists at the Weizmann Institute in Israel finally sat down and actually did the maths. Turns out the 10-to-1 figure traced back to a single 1972 estimate that nobody had bothered to double-check in forty years.

The real ratio is much closer to 1.3 to 1. About 38 trillion bacterial cells versus 30 trillion human cells in the average adult body.

Which, if you stop and think about it, is somehow worse. You are not a clear majority of yourself. You are a narrow win at best.

The vast majority of your bacteria live in your gut — specifically your large intestine, which functions less like a digestive tube and more like a dense, warm, fermenting ecosystem. Over a thousand different species are at work in there right now. Some scientists estimate the number could be higher than 2,000 if you count the rarer strains.

But the gut is not the only neighbourhood. You have distinct bacterial colonies on your skin, in your mouth, in your nose, in your armpits, between your toes, in your ears. Each region hosts a different community adapted to different conditions — and each of those communities is as unique to you as a fingerprint.

The bacteria on your left hand are genuinely different from the bacteria on your right. Two people standing next to each other in a lift are carrying two completely separate microbial cities on their skin.

If you could somehow sterilise yourself overnight — kill every bacterium in and on your body — you would not feel free. You would feel catastrophically broken.

Gut bacteria do most of the heavy lifting when it comes to digestion. They break down the fibre your own enzymes can’t touch. They ferment complex carbohydrates into short-chain fatty acids, which your colon cells use for energy. Without them, a large fraction of what you eat would pass through you essentially untouched.

They also produce vitamins. Vitamin K, biotin, and several of the B vitamins are synthesised by your gut bacteria and absorbed straight into your bloodstream. Your body has quietly outsourced a chunk of its chemistry to tenants it pays in leftover pizza crusts.

And then there’s the immune system. Your immune cells are trained, from the moment you are born, by constant exposure to the friendly bacteria living inside you. They learn what’s safe and what isn’t largely by watching the microbiome — which is part of why children raised in sterile environments tend to develop more allergies and autoimmune conditions later in life.

This is the part that sounds like science fiction and isn’t.

Your gut is connected to your brain by a thick bundle of nerve fibres called the vagus nerve — and the traffic on it runs in both directions. Gut bacteria produce neurotransmitters. Real ones. The same chemicals your brain uses.

Around 90 percent of your body’s serotonin — the neurotransmitter most associated with mood and happiness — is produced in the gut, not the brain. Gut bacteria are directly involved in its manufacture. They also produce dopamine, GABA, and acetylcholine. These chemicals influence how you feel, how well you sleep, and how anxious you get on a Tuesday afternoon for no obvious reason.

Mice raised in germ-free conditions — with no bacteria at all — show abnormal stress responses and altered behaviour. Transplant the gut bacteria of an anxious mouse into a calm mouse, and the calm mouse starts behaving like an anxious one. The reverse works too.

This is where the whole thing gets properly strange. Bacteria have been here for about 3.5 billion years. Multicellular animals — the lineage that eventually produced you — showed up around 600 million years ago. Bacteria were already running the planet’s chemistry for three billion years before anything resembling a human ancestor was even a faint possibility.

When your lineage finally turned up, the bacteria didn’t exactly step aside. They moved in. Your gut, your skin, your mouth — all of it is real estate they colonised a very long time ago, and every single human being since has been, essentially, a rental property.

You don’t own your body. You occupy it. And the tenants that were there before you are still there, running most of the chemistry that keeps you alive.

38 trillion bacteria. A thousand species. Two kilos of microbes. A gut that makes your serotonin. A skin that fingerprints you every time you touch a doorknob. A body that you share, whether you agreed to or not.

You were never just yourself. You never could have been.

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SOURCES

Sender R., Fuchs S. & Milo R. — “Revised Estimates for the Number of Human and Bacteria Cells in the Body”, PLOS Biology (2016)

NIH Human Microbiome Project — hmpdacc.org, reference data and species counts

Nature — “Structure, function and diversity of the healthy human microbiome” (2012)

Franzosa E. et al. — “Identifying personal microbiomes using metagenomic codes”, PNAS / Harvard T.H. Chan School of Public Health (2015)

Yano J.M. et al. — “Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis”, Cell (2015)

Cryan J.F. & Dinan T.G. — “Mind-altering microorganisms: the impact of the gut microbiota on brain and behaviour”, Nature Reviews Neuroscience (2012)

Sonnenburg J. & Sonnenburg E. — The Good Gut (Penguin, 2015)