The Microbiome
You Are Not Mostly You
The cells in your body are outnumbered — and the organisms doing the outnumbering have been quietly shaping your mood, your immunity, and possibly your decisions for your entire life.
The Idea
For most of medical history, bacteria were the enemy — things to be killed, scrubbed away, or kept outside the body. The discovery of the microbiome forced a rethink so radical it borders on philosophical: your body is not a discrete individual organism but a walking ecosystem, host to trillions of microbes that have co-evolved with humans for hundreds of thousands of years. The old figure — that bacterial cells outnumber human cells ten to one — turned out to be a rough estimate, but the revised number is still striking: roughly equal, and highly variable depending on whether you've recently used the bathroom. What matters more than the count, though, is the function. Your gut microbiome alone performs tasks your own genome cannot: breaking down certain plant fibres, synthesising vitamins your body can't produce, training your immune system to distinguish friend from foe, and producing neurotransmitters including serotonin and dopamine. What makes this genuinely strange is the axis of influence. The gut-brain axis — a bidirectional communication highway running via the vagus nerve, the immune system, and the bloodstream — means your intestinal bacteria can send signals that affect mood, stress response, and cognition. This isn't fringe science anymore. It's a rapidly hardening area of research with real clinical implications. The microbiome doesn't just live inside you. In a meaningful sense, it participates in being you.
In the World
In 2011, a young woman referred to in published case studies as 'Patient C' was suffering from recurrent, life-threatening Clostridioides difficile infections — a bacterial overgrowth that causes severe intestinal damage and had resisted round after round of antibiotics. Her doctors at a hospital in the Netherlands proposed something that sounded almost medieval: a faecal microbiota transplant, or FMT, in which stool from a healthy donor is introduced into the patient's gut to reestablish a diverse microbial community. Patient C received the transplant from her overweight daughter. Within weeks, her C. difficile infections resolved — a success. But over the following months, something unexpected happened: she began gaining significant weight, despite no meaningful change in her diet. She eventually became obese, a condition she had never experienced before. Her daughter's microbiome, it seemed, had transferred not just the ability to fight infection but also metabolic tendencies. The case became one of the most-cited examples in microbiome research — not because it proved anything definitive, but because it crystallised a disquieting possibility: that the microbial communities living in our guts are not passive passengers but active contributors to our metabolic identity. FMT is now an approved therapy for recurrent C. difficile in several countries, with clinical trials underway for conditions ranging from inflammatory bowel disease to depression. The cautionary weight-gain story didn't stop the field — it deepened it.
Why It Matters
Most of us walk around with a mental model of health that is essentially mechanical: eat well, sleep enough, exercise, avoid toxins. The microbiome research doesn't invalidate that model, but it adds a layer of complexity that changes how you might interpret your own body. That craving for sugar after a course of antibiotics? Possibly not weakness — possibly a destabilised gut community signalling for fast fuel. The anxiety that spiked during a period of poor eating? There may be more going on than stress alone. This isn't an invitation to outsource your choices to your bacteria or to fall into pseudoscientific rabbit holes about 'gut healing' supplements. The commercial world has rushed far ahead of the science here. But the genuine insight is worth holding: you are, biologically, a collaborative project. Health is less about maintaining a clean machine and more about tending an ecosystem. Diversity — in diet especially, since different fibres feed different microbial species — seems to be the most robust variable under your control. The more varied what you eat, the richer the community inside you. That's not a diet tip. It's a reframe of what a body actually is.
A Question to Ponder
If the microbes in your gut genuinely influence your mood and cognition, where exactly is the boundary between your choices and theirs?
Get a new one of these every morning.
Start learning with Thinkable