Think you’re fully human? Think again. 

We’re only 43% human cells. Only 1% uniquely human DNA. 

The rest? Microbes. Bacteria, fungi, viruses. Single-celled organisms called ‘archaea’ that are like bacteria, but not.

And most of them – a 100 trillion! – are found in the dark, oxygen-deprived recesses of our gut. A powerful four pound “chamber of secrets” that could not only be a major defence against infection, but the master puppeteer of our bodies and brains.

This is the universe pioneering neurobiologist John Cyran is on a mission to crack open. 

Over the past two decades, concrete evidence has piled up. That it’s no longer a simple relationship between food and mood, diet and digestion. That the “second brain” thriving in our gut – thanks to a network of nerve cells lining the gastrointestinal tract – isn’t just talking to its sister nerve centre in the brain on its own. 

Instead, playing a starring role are armies of microscopic colonists – and they’re affecting everything from how our brain develops and functions to how it ages. Mood. Motivation. Mental Health. Behaviour. Cognition. Development of disease. That gut feeling you have? It may not be your subconscious mind but signalling microbes. 

In short: it’s no longer a gut-brain axis in action – but the microbiome-gut-brain axis. 

Want proof? Take Cryan and his lab’s experiments. Over the past decade and more, his lab has shown how mice that are raised in a completely sterile environment – so that they grow up without any bacteria in their gut – do not behave like those who grow up in normal, bacteria-riddled environments. Their fear responses aren’t the same. Their pain processing is different. In short, their brains don’t develop as they would normally. 

Or take the lab’s study where they transplanted – hold your breath – fecal matter into mice from a human being with symptoms of severe depression. Guess what happened in the coming days and months: the mice stopped deriving joy from food. 

Or another set of experiments where they have fed microbes from younger animals into older models and reversed the effects of ageing on brain health. 

Why? How? That’s exactly what Cryan – who has always been fascinated by the ‘why’ in everything – is looking to answer: who these cells are. Their family tree. What they’re doing, what they’re making. (Apparently, it’s gut microbes that secrete a chemical whose lack in the brain leads to depression. Other microbes secrete dopamine, serotonin, oxytocin: powerful mood and happiness regulators.) 

And the highways of communication between the gut microbiome and the brain. Is it the spinal cord? Neurons and hormones? The vagus nerve, the longest nerve that carries signals between the brain, heart, and digestive system? (For example, he and his collaborators showed in yet another experiment that if you cut this core nerve in mice, the effects of a certain bacteria do not materialise on brain and behaviour.) 

As the “microbiome revolution” gains traction, researchers are parsing the good from the bad. Some are exploring the link between microbes and cancer. Others, how gut bacteria could help diagnose autism. And yet others the “microbiome signatures” of people suffering from multiple sclerosis and Parkinson’s. 

Some are calling the microbiome an “organ”. Others are discovering how it is a “battleground” – with rival tribes of bacteria that often switch sides thanks to their own DNA makeup – with spillover effects for their hosts. And yet others are focusing on germ habitats in specific regions of the body – like the brain, which may host as many as 100,000 species of bacteria, viruses, and fungi, contrary to accepted wisdom of a pathogen-free cranium. 

As for Cryan, he is currently leading the emerging field of “psychobiotics”: leveraging microbes for mental health – to prevent, improve, even reverse. Exploring the effects of medicines on the microbiome. Tweaking “viromes” – viruses that teem in our gut – to find new ways of treating stress and anxiety. 

Facets he’s explored in his best-selling book The Psychobiotic Revolution: Mood, Food, and the New Science of the Gut-Brain Connection.

Fascinatingly too, each of us has our own unique microbiome – making up 99% of our DNA. Which is leading scientists to ask just what it means to be “human” in the first place. 

At SYNAPSE 2025, he will help us meet our gut microbiome: the “little factories” that produce an array of weird and wonderful chemicals that our bodies would not make without them – and how these interact with human health, disease, immunity, and infection. He will unpack what came first, the human or the microbiome. Share how to cultivate a healthy inner ecosystem. And why the maxim of the day isn’t “you are what you eat” – but “you are what your microbes eat.” 

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