It's an intriguing concept - that an imbalance in the gut microbiome could be involved in depression.
So scientists at the APC Microbiome centre, at University College Cork, started transplanting the microbiome from depressed patients to animals. It's known in the biz as a trans-poo-sion.
It showed that if you transfer the bacteria, you transfer the behaviour too.
Prof John Cryan told the BBC: "We were very surprised that you could, by just taking microbiome samples, reproduce many of the features of a depressed individual in a rat."
This included anhedonia - the way depression can lead to people losing interest in what they normally find pleasurable.
For the rats, that was sugary water they could not get enough of, yet "when they were given the microbiome from a depressed individual, they no longer cared", says Prof Cryan.
It is clearly a brain disorder. Patients lose control over their muscles as brain cells die and it leads to a characteristic tremor.
But Prof Sarkis Mazmanian, a medical microbiologist from Caltech, is building the case that gut bacteria are involved.
"Classical neuroscientists would find this as heresy to think you can understand events in the brain by researching the gut," he says.
He has found "very powerful" differences between the microbiomes of people with Parkinson's and those without the disease.
Studies in animals, genetically hardwired to develop Parkinson's, show gut bacteria were necessary for the disease to emerge.
And when stool was transplanted from Parkinson's patients to those mice, they developed "much worse" symptoms than using faeces sourced from a healthy individual.
Prof Mazmanian told the BBC: "The changes in the microbiome appear to be driving the motor symptoms, appear to be causal to the motor symptoms.
"We're very excited about this because it allows us to target the microbiome as an avenue for new therapies."
The evidence linking the microbiome and the brain is as fascinating as it is early.
But the pioneers of this field see an exciting prospect on the horizon - a whole new way of influencing our health and wellbeing.
If microbes do influence our brains then maybe we can change our microbes for the better.
Can altering the bacteria in Parkinson's patients' guts change the course of their disease?
There is talk of psychiatrists prescribing mood microbes or psychobiotics - effectively a probiotic cocktail of healthy bacteria - to boost our mental health.
Dr Kirsten Tillisch, at University of California, Los Angeles, told me: "If we change the bacteria can we change the way we respond?
But she says we need far bigger studies that really probe what species, and even sub-species, of bacteria may be exerting an effect on the brain and what products they are making in the gut.
Dr Tillisch said: "There's clearly connections here, I think our enthusiasm and our excitement is there because we haven't had great treatments.
"It's very exciting to think there's a whole new pathway that we can study and we can look and we can help people, maybe even prevent disease."
And that's the powerful idea here.
The microbiome - our second genome - is opening up an entirely new way of doing medicine and its role is being investigated in nearly every disease you can imagine including allergies, cancer and obesity.
I've been struck by how malleable the second genome is and how that is in such stark contrast to our own DNA.
The food we eat, the pets we have, the drugs we take, how we're born… all alter our microbial inhabitants.
And if we're doing that unwittingly, imagine the potential of being able to change our microbiome for the better.
Prof Cryan said: "I predict in the next five years when you go to your doctor for your cholesterol testing etc, you'll also get your microbiome assessed.
"The microbiome is the fundamental future of personalised medicine."