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If there is something that makes us human, it is our minds, thoughts and emotions.
However, a new and controversial theory is emerging that holds that intestinal bacteria alter our brain invisibly.
Science is seeking to understand how the trillions of microbes that live inside and outside our microbiome affect our physical health.
And now even conditions like depression, autism and neuro-degenerative diseases are being linked with these little creatures.
We have known for centuries that the way we feel affects our gut – just think about what happens before an exam or a job interview – but now this is being seen as a two-way street.
Groups of researchers believe they are on the cusp of a revolution that uses “mood microbes” or “psychobiotics” to improve mental health.
The research that led to the concept was carried out at the University of Kyushu in Japan.
The scientists showed that “germ-free” mice-those that never came into contact with microbes-generated twice as much stress hormone when they were distressed as normal mice.
The animals were identical except for their microbes. It was a strong indication that the difference was the result of their microorganisms.
“All the neuroscientists who study microbes go back to that first article,” says Dr. Jane Foster, a neuropsychiatrist at McMaster University in Canada.
“It really was very powerful for those of us who were studying depression and anxiety.”
It was the first indication of microbial medicine in mental health.
How bacteria could be altering your mind
The brain is the most complex object in the universe we know, so how could it be reacting to the bacteria in the intestine?
- A possible route is the vagus nerve, it is an information highway that connects the brain and the intestine.
- Bacteria break down fiber in the diet by transforming it into chemicals called short-chain fatty acids, which can have effects throughout the body.
- The microbiome influences the immune system, which has also been implicated in brain disorders.
- There is even emerging evidence that intestinal bacteria may be using tiny strips of genetic code called microRNAs to alter how DNA works in nerve cells.
Now there is a lot of research that links germ-free mice with changes in behavior and even brain structure.
But the completely sterile upbringing of these creatures is nothing like the real world. Humans are constantly in contact with microbes in our environment, none of us is free of germs.
At the University Hospital of Cork, in Ireland, Professor Ted Dinan tries to discover what happens to the microbiome of his depressed patients.
In general, a healthy microbiome is a diverse microbiome, containing a wide variety of different species that live throughout our body.
Professor Dinan explains: “If you compare someone who is clinically depressed with someone who is healthy, there is less diversity of microbiota.
“I am not suggesting that it is the only cause of depression, but I do believe that for many people it does play a role in the genesis of depression.”
He also argues that some lifestyles that weaken our intestinal bacteria, such as a diet low in fiber , can make us more vulnerable.
- You are more microbe than human : if you count all the cells in your body, only 43% are human.
- The rest is our microbiome and includes bacteria, viruses, fungi and unicellular archaea.
- The human genome – the complete set of genetic instructions for a human being – consists of 20,000 instructions called genes.
- But if you add all the genes in our microbiome, the number reaches between two million and 20 million microbial genes.
- It is known as the second genome and is linked to diseases that include allergy, obesity, inflammatory bowel disease, Parkinson’s, depression and autism. You can even determine if the drugs for cancer work.
It is an intriguing concept: that an imbalance in the gut microbiome could be related to depression.
So scientists at the APC Microbiome Center, University College Cork, began transplanting the microbiome from depressed patients to animals.
What they found was that if the bacteria is transferred, the behavior is also transferred.
Professor John Cryan told the BBC: “We were very surprised that we could, by simply taking microbiome samples, reproduce many of the characteristics of a depressed individual in a rat.”
This included “anhedonia,” the way in which depression can lead people to lose interest in what they normally find pleasurable.
In rats, this was seen in the form of sugar water, which they normally consume obsessively. However, “when they were given the microbiome of a depressed individual, they did not care anymore,” says Professor Cryan.
Similar evidence – which connects the microbiome with the intestine and the brain – is emerging in relation to Parkinson’s disease.
It is clearly a brain disorder. Patients lose control over their muscles as brain cells die and this leads to the tremor that characterizes the disease.
But Professor Sarkis Mazmanian, a medical microbiologist at the California Institute of Technology (Caltech), in the United States, postulates that intestinal bacteria are involved .
“Classical neuroscientists would consider it a heresy to think that you can understand the events in the brain by investigating the intestine,” 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 modified to develop Parkinson’s, showed that intestinal bacteria were necessary for the disease to emerge.
And when the stools of Parkinson’s patients were transplanted, those mice developed “much worse” symptoms than if they received stool from a healthy individual.
Professor Mazmanian told the BBC: “The changes in the microbiome seem to be driving motor symptoms, they seem to be causal of those symptoms.”
“We are very excited about this because it allows us to target the microbiome as a way for new therapies .”
The evidence that unites the microbiome and the brain is as fascinating as it is incipient .
But the pioneers in this field see an exciting prospect on the horizon: a completely new way of influencing our health and well-being.
If microbes influence our brains then maybe we can improve our microbes.
Can the alteration of bacteria in the bowels of Parkinson’s patients change the course of their disease?
There is talk of psychiatrists prescribing mood microbes or “psychobiotics” – essentially a probiotic cocktail of healthy bacteria – to improve our mental health.
Dr. Kirsten Tillisch of the University of California, Los Angeles, told me: “If we change bacteria, can we change the way we respond to them?”
But she says that we need much broader studies that really investigate what species, and even which subspecies, of bacteria may be exerting an effect on the brain and what they are producing in the intestine.
” There are clearly connections and I think our enthusiasm and emotion are due to the fact that we have not found good treatments,” said Dr. Tillisch.
“It is very exciting to think that there is a completely new path that we can investigate and we can help people, even to prevent diseases.”
And that is the most powerful idea here.
The microbiome, our second genome , is opening up a whole new way of doing medicine and its function is being investigated in relation to almost every disease you can imagine, including allergies, cancer and obesity.
I was surprised at how malleable the second genome is and how that is in stark contrast to our own DNA.
The food we eat, the pets we have, the medicines we take, how we are born … everything alters our microbial inhabitants.
And if we do everything without realizing it, imagine the potential of being able to change our microbiome for the better.
“I predict that in the next five years when you go to your doctor to measure your cholesterol, etc., they will also evaluate your microbiome,” Professor Cryan tells me.
“The microbiome is the fundamental future of personalized medicine.”