What is the microbiome and how does it affect our brain?

The term is commonly used to describe the trillions of bugs that live on and in the human body.

From bacteria and viruses to fungi and protozoa (single-cell organisms), the human body is teeming with microscopic life. 

We are only 43% human

In fact, more than half of your body isn’t human – but made up of microbes. 

Scientists estimate that human cells make up just 43% of the body’s cell count.  The other 57%, some 39 trillion cells – is microbial.

Most of these tiny tenants are in our gut where, together, they weigh 1.5kg – roughly as much as the average human brain.

Where do they come from?

We are first exposed to microorganisms as a baby, during birth and from our mother’s breast milk. 

Later on, everything from our genes to our diet, stress and antibiotics and other medicines can alter the composition of our microbiome.

You may also hear the term microbiota and often, microbiome and microbiota are used interchangeably.  But, strictly speaking, there is a difference. 

The term microbiota refers to all the bugs in the human body, while microbiome refers to the microbes’ genetic material.

Aren’t bugs bad for us?

Not necessarily. The bacteria in your microbiome help digest your food, regulate your immune system and produce vitamins, including some B vitamins.

In a healthy gut, these ‘good’ bacteria keep any ‘bad bacteria’ in check. But, if something disturbs this delicate balance, the body can become more vulnerable to disease.

As a result, the bugs in our gut have been linked to a host of conditions, including obesity, diabetes and irritable bowel disease.

Studies show, for instance, that the gut microbiome differs between lean and obese twins, with the lean twin having a greater diversity of bacteria. And when lean mice are given transplants of bacteria from obese mice, they put on weight.

What about the brain?

Perhaps more surprisingly, the bugs in our gut may affect the health of our brain.

Autism, multiple sclerosis, Parkinson’s disease, depression and Alzheimer’s, for example, have all been associated with changes in the gut microbiome.

While it might seem odd that what goes on in our digestive system influences our brain, the two are connected by millions of nerves. 

Gut bacteria also make hundreds of chemicals that act on the brain, including at least 90% of serotonin, the ‘happy chemical’ that is key to mood.

Recent studies show that people with dementia have a difference balance of bacteria in their gut to those without the condition.

Some scientists believe this disruption to the gut microbiome accelerates, or even triggers, Alzheimer’s by fuelling the growth of beta-amyloid, the sticky brain protein that is a hallmark of the condition.