Did you know that you’re never alone? In fact your body provides housing for a dizzying array of microorganisms. These tiny tag-alongs colonise a number of different ecosystems within the human body, including a whole host which make their home in our digestive system. It is estimated that there are around 1014 microorganism living in the gut, meaning that our guts actually contain around 10-times more microbes than human cells! It is therefore not surprising that scientists are starting to uncover a wide range of effects these internal residents have on our development and overall health.
Historically, neuroscientists have remained skeptical regarding the effects of gut microbes on our mental well-being. However, recent clinical observations and animal studies suggest that microbes in the gut can influence behaviour through alterations in brain physiology and neurochemistry – and now the neuroscience community is starting to take notice.
The most compelling evidence so far of a brain-gut link comes from mice raised in sterile germ-free conditions – these mice are born by cesarean (to prevent them from picking up microbes that reside in their mothers’ birth canals) and raised in a strictly sterile environment, meaning they don’t come into contact with microbes present in the normal mouse digestive track. Studies in these animals show that germ-free mice have an altered response to stress compared to mice harbouring a normal compliment of microbes. Interestingly, when germ-free mice are moved away from their sterile homes and back into normal cages (where they will be exposed to many microbes), their behaviour does not revert back to that of normal mice, however the behaviour of their offspring does. This suggests that there may be a critical time window, early in development, where microbes have the greatest effect on brain chemistry. This may prove to be an important consideration for anyone having a cesarean birth. Indeed, a small number of hospitals will rub the mouth and skin of babes born through cesarean section with a piece of gauze from the mother’s vagina, to ensure the baby inherits the same vaginal microbes it would have gained through a natural birth.
In addition to this, in 2011, a team of scientists from McMaster University in Hamilton, Canada, found that they could transfer behavioural characteristics between different mouse strains by simply transplanting gut microbes from one animal to another. For example, it was possible to make shy mice more outgoing by transplanting them with gut microbes from their more outgoing counterparts. Also, some research suggests that transplanting faecal bacteria from humans with IBS and anxiety into mice can cause these mice to become more anxious – a finding which is simultaneously gross and amazing.
It has long been recognised that the brain and gut are connected – indeed, anyone suffering from anxiety is likely to have numerous tales of the negative impact this has on their digestive system. But, how does the gut communicate with the brain? Well, researchers are now starting to find answers to this question.
Firstly, microbes in the gut break down complex carbohydrates into short-chain fatty acids, many of which influence the structure of the blood-brain barrier (a semi-permiable barrier controlling the passage of cells, particles and large molecules into the brain). This means that gut microbes may be able to control what passes into the brain. It has also been found that gut microbes can directly alter neurotransmitter levels – perhaps providing a conduit by which they can communicate with neurones. Specifically, certain metabolites from gut microbes cause cells lining the colon to produce Seratonin (a neurotransmitter often targeted by antidepressant ‘SSRI-type’ drugs). These finding may point towards new and promising research areas in the fight again mental illness.
However, we must be aware that scientists still don’t know how well this research will actually translate into humans. In fact, as we might expect, preliminary research into the brain-gut connection in humans suggests significant complexity and a need for further research. Neuroscientist Rebecca Knickmeyer who is currently working in this field says “There’s probably more speculation than hard data now. So there’s a lot of open questions about the gold standard for methods you should be applying. It’s very exploratory”. So, there may still be a long way to go before we fully understand how your internal ecosystem affects your mental well-being. But, perhaps in the future we may see probiotics prescribed alongside more traditional treatments for mental health problems.
Post by: Sarah Fox