Bacteria that influences happiness

In a recent study from the Alimentary Pharmabiotic Center, scientists showed that the bacteria that live in our gut during development can affect adult brain function and emotional states. Specifically, these little microbes are able to affect levels of the chemical serotonin in our brains. Serotonin, a neurotransmitter, plays an important role in the regulation of mood and emotion. Research has found that serotonin levels are altered during stress, anxiety and depression and many antidepressant drugs are designed to target this neurotransmitter. 

In this study, the scientists were able to demonstrate that mice that were raised in a germ-free environment and therefore had very little gut flora or microbiota during early life. This early absence of gut bacteria significantly elevated concentrations of serotonin in the adult brain, specifically in the hippocampus, a neuronal structure that has important roles in episodic and emotional memory. Most importantly, these germ-free mice with increased levels of serotonin during development displayed less anxious behavior when tested as adults, as compared to mice that were raised in normal conditions and had normal gut flora. Interestingly, this effect seemed to be sex-specific; male mice with reduced gut flora and subsequent elevated levels of hippocampal serotonin showed a much more marked effect than female mice. 

Intriguingly, the neurochemical effect seems to be irreversible. When the scientists recolonized the young germ-free mice in a normal environment that would allow the restoration of intestinal microbiota, they found that hippocampal serotonin levels continued to be elevated in these mice. Paradoxically, the restoration of the gut flora was able to reverse the behavioral effect, such that the germ-free mice no longer showed reduced anxiety compared to the control animals. 

This study follows  earlier work from several groups, showing that a microbiome-gut-brain axis exists and that it is essential for maintaining normal health which can affect brain and behaviour. The research was carried out by Dr Gerard Clarke, Professor Fergus Shanahan, Professor Ted Dinan and Professor John F Cryan and colleagues at the Alimentary Pharmabiotic Centre in UCC.

“As a neuroscientist these findings are fascinating as they highlight the important role that gut bacteria play in the bidirectional communication between the gut and the brain, and opens up the intriguing opportunity of developing unique microbial-based strategies for treatment for brain disorders”, said Professor John F Cryan, senior author on the publication and Head of the Department of Anatomy & Neuroscience at UCC.

The results from this study have many implications, as it shows that manipulating the natural microbiota that exist in our body, whether it be through infection, antibiotics or diet, can profoundly affect other bodily functions, including brain function and behavior. “We’re really excited by these findings” said lead author Dr Gerard Clarke. “Although we always believed that the microbiota was essential for our general health, our results also highlight how important our tiny friends are for our mental wellbeing.”