Microbiome & psychobiotics, a scientist’s opinion
Interview with Dr. Ruairi Robertson, Wellcome Trust Fellow at Queen Mary University of London.
How might bacteria be affecting our mental health? What are the possible communication routes?
Ruairi Robertson: There are a number routes through which the gastrointestinal tract and its microbiome interact with the brain. The gut is home to a large proportion of the peripheral nervous system, known as the enteric nervous system (ENS). This extensive network of peripheral neurons allows the brain to regulate gut functions such as motility i.e. the movement of contents through the intestinal tract. Arguably the most important connection in this system is the vagus nerve which is responsible for a huge proportion of the electrochemical signals that are sent between the gut and brain. One of the breakthrough discoveries in this field is that these networks are bi-directional, meaning that not only does the brain influence gut function but also vice versa. Gut cells form physical connections with peripheral nerves through structures called ‘neuropods’, which were recently found to send extremely fast signals to the brain. The vast ecosystem of microbes in the intestine hugely influence the messages that are sent from the gut to the brain through this system. They do this by producing thousands of different chemicals on a constant basis which are either transported directly to the brain through the bloodstream, or which influence gut brain signalling through communication with gut cells and the ENS. One of these groups of chemicals produced by gut bacteria are neurotransmitters, which are typically produced in the brain, and which influence brain function and mood. Gut bacteria can either themselves produce, or stimulate gut cells to produce, neurotransmitters such as serotonin and GABA. It is estimated that 90% of serotonin, which is an antidepressant neurotransmitter, is produced in the gut, although it typically does not travel to the brain itself. Finally, gut bacteria play an important role in the immune response as a huge proportion of the human immune system is based in the intestinal tract. An overload of disease-causing bacteria in the gut can trigger inflammation through the body. Many mental health disorders are associated with inflammation which potentially could be derived from the gut.
What’s next on the agenda to discover to help us along the path of potentially realising the psychobiotics potential in humans?
Ruairi Robertson: In order to realise the potential of psychobiotics in humans, scientists must first further understand the huge variability in the microbiome between individuals. Our microbiomes are as unique as our own fingerprints meaning that you and your neighbour may respond very differently to a specific probiotic, prebiotic or any other treatment. With the help of AI and machine learning, the reality of personalised medicine has rapidly advanced in recent years. This means that, in the future, a doctor may be able to sequence a patient’s gut microbiome very quickly and, using a computer algorithm, know whether that person is likely to respond to a certain psychobiotic, or choose the most suitable psychobiotic for that individual. Next generation probiotics are also warranted in this field. At the moment, probiotics are restricted to a very small number of species that have been approved for consumption. Future developments in this field will depend on genetically modified organisms that possess specific functions, such as producing certain neurotransmitters. However, a number of ethical and regulatory issues surrounding GMOs will have to be tackled.
What are the potential problems/blocks that may hinder progress in this research area? Do you think psychobiotics will really become a reality one day?
Ruairi Robertson: There are a number of barriers that must be overcome in order for this field to be brought to clinical practice. Firstly, as happens with many new exciting areas of science, such as the Human Genome Project back in the 1990s, the hype and hope surrounding the microbiome has begun outpacing the scientific reality. This hype leads to unscientific claims being spread on the internet and products on the market that claim to benefit the gut-brain axis but have no scientific basis. All of this hinders scientific progress as it leads to confusion for the general public and mistrust in the scientific method. The general public are eager for research in the gut-brain axis to be translated to novel therapies quickly, however science by default is a slow and rigorous process. It is essential that scientists continue to communicate the potential of the gut-brain axis whilst simultaneously communicating the current limitations to its clinical applicability.
Secondly, mental health disorders are multifactorial and complex. Psychobiotics will never be standalone treatments for any mental health disorder. Progress will be restricted in the field of the microbiome and probiotics/psychobiotics if it is viewed as a panacea. With further research however, psychobiotics could become a reality by acting as suitable adjunctive therapies to medication, psychotherapy and other forms of treatments for brain and mental health disorders.