The phrase ‘gut microbiome’ refers to the bacterial colonies which live in our gut, forming a symbiotic relationship with their host. The host provides the bacteria with a safe place to inhabit and multiply, whilst the bacteria provide the host with key compounds and nutrients via their own metabolic processes, as well as helping to defend the host from opportunistic pathogens, assisting the immune response (Lee et al, 2021). In fact, this symbiosis is ingrained and developed enough that humans can be classified as ‘superorganisms’, meaning that they exhibit metabolic processes which are intrinsically linked with those of their microbiome (Li et al, 2008)
It is estimated that ¼ of the UK population has some form of mental health disorder, and that 17% of UK adults are prescribed antidepressants (McManus et al, 2016; Prescribed medicines review, 2020). This number has only increased in recent years with the global reaction to the Covid-19 pandemic. Multiple lockdowns and health anxiety has served to add pressure to already overwhelmed mental health services.
Approximately 17% of the population of the United Kingdom suffer from either anxiety or depression, and these are referred to as ‘common mental disorders’ (McManus et al, 2016). A key factor for these disorders is a lower amount of serotonin in the brain, which is colloquially known as the happiness neurotransmitter. Therefore, their treatment usually centres on trying to increase the neural concentrations of serotonin, via the prescription of medications such as SSRIs (selective serotonin reuptake inhibitors).
These medications increase the brain’s ability to absorb serotonin, although they cannot increase the overall level of serotonin present. This is partly because an estimated 95% of our serotonin is actually produced in our gut, and travels to our brain via our bloodstream (Camilleri, 2009). A large amount of this serotonin from the gut is produced by our gut microbiome, as a part of our symbiotic relationship. Specifically, the phyla of Streptococcus, Enterococcus and Escherichia, although other specific bacteria also have been shown to have involvement in our serotonin production mechanisms, with Turibacter sanguinis helping the trigger the gut itself to release the serotonin synthesised there, as part of their intrinsically linked metabolic processes (Kaur et al, 2019).
This would suggest, that in cases of dysbiosis, where the levels of various bacterial phyla and genera are different to what would be expected in a healthy gut microbiome, then the amount of serotonin available within the body would be limited, perhaps contributing to diagnoses of anxiety and depression.
So, could this be used as a novel treatment for mental disorders? If the dysbiosis could be identified, and repaired, would it reduce anxiety and depression symptoms without SSRIs? This is particularly interesting as it has been suggested that anti-depressants only act as an effective treatment in 40-60% of cases, whilst a placebo group showed an improvement in 20-40% of the patients. This means that under experimental conditions, there was only an improvement of 20% due to the use of antidepressants (IQWIG, 2006). Therefore, there is clearly a need for the development of new methods of treating mental illness, and the exploration of the gut-brain axis presents an interesting avenue of research.
References
IQWIG-Institute for Quality and Efficiency in Health Care (2006) Depression: How effective are antidepressants? [Updated 2020 Jun 18]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK361016/ [Accessed 10 November 2022]
Li, M., Wang, B., Zhang, M., et al. (2008) Symbiotic gut microbes modulate human metabolic phenotypes. Proceedings of the National Academy of Sciences [online], 105 (6): 2117–2122. Available from: https://dx.doi.org/10.1073/pnas.0712038105 [Accessed 10 November 2022]
Camilleri M. (2009). Serotonin in the gastrointestinal tract. Current opinion in endocrinology, diabetes, and obesity, 16(1), 53–59, https://doi.org/10.1097/med.0b013e32831e9c8e
McManus et al, 2016 [online] https://webarchive.nationalarchives.gov.uk/ukgwa/20180328140249/http://digital.nhs.uk/catalogue/PUB21748 [Accessed 10 November 2022]
Kaur Harrisham, Bose Chandrani, Mande Sharmila S, Tryptophan Metabolism by Gut Microbiome and Gut-Brain-Axis: An in-silico Analysis, Frontiers in Neuroscience 13, 2019, https://www.frontiersin.org/article/10.3389/fnins.2019.01365, 10.3389/fnins.2019.01365
Lee Learn-Han, Wong Sunny Hei, Chin Siok-Fong, Singh Vishal, Ab Mutalib Nurul-Syakima, Editorial: Human Microbiome: Symbiosis to Pathogenesis, Frontiers in Microbiology, 12, 2021, https://www.frontiersin.org/articles/10.3389/fmicb.2021.605783, 10.3389/fmicb.2021.605783, 1664-302X
GOV.UK. 2022. Prescribed medicines review: summary. [online] Available at: <https://www.gov.uk/government/publications/prescribed-medicines-review-report/prescribed-medicines-review-summary> [Accessed 10 November 2022].
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