Bacteria That Cures Mental Illness

An Exploration into Probiotics as a Cure for Anxiety and Depression

Written by: Sean Park '25
Edited by: Elizabeth Ding '24

​​Following weeks of anxiety, you decide to visit your local psychologist to discuss possible methods of treatment. Curious as to what your psychologist may recommend, you think of a few yourself: self-care, anti-anxiety medications, and psychotherapy. To your surprise, your psychologist prescribes Greek Yogurt. With much skepticism, you accept the prescription, pondering its clinical potential.

Within your Greek Yogurt lies over billions of microorganisms known as probiotics. Contrary to the negative connotation typically associated with microorganisms, probiotics, when ingested in limited quantities (10-20 billion colony forming units), confer a plethora of health benefits, including improved intestinal microbial balance and intestinal function. The Greek yogurt itself simply serves as an envoy, delivering these beneficial probiotics to your system [1]. But, how were these probiotics first discovered? In the early 1900s,  Lactobacillus was isolated from the human GI tract (gastrointestinal tract) making it one of the first probiotics to be discovered. Since then, numerous species have also been identified by scientists in foods like kimchi, miso, and pickles! 

At this point, some of you may be asking the correlation between probiotics and mental illnesses such as anxiety and depression. As of 2017, research on the relation between gut microbiota and mental illnesses seems to be growing significantly with scientists particularly focused on the gut-brain-axis—the bidirectional pathway of communication between the brain and gut microbiota [2]. Attempts at understanding methods of communication between the two have included 1) investigating the effects of gut microorganism composition on the brain and 2) the exploration of how certain metabolic processes undertaken by gut microorganisms affect the brain. Among these recent attempts is determining the effects of microbial imbalances within the gut on the gut-brain axis and discovering a solution to this problem: probiotics. 

With the human microbiome, consisting of over one trillion microbes, microbial balance plays a significant role in regulating various bodily processes. In fact, a balance of microbial species within the gut allows resident flora to mutually reside with one another. This mutual interaction, also known as symbiosis, results in the production of inflammomodulatory compounds—compounds that regulate immune response by activating or suppressing its components [3]. On the other hand, dysbiosis, or microbial imbalance within the gut, is found to result in elevated levels of certain molecules such as pro-inflammatory cytokines within the blood that could influence brain functions that promote the onset of mental illnesses [2]. 

Recognized as a critical component of the immune system, cytokines are small molecules that result in many of the known symptoms that are present in many illnesses, including mental ones: inflammation, fever, fatigue, aches, etc. [4]. Noted in the name, pro-inflammatory cytokines, including TNF-ɑ (Tumor Necrosis Factor Alpha) and IL-6 (Interleukin 6) promote inflammatory response. Proinflammatory cytokines most certainly play a role in the onset of anxiety and depression as a specific study demonstrated how levels of TNF-ɑ and IL-6 increase with increasing severities in the symptoms of depression and anxiety. This increase in the number of proinflammatory cytokines, increases the permeability of the blood-brain barrier—a layer of epithelial cells that prevents circulating blood from interacting with the cerebrospinal fluid of the brain—enabling rogue molecules from the gut to enter the brain, which influence brain function leading to the onset of anxiety and depression [5]. The solution to this problem is quite obvious: put simply, replenish gut flora to restore microbial balance; This is where probiotics play a significant role. 

So, how do probiotics work? Within probiotics are billions of beneficial microbial species such as Lactobacillus (Image Above) or Bifidobacterium lactis. Upon intake of probiotics, these microbial species will repopulate the intestinal system, crowding out the “bad bacteria” with the good ones, and restoring our intestinal balance of microbial species [6]. However, when it comes to utilizing probiotics to treat mental illnesses, scientists are more interested in the molecular effects probiotics have on the human body. Recent studies have shown that probiotics prevent stress-induced synaptic dysfunction, normalize cortisol levels, and stimulate pre-existing gut microbiota [2]. 

Scientists’ understanding of the role gut microbiota play in mental health may still be relatively vague and uncertain. However, recent discoveries (since 2017) have shown that gut microbiota possess the potential to influence not one but multiple systems of the human body. When it comes to mental illnesses, the relationship between dysbiosis and inflammation of neural systems still needs to be further studied prior to establishing a direct correlation or causation relationship between the two. Until then, probiotics will simply remain an over-the-counter health supplement. However, are we far from the point in time where psychologists prescribe Greek Yogurt to a patient experiencing anxiety? Simply stated, no. 



References:

[1] Sadick B, Upham B, Patino E, Rapaport L, Thrasybule L, Phillips Q, et al. Know-gurt: A guide to probiotics and yogurt - digestive health center - everyday health [Internet]. EverydayHealth.com. [cited 2021Oct24]. Available from: https://www.everydayhealth.com/digestive-health/knowgurt-a-guide-to-probiotics-and-yogurt.aspx

[2] Clapp M, Aurora N, Herrera L, Bhatia M, Wilen E, Wakefield S. Gut Microbiota's effect on Mental Health: The gut-brain axis [Internet]. Clinics and practice. PAGEPress Scientific Publications, Pavia, Italy; 2017 [cited 2021Oct24]. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5641835/

[3]Johnson CL, Versalovic J. The human microbiome and its potential importance to pediatrics [Internet]. Pediatrics. American Academy of Pediatrics; 2012 [cited 2021Oct24]. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3340594/

[4] mDantzer R. Cytokine, sickness behavior, and Depression [Internet]. Immunology and allergy clinics of North America. U.S. National Library of Medicine; 2009 [cited 2021Oct24]. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2740752/

[5] Berk M, Williams LJ, Jacka FN, O'Neil A, Pasco JA, Moylan S, et al. So depression is an inflammatory disease, but where does the inflammation come from? [Internet]. BMC medicine. BioMed Central; 2013 [cited 2021Oct24]. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3846682/

[6] Do probiotics restore digestive balance? [Internet]. Center for Science in the Public Interest. 2016 [cited 2021Oct24]. Available from: https://www.cspinet.org/tip/do-probiotics-restore-digestive-balance