Microbiome and Mental Health

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Introduction to the Gut Microbiome[edit]

The human gastrointestinal tract alone contains a delicately balanced ecosystem of 100 trillion microorganisms, nearly ten times the number of cells in the entire human body. [1] These bacteria in our gut, which are collectively called the gut microbiome, play many physiological roles in the body, for instance synthesizing vitamins, developing the immune system, aiding digestion to name a few, and management of the stress response.[1] Beyond involvement in somatic processes, bacteria within the body are so interwoven in our systems that they have impact on our behavior and cognition. One study even found that when the gut contents of two mice were swapped, including all of their gut microbiome, the mice's personalities switched; for example, stress-prone mice became calm and calm mice became stress-prone.[1]

Even without the context of disease, humans and animals alike have very diverse interpersonal compositions of their microbiomes. Thus, it has been difficult for researchers to discern the difference between unbalanced, or dysbiotic, microbiome and a healthy microbiome.[2] Over the past decades, researchers have found hundreds of bacteria strains in the human gut, with only a handful amongst such bacteria ubiquitous[2]. Some of these ubiquitous bacteria include: anaerobic cocci and Bacteroides--which are prevalent in high abundance--and Clostridium, Bifidobacterium, Eubacterium, Lactobacillus, Escherichia coli and Streptococcus--which are prevalent in lower abundance[2].

Acquisition and Development in Humans[edit]

Bacteria begin to form an inextricable link to us shortly before birth when they colonize our guts within the womb.[1] By the time we are one year old, we have developed a full adult microbiome and a gut-brain axis.

More on the timeline.

The Gut-Brain Axis[edit]

The gut-brain axis (GBA) is a set of mechanisms through which the gut and brain communicate bidirectionally. Three of the main communication routes are the immune system, the nervous system, and the endocrine system. Through these mechanisms, the information in the gut can affect behavior and cognition, although not necessarily in a negative way. For instance, flight or flight response is a highly evolutionarily conserved and beneficial response system that enables humans to react to threatening stimuli in a way that preserves survival. In an experiment conducted by _________, mice..

Immune System Interaction[edit]

Nervous System Interaction[edit]

Through various mechanisms, including the vagus nerve and through the release of neurotransmitter precursors, the enteric nervous system is connected bidirectionally with the central nervous system.

Endocrine System Interaction[edit]

Microbiome in Physical Illness[edit]

Microbiome and Mental Illness[edit]

Anxiety and Depression[edit]

Depression is known to be closely related to elevations in C-reactive proteins, inflammatory cytokines, and oxidative stress.[3] Current research is being done on the relationship between fecal bacteria (which served as a proxy to analyze the gut microbes) and depression, showing that the presence of certain bacteria are correlated to symptoms of depression.[4]


Schizophrenia is a neuropsychiatric disorder which can appear during adolescence and usually persists throughout an individuals' life. There are varying degrees of Schizophrenia, with characteristic symptoms such as hallucinations, delusions, apathy, and social withdrawal. Studies have shown that Schizophrenia is associated with alterations of the immune system.

Bipolar Disorder[edit]

Bipolar disorder (BD) is a complex and multifaceted issue with a wide range of manifestations. Bipolar Disorder varies greatly and is defined by the presence of mania or depression in different regards. The role


Links between particular bacteria and phenotypes relevant to ASD raise thequestion of whether microbial dysbiosis (imbalances of the microbiome) plays a role in the development or presentation of ASD symptoms.[5]

Probiotic Interaction with Microbiome[edit]

Anxiety and Depression

Lactobacillus and Bifidobacteria are common anti-inflammatory probiotics that have been shown to reduce anxiety and behavioral signs of distress.[6] A recent study found that probiotics decrease ruminative, negative thoughts in humans, and that the introduction of prebiotics (or fibers that promote the growth of beneficial bacteria decrease anxiety.[7]

Influence of Diet on Microbiome[edit]

  1. 1.0 1.1 1.2 1.3 Kramer, Peter; Bressan, Paola (2015-07-14). "Humans as Superorganisms". Perspectives on Psychological Science10 (4): 464–481. doi:10.1177/1745691615583131.
  2. 2.0 2.1 2.2 Lloyd-Price, Jason; Abu-Ali, Galeb; Huttenhower, Curtis (2016-04-27). "The healthy human microbiome". Genome Medicine8: 51. ISSN 1756-994X. doi:10.1186/s13073-016-0307-y.
  3. Logan, Alan C.; Jacka, Felice N.; Craig, Jeffrey M.; Prescott, Susan L. (2016-05-31). "The Microbiome and Mental Health: Looking Back, Moving Forward with Lessons from Allergic Diseases". Clinical Psychopharmacology and Neuroscience14 (2): 131–147. ISSN 1738-1088. doi:10.9758/cpn.2016.14.2.131.
  4. Naseribafrouei, A.; Hestad, K.; Avershina, E.; Sekelja, M.; Linløkken, A.; Wilson, R.; Rudi, K. (2014-08-01). "Correlation between the human fecal microbiota and depression". Neurogastroenterology & Motility26 (8): 1155–1162. ISSN 1365-2982. doi:10.1111/nmo.12378.
  5. Vuong, Helen E.; Hsiao, Elaine Y. "Emerging Roles for the Gut Microbiome in Autism Spectrum Disorder". Biological Psychiatry81 (5): 411–423. doi:10.1016/j.biopsych.2016.08.024.
  6. Deans, Emily (2016-06-27). "Microbiome and mental health in the modern environment". Journal of Physiological Anthropology36: 1. ISSN 1880-6805. doi:10.1186/s40101-016-0101-y.
  7. Steenbergen, Laura; Sellaro, Roberta; Hemert, Saskia van; Bosch, Jos A.; Colzato, Lorenza S. "A randomized controlled trial to test the effect of multispecies probiotics on cognitive reactivity to sad mood". Brain, Behavior, and Immunity48: 258–264. doi:10.1016/j.bbi.2015.04.003.