Franklin 
As the “gut-brain” concept becomes an ever more embedded part of culture, researchers are rapidly gaining insight into bidirectional communication pathways that shape gut physiology and behavior; various factors affect these communication paths including diet and microbiota.
One such discovery is the finding that certain strains of gut bacteria alter fear learning and extinction, two key processes associated with anxiety disorders like posttraumatic stress disorder (PTSD).
Stress and the Gut Microbiome
Stress response activates physiological systems across the board – including nervous, cardiovascular, endocrine and immune. Energy from non-essential activities is diverted towards fighting or fleeing perceived threats – as your body prepares to fight or flee in preparation of fight or flight response.
Research has demonstrated the ability of gut microbiome to influence how an individual responds to stress.
Researchers have discovered that people with diverse microbiomes are more resistant to stress. Furthermore, this diversity can alter how a person responds to stressful events by altering availability of certain metabolites like 5-HT from Trp in epithelial cells.
The Gut Microbiome and Depression
Researchers have recently identified that people suffering from depression, autism and other mood disorders tend to lack certain gut microbes that produce and consume metabolites, short-chain fatty acids and neurotransmitters that send signals back to the brain through the vagus nerve, intestinal endocrine cells or other means.
These metabolites disrupt the hypothalamic-pituitary-adrenal axis and increase inflammation, increasing intestinal barrier permeability to gram-negative bacteria and permitting pro-inflammatory molecules into the bloodstream; depression is commonly linked with dysregulation of this axis.
Studies have demonstrated that transplanting human fecal samples from depressed individuals onto germ-free mice result in depressive symptoms, suggesting gut microbes can directly impact our minds. A separate research project published this month by Nature Mental Health showed that people who exhibit resilience to stress have distinct signature microbial profiles in their microbiomes; specifically Morganella bacteria have been implicated as responsible. These results could have profound psychiatric and neurological implications such as stroke which is also connected with this axis.
The Gut Microbiome and Anxiety
Your gut microbiome is an intricate web of bacteria, archaea, fungi and viruses that plays an integral part in maintaining good metabolism, immunity and mental health. Many physical illnesses have been linked to imbalanced gut microbiomes; anxiety disorders are no exception.
Researchers have discovered that people suffering from anxiety disorders contain different species of bacteria in their gut than healthy people. Furthermore, they tend to have lower levels of short-chain fatty acids and serotonin neurotransmitter in their blood, as well as interaction between bacteria products and the endocrine cells in their gut lining regulating our hormones.
The gut-brain axis transmits signals through millions of nerve endings to the brain, with microbes living in our digestive tract having an influence over which signals reach our minds. Studies are currently exploring this connection further and suggest that gut microbiome could play an integral role in psychological conditions like anxiety. By improving its health and taking steps to do so can also reduce symptoms.
The Gut Microbiome and Bipolar Disorder
The microbiota-gut-brain (MGB) axis plays a vital role in neuromodulation and endocrine functions (e.g. regulating serotonin production or activating hypothalamic-pituitary-adrenal axis stimulation), as well as intestinal, systemic, and brain inflammation (neuroinflammation).
Research suggests a correlation between gut microbiome composition and mental health conditions such as bipolar disorder. People diagnosed with this condition appear to have different gut bacteria composition than non-demented individuals and more digestive ailments like IBS.
Shotgun metagenomic sequencing studies have demonstrated that the bacterial community in those suffering from depression differs substantially from healthy controls (HCs). Faecalibacterium, for instance, was less prevalent amongst BD patients compared with HCs; other clades associated with inflammation status, serum lipid levels, tryptophan levels, and oxidative stress were identified as well. Furthermore, multiple studies have illustrated how gut microbiota contributes to synaptic pruning, and these changes may be modified through antidepressant drugs.
