HOW IT WORKS
Faced with important decisions, we often listen to what our “gut” is telling us, believing that our true intuition originates from somewhere deep within.
There is indeed truth to this feeling. The brain has long been considered the source of our cognition and the control center of our body. However, modern science has confirmed that our gut, or gastrointestinal (GI) tract, might be just as influential in how we think and feel.
A "Gut Feeling"
The “second brain” theory  suggests that we actually have two “brains,” one in our head and one harbored within our gut. In addition to our central nervous system (CNS), which is comprised of our brain and spinal cord, our GI tract has its own independent neural network. During embryonic development, both systems originate from the same tissue and remain physically linked together throughout life by the vagus nerve. This gut-brain axis involves both “brains” functioning independently, yet still maintaining constant, bidirectional communication.
The Gut Microbiome
Gut microbiota are the thousands of species of microorganisms living inside our GI tract. They can outnumber the human cells in our body and weigh as much as our own brain. This environment - composed of our own epithelial, immune, and nerve cells; enteric microbes; and a countless range of molecules - is our gut microbiome.
This microbiome is responsible for much of our gut-brain messaging, and its biotic diversity profoundly influences our health. These gut microbes induce communication between our two “brains” using molecules such as neurotransmitters, hormones, fatty acids, and other metabolites.
Researchers compared germ-free rats (with a sterile gut) and those with normal microbiota 
In germ-free rats, neuron-enhancing brain-derived neurotrophic factor (BDNF) and happy hormone serotonin concentrations lower, while stress hormone concentrations higher compared to normal rats
Physiological conditions of germ-free animals paralleled those in people suffering from anxiety or depression
Scientists Ted Dinan and John Cryan  studied how a strain of Bifidobacterium longum affected neural activity of mice 
Proposed active pathways included neuro-endocrine interaction and direct gut-brain neural communication via vagus nerve
Reduced anxiety and stress hormone
Enhanced learning and memory ability and levels of tryptophan (precursor of serotonin)
Proposed psychobiotics may be used to reduce depression symptoms (since serotonin is the neurotransmitter increased by many common depression medications)
Clinical trial tested effects of the same B. longum strain on humans 
Stress perception and stress hormone levels dropped
Memory and cognitive performance enhanced significantly
Our gut microbiota-gut-brain axis functions bi-directionally, with the enteric microbiome neurons in constant interaction with our CNS. The following are some of the mechanisms by which this communication occurs.
Microbes in the gut can induce immune cells within the intestine to secrete a range of cytokines. These circulate via the bloodstream throughout the body. Though typically not able to pass directly into the brain, cytokines are still able to stimulate neurological responses through interaction at weaker, more susceptible regions of the blood-brain barrier (BBB).
The vagus nerve runs from the brain to the gastrointestinal tract, a direct communication pathway between the CNS and the gut. A 2011 study in mice  found that only when the vagus nerve was intact did a specific B. Longum strain show an anxiolytic effect in mice with induced colitis; post-vagotomy, the probiotic-treated mice behaved no differently than a control group.
Gut microbiota can help generate the precursors of neurotransmitters and other metabolites. These circulate through the bloodstream, and some, such as short-chain fatty acids, even pass directly through the BBB.
The hypothalamic–pituitary–adrenal (HPA) axis is another key pathway of microbiota-gut-brain interaction. Upon feeling stress, the HPA axis stimulates the adrenal gland to secrete cortisol (stress hormone), which causes a sympathetic nervous system “fight or flight” response. Multiple body systems are either triggered or demobilized in order to react effectively to the external situation. Certain microbes in our gut can directly reduce circulating cortisol levels. Normalization of this stress hormone has the effect of reversing the HPA axis feedback loop to the brain, which diminishes secretion of the hormone and further lowers its concentration.
PS128: Nervous System Regulation
Certain neurotransmitters known to directly affect our mood are often referred to as happy hormones. Dopamine and serotonin are two of the most influential of these.
Proper regulation of these hormones is necessary for mental health and overall wellbeing. Their imbalance can result in a wide range of mental, emotional, and physical disorders, from depression or autism spectrum disorder to insomnia or Parkinson’s disease.
More Happy, Less Stress
Preclinical studies  have demonstrated PS128’s ability to modulate dopamine and serotonin, as well as the primary stress hormone, corticosterone (cortisol in humans). This is likely carried out via one or more of the pathways described above. PS128 was found to raise the concentration of happy hormones dopamine and serotonin in most situations, while effectively reducing stress hormone levels. As a result, laboratory mice were found to respond better in stressful situations.
PS128 raises happy hormones
PS128 lowers stress hormones
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