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The possibilities are exciting. See my earlier posts on psychobiotics for more information. From Medical Xpress:

How gut bacteria ensures a healthy brain – and could play a role in treating depression

But medicine in the 21st century is rethinking its relationship with bacteria and concluding that, far from being uniformly bad for us, many of these organisms are actually essential for our health. Nowhere is this more apparent than in the human gut, where the microbiome  – the collection of bacteria living in the gastrointestinal tract - plays a complex and critical role in the health of its host.

The microbiome interacts with and influences organ systems throughout the body, including, as research is revealing, the brain. This discovery has led to a surge of interest in potential gut-based treatments for neuropsychiatric disorders and a new class of studies investigating how the gut and its microbiome affect both healthy and diseased brains.

The lives of the bacteria in our gut are intimately entwined with our immune, endocrine and nervous systems. The relationship goes both ways: the microbiome influences the function of these systems, which in turn alter the activity and composition of the bacterial community. We are starting to unravel this complexity and gain insight into how gut bacteria interface with the rest of the body and, in particular, how they affect the brain.

The microbiome-immune system link is established early on. Over the first year of life, bacteria populate the gut, which is largely sterile at birth, and the developing immune system learns which bacteria to consider normal residents of the body and which to attack as invaders. This early learning sets the stage for later immune responses to fluctuations in the microbiome's composition.

When a normally scarce strain becomes too abundant or a pathogenic species joins the community of gut bacteria, the resulting response by the immune system can have wide-reaching effects. Depression has been linked with elevated levels of such molecules in some individuals, suggesting that treatments that alter the composition of the microbiome could alleviate symptoms of this disorder.

Such an intervention could potentially be achieved using either prebiotics – substances that promote the growth of beneficial bacteria – or probiotics – live cultures of these bacteria. It is even possible that the microbiome could be manipulated by dietary changes.

In one experiment, researchers transplanted the human microbiome into germ-free mice (animals that have no gut bacteria) in order to study it in a controlled setting. They found that, simply by changing the carbohydrate and fat content of the mice's food, they could alter basic cellular functions and gene expression in the microbiome.

Depression is not the only psychiatric disorder in which the microbiome may play a role.Research in rodents, as well as a few preliminary studies in humans, indicate that the state of our resident microbes is tied to our anxiety levels.

This research also reveals the complexity of the relationship between the microbiome and psychological state. ...Researchers have shown that the presence or absence of microbes in young mice affects the sensitivity of the hypothalamic-pituitary-adrenal (HPA) axis – a key pathway in the body's stress response system. The activity of the microbiome during development thus sways how we respond to future stressors and how much anxiety they cause us.

How do the bacteria in our gut wield such influence over our brains and bodies? The mechanisms of microbiome-host interactions appear to be as numerous and varied as the interactions themselves.

Here are some more articles that I found regarding psychobiotics or the use of probiotics to affect behavior and treat psychiatric disorders.  A probiotic is a microorganism introduced into the body for its beneficial properties. Even though the articles are from 2013, they all give slightly different information about this emerging and exciting new field. Please note that psychotropic means having an effect on how the mind works (and it usually refers to drugs that affect a person's mental state). Remember that this area of research and terminology used is in its infancy. From Medscape (November 2013):

Probiotics a Potential Treatment for Mental Illness

Probiotics, which are live bacteria that help maintain a healthy digestive system, are now often promoted as an important part of dietary supplements and natural food products. "Many of the numerous health-improvement claims have yet to be supported scientifically..."

They note that the term "psychobiotic" was created as recent studies have begun to explore a possible link between probiotics and behavior.  "As a class of probiotic, these bacteria are capable of producing and delivering neuroactive substances such as gamma-aminobutyric acid [GABA] and serotonin, which act on the brain-gut axis," they write.

For this review, the investigators sought to examine studies that assessed whether ingesting these bacteria "in adequate amounts" could potentially lead to an effective treatment for depression and other stress-related disorders. In 1 of the preclinical studies examined, mice that ingested L rhamnosus showed reduced anxiety scores and "altered central expression" on both the GABA type A and type B receptors.

And a study of human patients with chronic fatigue syndrome showed that those who consumed an active strain of L casei 3 times a day had significantly higher improvement scores on anxiety measures than did those who received matching placebo. This provides "further support for the view that a probiotic may have psychotropic effects," write the researchers.

Still, Dr. Dinan called for caution. "What is clear at this point is that, of the large number of putative probiotics, only a small percentage have an impact on behavior and may qualify as psychobiotics," said Dr. Dinan. He added that for now, the field needs to wait for large-scale, placebo-controlled trials to provide definitive evidence of benefit and to detect which probiotics have psychobiotic potential.

Dr. Camille Zenobia wrote this in August 2013. From Real Clear Science:

Can 'Psychobiotic' Bacteria Affect Our Mood?

But what about your brain? Apparently, bacteria influence what’s going on up there, too. Within the last several years, a blossoming field of study called “microbial endocrinology” has provided some provocative insights about the relationship between our GI microbiota and our mood and behavior.

Studies in the field of microbial endocrinology have implicated GI microbes as a factor that can regulate the endocrine system. This could have both good and bad effects since the endocrine system is responsible for the production of hormones and coordinates metabolism, respiration, excretion, reproduction, sensory perception and immune function.

From Nov. 2013 Popular Science:

Forget Prozac, Psychobiotics Are The Future Of Psychiatry

The answer lies in the fact that many psychiatric illnesses are immunological in nature through chronic low level inflammation. There is a plethora of evidence showing the link between gut microbiota and inflammation and studies on probiotic strains have revealed their ability to modulate inflammation and bring back a healthy immunological function.  In this regard, by controlling inflammation through probiotic administration, there should be an effect of improved psychiatric disposition.

The authors bring up another reason why psychobiotics are so unique in comparison to most probiotics.  These strains have another incredible ability to modulate the function of the adrenal cortex, which is responsible for controlling anxiety and stress response. Probiotic strains, such as Lactobacillus helveticus and Bifdobacterium longum have shown to reduce levels of stress hormones and maintain a calmer, peaceful state.  There may be a host of other probiotic bacteria with the same ability although testing has been scant at best.

Finally, the last point in support of psychobiotics is the fact that certain strains of bacteria actually produce the chemicals necessary for a happy self.  But as these chemicals cannot find their way into the brain, another route has been found to explain why they work so well.  They stimulate cells in the gut that have the ability to signal the vagus nerve that good chemicals are in the body.  The vagus nerve then submits this information to the brain, which then acts as if the chemicals were there.