The way that we conceptualize the many foreign microbial bodies that thrive in our own systems has shifted in a dramatic way. Starting at birth and throughout adulthood, the human body is a welcoming host to thriving microscopic life.
In fact, the total number of foreign microbial cells in the human body is roughly equivalent to the number of human cells. The human body is like any other ecosystem – the survival of one species is dependent upon the functions of many others.
The variety foreign microbial cells throughout our bodies serve to maintain healthy daily processes and a growing body of evidence suggests that they play a major role in our moods and cognitive processes.
Diverse species of bacteria, fungi and yeast reside on every square centimeter of skin exposed to the outside world. Every crevice and fold of our skin are tiny habitats that foster the life of bacteria like staph and streptococcus, surviving symbiotically by producing antimicrobial substances that ward off more pathogenic invaders.
Internally, the bacteria that occupy the lower parts of our gastrointestinal tracts aid in digestion. These bacteria live in us and work for us every day recycling waste and protecting delicate tissue that could easy be overtaken by microbes with more malicious intent.
These microbes also work on our behalf beyond local symbiosis, elicitingwidespread influences on human health and behavior. The development of immunity, metabolism and body type, and even cognitive development and functioning have all been linked to relative health of bacteria in the GI tract.
Elegant animal studies have highlighted these relationships, which has since spurred a wave of funding by groups like the National Institute of Health (NIH), giving millions of dollars to the studying the “mind-gut-brain” axis.
Despite the growing research initiative, illuminating a causal link that microbiota are responsible for other realms of human health has been challenging, as many of the relationships between gut health and overall well being are not immediately evident.
The Mind-Gut Connection
The relationship between the mind and the gut may not seem obvious until we take a moment to examine our most common cognitive states in relation to the functioning of our digestive system.
Anxiety and nervousness are both feelings that can manifest throughout the body and often localize to the belly. This connection is facilitated by a large interconnected body of nervous tissue, known as the enteric nervous system.
The enteric nervous system is responsible for initiating the rhythmic peristaltic movement of the gut, as well as relaying important signals to and from the brain.
The basic sensations of hunger, pain and satiety are examples of signals that require efficient bidirectional communication between the enteric nervous system and the brain.
These signals are strong enough to drive behavior necessary for survival and subsequently influence mood and affect.
The mind-gut connection becomes more complex when the role of microbes is considered. The initial colonization of the gut during infancy is likely to have the most profound influence on developmental processes.
As an example, a study that compared mice born by cesarean section to those delivered vaginally showed distinctly different mircobiomes that correlated with differences in behavior (1).
Mice born by cesarean section with sterile microbiota were significantly more anxious and showed more depressive symptoms during development compared to the vaginally delivered mice with microbiota passed on from their mother.
Other studies have shown similar results by wiping out gut bacteria and studying the resulting behavior. This research found that both gut infections and the antibiotic treatment used to resolve those infections can induce cognitive impairments (2).
Eliminating our natural gut microbiota reduces circulating lipid metabolites and neuronal signaling molecules known to be instrumental in dynamic cognitive processes, including memory formation (3).
The mechanisms by which gut flora influence mood and shape personality remain largely undescribed. The chemical byproducts of certain bacterial species are hypothesized to be large contributors.
For example, two species of gut bacteria are known to produces the anxiolytic neurotransmitter, GABA, which may influence the brain through the vagus nerve (4).
The knowledge base of the adult gut microbiome is growing exponentially. In 1978, the diarrhea producing illness, pseudomembranous colitis, was shown to be caused by the colonization of the virulent bacteria C. difficile in the gut following eradication of normal gut flora by antibiotics.
This highlighted the importance of normal gut bacteria and initiated an effort to improve the natural protective lining through fecal transplantation (5). In this procedure, individuals who have suffered from recurrent
Pseudomembranous colitis may elect to utilize the living contents in waste from screened, healthy humans. Though now widely implemented with successful results reported in many studies, the regulation of the therapy continues to be a challenge for the FDA and the use is confined to recurrent pseudomembranous colitis.
Probiotics & Gut Health
Fortunately there are easier, less invasive ways to improve one’s microbiota. Probiotics are forms of bacteria and yeast that confer benefits to the digestive tract. Many food products, such as yogurt and kombucha, contain living cultures that take up residence in the GI tract.
Encapsulated probiotic supplements are also teeming with similar health benefits that can directly influence individuals microbiomes. Probiotics are effective in preventing the inoculation of harmful bacteria, reducing the need for antibiotics and averting associated cognitive dysfunctions.
Administration of probiotics has also been shown to directly influence psychological processes in human studies. In a randomized, double-blind, placebo-controlled study, volunteers who took probiotics for 30 days significantly alleviated factors of psychological distress, exhibiting decreased symptoms of anxiety, depression and anger (6).
The relationship between a sound mind and a healthy gut is becoming clearer each day. The reliance upon healthy microbiota and the influence on widespread functional processes, such as mood and personality, demonstrate the complexity of the dynamic ecosystem of the human body.
As genomic sequencing accelerates and research on the mind-brain-gut axis grows, we will yield to an age of personalized health information useful in determining microbiota insufficiencies with specific remedies.
Until that time arrives, a diet rich with living cultures and probiotics are ideal to foster the life of the inner gut and keep the mind in harmony.
1. Clarke G, O’mahony SM, Dinan TG, Cryan JF. Priming for health: gut microbiota acquired in early life regulates physiology, brain and behaviour. Acta Paediatr. 2014;103(8):812-9
2. Gareau MG, Wine E, Rodrigues DM, et al. Bacterial infection causes stress-induced memory dysfunction in mice. Gut. 2011;60(3):307-17.
3. Fröhlich EE, Farzi A, Mayerhofer R, et al. Cognitive Impairment by Antibiotic-Induced Gut Dysbiosis: Analysis of Gut Microbiota-Brain Communication. Brain Behav Immun. 2016;
4. Bravo JA, Forsythe P, Chew MV, et al. Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve. Proc Natl Acad Sci USA. 2011;108(38):16050-5.
5. Allegretti JR, Korzenik JR, Hamilton MJ. Fecal microbiota transplantation via colonoscopy for recurrent C. difficile Infection. J Vis Exp. 2014;(94)
6. Messaoudi M, Lalonde R, Violle N, et al. Assessment of psychotropic-like properties of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in rats and human subjects. Br J Nutr. 2011;105(5):755-64.