How Your Bacteria Controls You?
The Influence of Intestinal Flora on Personality and Mental Health
We Have More Bacteria in our Bodies than Cells?
The normal flora or microbiota are bacteria which are found in or on our bodies on a semi-permanent basis without causing disease. There are more bacteria living in or on our bodies, than we have cells of our own.
A human body contains around 1013 cells. However, the human body is home to around 1014 bacteria.
The normal flora protects our bodies from disease by:
- Competing with invaders for space and nutrients.
- Producing compounds (bacteriocins) which kill other bacteria.
- Lowering the pH so that other bacteria can’t grow.
Resident normal flora are found in sites exposed to the outside world (external environment) like the skin, all body openings, and mucous membranes that line the digestive and genitourinary tracts. (1)
The Intestinal Flora: A Forgotten Organ?
The idea that there is a relationship between your brain and your gut has been known since hundreds of years ago and is described as the “gut feeling”, suggesting that your gut plays a role in your mood and way of thinking. However, only recently have scientists been able to know the exact connection between the gut and the brain; Microbiomes.
The human intestine houses an astounding number and species of microorganisms, estimated at more than 1014 gut microbiota and composed of over a thousand species. An individual’s profile of microbiota is continually influenced by a variety of factors including but not limited to genetics, age, sex, diet, and lifestyle. Although each person’s microbial profile is distinct, the relative abundance and distribution of bacterial species is similar among healthy individuals, aiding in the maintenance of one’s overall health (2). The latest studies have shown that the state of our microbiome affects not only diseases of the gut but also autoimmune disease, obesity, and even how well the liver metabolizes alcohol (3).
The gut microbiome has played a crucial role in the bidirectional gut–brain axis that integrates the gut and central nervous system (CNS) activities, and thus the concept of microbiome–gut–brain axis is emerging. Studies are revealing how diverse forms of neuro-immune and neuro-psychiatric disorders are correlated with or modulated by variations of microbiome, microbiota-derived products and exogenous antibiotics and probiotics. (4)
The Microbiota-Gut-Brain Axis: Crosstalk Between Bacteria and Your Brain?
The microbiota-gut-brain axis is a rich topic of conversation at the moment. The gut-brain axis (GBA) consists of bidirectional communication between the central and the enteric nervous system, linking emotional and cognitive centres of the brain with peripheral intestinal functions. Recent advances in research have described the importance of gut microbiota in influencing these interactions, suggesting that microbes can influence brain chemistry.
This interaction between microbiota and GBA appears to be bidirectional, namely through signalling from gut-microbiota to brain and from brain to gut-microbiota by means of neural, endocrine, immune, and humoral links.
This bidirectional communication network consists of:
- The central nervous system (CNS), both brain and spinal cord.
- The autonomic nervous system (ANS): with the sympathetic and parasympathetic limbs, drives both afferent signals, arising from the lumen and transmitted though enteric, spinal and vagal pathways to CNS, and efferent signals from CNS to the intestinal wall.
- The enteric nervous system (ENS).
- The hypothalamic pituitary adrenal (HPA) axis: The HPA axis is considered the core stress efferent axis that coordinates the adaptive responses of the organism to stressors of any kind. It leads to release of cortisol from your adrenal glands. Cortisol is a major stress hormone that affects many human organs, including the brain.
Thus, both neural and hormonal lines of communication combine to allow brain to influence the activities of intestinal functional effector cells, such as immune cells, epithelial cells, enteric neurons, smooth muscle cells, interstitial cells of Cajal and enterochromaffin cells. These same cells, on the other hand, are under the influence of the gut microbiota. (5)
Recent studies have proven that any disturbances in these microbiota (dysbiosis) will not only lead to gastrointestinal disorders, but can also be the cause of a number of central nervous disorders (i.e. autism, anxiety-depressive behaviors). In addition to a number of immune disorders including allergies, obesity, Alzheimer’s, diabetes, and systemic lupus erythematosus. Some studies have even suggested that gut microbiomes play a role in atherosclerosis.
Normal Flora Affects Your Personality?
- A research team led by Nobuyuki Sudo, a professor of internal medicine at Kyushu University in Japan, restrained germ-free mice in a narrow tube for up to an hour and then measured their stress hormone output. The amounts detected in the germ-free animals were far higher than those measured in normal control mice exposed to the same restraint. These hormones are released by the hypothalamic-pituitary-adrenal axis, which in the germ-free mice was clearly dysfunctional. But more important, the scientists also found they could induce more normal hormonal responses simply by pretreating the animals with a single microbe: a bacterium called Bifidobacterium infantis. This finding showed for the first time that intestinal microbes could influence stress responses in the brain and hinted at the possibility of using probiotic treatments to affect brain function in beneficial ways. (6)
- A research team at McMaster University in Ontario led by microbiologist Premsyl Bercik and gastroenterologist Stephen Collins discovered that if they colonized the intestines of one strain of germ-free mice with bacteria taken from the intestines of another mouse strain, the recipient animals would take on aspects of the donor’s personality. Naturally timid mice would become more exploratory, whereas more daring mice would become apprehensive and shy. These tendencies suggested that microbial interactions with the brain could induce anxiety and mood disorders. (7)
Gut Microbiota and Autism Spectrum Disorder:
Autism spectrum disorders (ASDs) are considered a heterogenous set of neurobehavioral diseases, with the rates of diagnosis dramatically increasing in the past few decades. As genetics alone does not explain the underlying cause in many cases, attention has turned to environmental factors as potential etiological agents. Evidence is mounting that intestinal microbes exacerbate or perhaps even cause some of autism’s symptoms.
- In a 2013 study published in PLOS ONE, Italian researchers reported that, compared with healthy kids, those with autism had altered levels of several intestinal bacterial species, including fewer Bifidobacterium, a group known to promote good intestinal health. (8)
- A study published in December 2013 in Cell supports the former idea. When researchers at the California Institute of Technology incited autism-like symptoms in mice using an established paradigm that involved infecting their mothers with a virus-like molecule during pregnancy, they found that after birth, the mice had altered gut bacteria compared with healthy mice. By treating the sick rodents with a health-promoting bacterium called Bacteroides fragilis, the researchers were able to attenuate some, but not all, of their behavioral symptoms. The treated mice had less anxious and stereotyped behaviors and became more vocally communicative.
A co-author of the previous study called Elaine Y. Hsiao, a microbiologist at Caltech, says that researchers do not yet know how exactly gut bacteria might influence behaviour, but one hypothesis is that a leaky gut may allow substances to pass into the bloodstream that harm the brain. In the mouse module, the probiotic may have helped reshape the microbial ecosystem and made the intestines more robust, preventing the leakage of such substances. (9)
In conclusion, intact and healthy microbiota plays a big role in the regulation of mood and behaviour. This area of research has been rising in the past few years as the link between microbiota & mental health has proposed new ways of treating many diseases including mental illnesses.
By: Hadil Khalid Ellafi – 3rd Year Medical Student