The human gut alone hosts approximately 100 trillion bacteria and other microbes of many different species, which are collectively known as the gut microbiome or microbiota (our body's overall microbiome also includes microbes living on the skin and in other parts of the body). Studies have shown that the gut microbiome plays a key role in regulating everything from digestion and metabolism to immune function and even mood, but the mechanisms of this action remain largely a mystery.
Microbes manipulate serotonin levels
Prior research has shown that a disrupted microbiome may contribute to the development of inflammatory disease, including inflammatory bowel diseases (IBD) such as Crohn's disease or ulcerative colitis. Research has also confirmed that people with IBD have a different gut microbiome composition than healthy people.
The current study was funded by the Foundation for the Study of Inflammatory Bowel Diseases in Aragón, Spain (ARAINF), in order to further study this connection. The researchers focused their investigations on a protein known as TLR2, which is a key marker of the presence of certain microbes in the intestines. Studies have also suggested that IBD may be triggered by the failure of TLR2 to function correctly.
In experiments conducted in cell cultures and in living mice, the researchers found that TLR2 actually helps regulate levels of the chemical serotonin. Although perhaps most well-known as a neurotransmitter that carries signals for the brain, serotonin also plays a key role in regulating bowel function.
The findings suggest that certain gut microbes can, through the action of TLR2, modulate levels of serotonin and therefore directly influence human physiology and brain chemistry.
Could the gut microbiome also modify serotonin levels to cause changes in mood or brain function? A 2014 review of the evidence into whether gut microbes can influence human emotions and behavior, published in the journal BioEssays, concluded that there is strong theoretical support for the idea but that evidence remains circumstantial. For example, studies suggest that some microbes can release chemicals that change the activity of the vagus nerve, which runs from the gut to the brain. Another study showed a different makeup of gut microbes in people who regularly crave chocolate, regardless of what they had recently eaten.
"Microbes have the capacity to manipulate behavior and mood through altering the neural signals in the vagus nerve, changing taste receptors, producing toxins to make us feel bad, and releasing chemical rewards to make us feel good," said senior author Athena Aktipis. (RELATED: Find more news about scientific discoveries at Scientific.news.)
Far-reaching effects
A 2015 study published in the journal Nature found another mechanism by which gut microbes might influence human physiology. That study showed that the common industrial food ingredients known as emulsifiers (detergents used to improve food's texture and shelf life) produce changes in the gut microbiome that lead to more of the inflammation associated with IBD and metabolic syndrome.
Metabolic syndrome is a cluster of physiological symptoms linked with a higher risk of heart disease, diabetes, liver disease and Alzheimer's disease. It is associated with high levels of systemic inflammation. IBD, in turn, is characterized by abnormal inflammation of the digestive tract. Both conditions have dramatically increased since the time period that saw the widespread adoption of chemical food additives.
Inflammation is an immune response, thus suggesting at least one mechanism by which gut microbes interact directly with the immune system.
Another recent study linked the gut microbiome with the development of Parkinson's disease, while others have linked a disrupted microbiome with the development of autism.
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