Scientists are investigating evidence that major depression may be, in part, a gut feeling orchestrated by the microbiome – trillions of microorganisms that live in and around our bodies that affect our health and well-being.
In a series of studies, researchers find that the microbial menagerie that lives in our digestive tract can help regulate brain function, including mental health. Recent findings from scientists in the US, Europe and China link our feelings of stress, anxiety and severe depression to disturbances among hundreds of microbe species living in our gut that some researchers have come to call the psychobiome.
Conversely, other bacteria in the gut seem to produce some of the same substances that doctors use to treat depression, and can naturally play a role in maintaining our emotional balance.
“The feeling of malaise, if you will, is often associated with gastrointestinal disturbances,” said microbiologist Jack Gilbert of the University of California, San Diego and the Scripps Institution of Oceanography, who pioneered the study of the human gut microbiome. It is “the chemical alteration of nerve signals going to the brain, which alter brain chemistry and thus behavior, mood and, we believe, depression and anxiety.”
As evidence, some scientists have been able to infect mice and rats with mental disorders, including depression and anxiety, by transplanting stool samples containing gut microbes from human patients into laboratory animals, several recent studies show. “When you give these mice the microbes of depression, they start to act depressed,” said psychiatrist Julio Licinio of the State University of New York Upstate Medical University in Syracuse. These behavioral changes in mice affect things like appetite, weight gain, and activities such as swimming. Dr. Licinio studies the biology of depression and helped design some experiments. “It’s actually transferable,” he said.
Bacteroides, seen here on a colored scanning electron microscope photo, are the most common bacteria found in the human intestinal tract.
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SCIENTIFIC PHOTO LIBRARY
Until now, no one has been able to distinguish specific types of microbes related to mental illness. This month, an international research team first identified dozens of types of gut microbes involved in depression by comparing patients diagnosed with the disorder to healthy people. These 47 species represent a tiny fraction of the gut’s microbial diversity, including other unicellular organisms, thousands of viruses and fungi.
The new research by neuroscientist Peng Xie at China’s First Affiliated Hospital of Chongqing Medical University and colleagues reveals a possible mechanism for a mental illness affecting an estimated 350 million people worldwide, several experts said. The research is published in the journal Science Advances.
Scientists are rushing to discover how such microbes interact with the human central nervous system, what signals they send to the brain, and how that changes a person’s behavior or risk of mental illness, in the hope of new treatments and diets for disorders of the mind.
“The great race has begun to understand the role all of these play in various brain diseases,” said Emeran Mayer, a medical psychologist at the University of California, Los Angeles who studies the microbiome of the brain and gut and “The Mind- Gut Connection ”has written. . He adds, “If you already have genetic risk factors for Parkinson’s, Alzheimer’s, or major depression, this is one factor that could push it over the edge into a disease.”
Not long ago, the only microbes that drew medical attention were germs that cause infections and illness.
But indiscriminate use of antibiotics and other sanitation measures eliminated the damage bacteria do at the expense of the protection they can provide. Unintended health consequences ranged from an increase in liver disease, type 2 diabetes and asthma to preterm birth and antibiotic-associated diarrhea, according to a 2019 review in the Journal of Experimental Medicine and many other microbiology studies.
Over the past decade, advances in low-cost, high-speed gene sequencing machines have allowed researchers to study millions of microorganisms that normally cannot be grown in a laboratory. In these studies, researchers can determine whether genetic material is from bacteria by means of a biomarker called the 16s ribosomal RNA gene, which is only found in microbes.
As a result, the study of the microbiome is one of the most exciting new fields in medicine, with more than 15,000 scientific papers published last year alone. “There is a lot of excitement in the field of psychiatry about this right now,” said John Cryan of University College Cork in Ireland, who studies the microbiome and neurobiology of stress.
Microbiologists calculate that the human gut contains more than 100 trillion microorganisms. Together, they weigh about five pounds – about as much as a large mango and a little more than the human brain, according to the European Society for Neurogastroenterology and Motility.
Additionally, where the human genome contains some 22,000 protein-coding genes, researchers estimate that the human microbiome contributes some eight million unique protein-coding genes, or 360 times more bacterial genes than human genes, according to the National Institutes’ Human Microbiome Project. Health. .
These microbes seem to be especially adaptable to changes in the environment, diet and the biochemistry of emotions. While no one knows exactly why, patients with a variety of psychiatric disorders, including depression, bipolar disorder, schizophrenia, and autism spectrum disorder, have significant perturbations in the composition of their gut microbiome.
The microbes seem to communicate with the brain almost constantly by affecting nerve signals and indirectly through chemicals that enter the bloodstream, said Dr. Gilbert, who is also a scientific advisor for a small microbiome company called Holobiome in Cambridge, Massachusetts. looks for new ways to treat depression, insomnia and other conditions.
For example, some common gut bacteria help generate neurotransmitters like serotonin, which affect neural activity related to mood and memory. It is often used to treat depression. Others make an amino acid called gamma-aminobutyric acid that naturally blocks certain brain signals. It is used in medicines to relieve anxiety and improve mood.
“The bacteria hijack parts of systems in the body that we know affect emotional regulation,” said Dr. Cryan. “This has led us to believe that we can have behavioral effects by targeting microbes in the gut that will affect overall well-being.”
Write to Robert Lee Hotz at [email protected]
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