Depression? The bacteria in your gut may have caused it or made you healthier

In a series of studies, researchers find that microbial menstruation that lives in our digestive tract can help regulate brain function, including mental health. Recent findings by scientists in the US, Europe and China link our feelings of stress, anxiety and severe depression to disorders of hundreds of species of microbes living in our gut, which some researchers have begun to call psychobiomes.

Instead, other bacteria in the gut seem to produce some of the same substances used by doctors to treat depression and can naturally play a role in maintaining our emotional balance.

“Feeling sick, if you will, is often associated with gastrointestinal disorders,” said microbiologist Jack Gilbert of the University of California, San Diego and the Scripps Institution of Oceanography, who helped initiate the study of the human gut microbiome. It is “the chemical change of nerve signals that enter the brain, which changes the chemistry of the brain and therefore behavior, mood and, we believe, depression and anxiety.”

As evidence, some scientists have managed to infect mice and rats with mental disorders, including depression and anxiety, by transplanting stool samples, which contain intestinal microbes, from human patients to laboratory animals, several recent studies show. “When you give these mice the germs of depression, they start behaving in a depressive way,” said psychiatrist Julio Licinio of New York State University Northwestern Medical School 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 experiments. “It’s actually transmissible,” he said.

So far, however, no one has been able to distinguish specific species of microbes related to a mental illness. This month, an international research team identified for the first time dozens of species of intestinal microbes involved in depression by comparing patients diagnosed with the disorder with healthy people. These 47 species represent a small part of the intestinal microbial diversity, which includes other single-celled organisms, thousands of virus species and fungi.

New research by neurologist Peng Xie at China’s first hospital in Chongqing Medical University and colleagues reveals a potential mechanism for a mental illness that affects about 350 million people worldwide, several experts said. The research was published in the journal Science Advances.

Scientists are rushing to discover how these microbes interact with the human central nervous system, what signals they send to the brain and how they change a person’s behavior or risk of mental illness, in hopes of new treatments and diets for mental illness.

“The big race is beginning to understand the role of all this in various brain diseases,” said Emeran Mayer, a medical psychologist at the University of California, Los Angeles, who studies the brain and gut microbiome and wrote The Mind-Gut Connection. He adds, “If you already have genetic risk factors for Parkinson’s or Alzheimer’s disease or major depression, this is a factor that could push you over the edge into a disease.”

Not many years ago, the only microbes that attracted medical attention were the germs that caused infections and disease.

But the non-discriminatory use of antibiotics and other sanitation measures has eliminated the damage caused by bacteria to the detriment of the protection they can provide. The unintended health consequences ranged from increased liver disease, type 2 diabetes and asthma to premature 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 cannot normally be grown in a laboratory. In these studies, researchers can determine if the genetic material belongs to bacteria, although a biomarker called the 16s ribosomal RNA gene, which occurs only in microbes.

As a result, the study of the microbiome is one of the hottest new fields in medicine, with over 15,000 scientific papers published last year alone. “There’s a lot of enthusiasm in psychiatry right now about this,” said John Cryan of University College Cork in Ireland, which studies the microbiome and neurobiology of stress.

Microbiologists estimate that the human gut contains more than 100 billion microorganisms. Together they weigh about 5 kilograms – about the size of a mango and slightly larger than the human brain, according to the European Society for Neurogastroenterology and Motility.

Moreover, where the human genome carries about 22,000 genes encoding proteins, the researchers estimate that the human microbiome contributes about eight million unique genes encoding proteins, or 360 times more bacterial genes than human genes, according to the project. National Institute of Health for the Human Microbiome. .

These microbes seem adaptable especially to changes in the environment, diet and the biochemistry of emotions. While no one still knows exactly why, patients with various psychiatric disorders, including depression, bipolar disorder, schizophrenia, and autism spectrum disorder have significant disturbances in the composition of the intestinal microbiome.

Microbes appear to be in almost constant communication with the brain directly by affecting nerve signals and indirectly through chemicals absorbed into the bloodstream, said Dr. Gilbert, who is also a scientific advisor for a small microbiome company called Holobiome in Cambridge. Massachusetts is looking for new ways to treat depression, insomnia and other conditions.

Some common intestinal bacteria, for example, help generate neurotransmitters, such as serotonin, which affect neuronal activity related to mood and memory. It is commonly used to treat depression. Others produce an amino acid called gamma-aminobutyric acid that naturally blocks some brain signals. It is used in medicines to relieve anxiety and improve mood.

“Bacteria hijack parts of the body’s systems that we know affect emotional regulation,” said Dr. Cryan. “This has led us to the idea that by targeting gut microbes, we can have behavioral effects that will have an impact on overall well-being.”

Write to Robert Lee Hotz at [email protected]