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The bacteria that colonize a newborn’s gut play a critical role in building a strong immune system, especially in preventing food allergies. a new study revealed that a specific type of good bacteria colonizes the intestines of newborns and produces the neurotransmitter serotonin (commonly known as the happy hormone). Those serotonin neurotransmitters teach gut immune cells that common foods like peanuts are harmless and also how to peacefully coexist with beneficial gut microbes.
In a press release, the study’s lead author, Dr. Melody Zeng, assistant professor of immunology at Weill Cornell Medicine, said: “The gut is now known as the second human brain, as it produces more than 90% of the neurotransmitters in the human body. While neurotransmitters like serotonin are best known for their role in brain health, neurotransmitter receptors are found throughout the human body.”
Previous studies have identified serotonin as an important neurotransmitter that is known to not only control mood regulation but also the movement of food in the gastrointestinal system. High levels of serotonin have been linked to flare-ups of inflammatory bowel diseases such as Crohn’s disease. But most of those studies were done in adult animals, and researchers still wondered whether there might be a direct relationship between serotonin and intestinal immune cells.
To investigate further, they studied the small intestine of newborn mice and what neurotransmitters were produced in the intestine. They then identified different microbial populations in the small intestine of newborn mice, known as lactobacillus and Lactobacillus murinus. These bacteria species were much more abundant in newborn mice than in their adult counterparts. They also played a crucial role in shaping the newborn’s immune system by producing neurotransmitters such as serotonin.
“Until now, almost all studies on intestinal neurotransmitters have been conducted in adult animals or humans, where a specific type of intestinal cells called enterochromaffin cells produce neurotransmitters,” Dr. Zeng explained. “However, we found that this is not the case in the newborn intestine, where most of the serotonin is produced by bacteria that are more abundant in the neonatal intestine.”
Zeng and his team were able to show that a similar process also occurs in newborn human babies. The researchers analyzed stool samples from human babies, thanks to a biobank that the Zeng laboratory had created in collaboration with the Alexandra Cohen Presbyterian Hospital for Women and Newborns in New York.
Their findings highlight that before the human intestine develops the capacity to produce its own neurotransmitters, different bacteria step in to play the crucial role of supplying essential neurotransmitters such as serotonin, necessary for the early development of the immune system.
“We found that gut bacteria in young mice not only directly produce serotonin, but also decrease an enzyme called monoamine oxidase that normally breaks down serotonin, thus keeping serotonin levels high in the gut,” said lead author Katherine Sanidad, associate postdoctoral fellow in pediatrics at Weill Cornell. Medicine, she said in a press release.
Health further explained that in the neonatal intestine, high levels of serotonin increase the number of immune cells called regulatory T cells. Those cells then prevent the immune system from overreacting and attacking food antigens or bacteria that are beneficial to the gut. “The neonatal intestine needs these serotonin-producing bacteria to keep the immune system under control,” added Dr. Sanidad.
The researchers emphasized the importance of babies having good bacteria in their gut immediately after birth. Particularly in developed countries, babies are much less exposed to various microbes because they live in cleaner environments than babies born in developing countries. Additionally, unhealthy diets packed with ultra-processed foods could significantly deplete the presence of serotonin-producing bacteria in the neonatal gut.
Zeng and his team said that because of these factors, babies born in rich countries may have fewer regulatory T cells and are at higher risk of suffering from food allergies or developing immune reactions to beneficial gut bacteria.
“If raised properly, babies’ immune systems would recognize that things like peanuts and eggs are okay and it doesn’t have to attack them,” Zeng said.
The study was published in the journal Scientific immunology on March 15, 2024.
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