Baby Gut Bacteria Found to Play Crucial Role in Immune Development

Researchers at Weill Cornell Medicine have shown that certain bacteria invade the stomach soon after birth and produce serotonin, a neurotransmitter that helps teach gut immune cells. In the early stages of growth, this stops allergic reactions to both food and the bacteria themselves.

The preclinical study, published in Science Immunology, revealed that serotonin, produced by bacteria plentiful in babies' stomachs, stimulates the growth of immune cells known as T-regulatory cells, or Tregs. To help prevent autoimmune illnesses and potentially fatal allergic reactions to innocuous foods or advantageous gut microorganisms, these cells dampen incorrect immune responses.

The gut is now known as the second human brain as it makes over 90 percent of the neurotransmitters in the human body. While neurotransmitters such as serotonin are best known for their roles in brain health, receptors for neurotransmitters are located throughout the human body.”

Dr. Melody Zeng, Assistant Professor and Study Senior Author, Department of Immunology, Weill Cornell Medicine

Gut Bacteria in Babies Provide a Helping Hand

The researchers observed that the neonatal mouse gut had much higher levels of neurotransmitters, including serotonin, than the adult gut.

So far, almost all studies of gut neurotransmitters were conducted in adult animals or human subjects, where a specific gut cell type called enterochromaffin cells produce neurotransmitters, however, we discovered that this is not the case in the newborn gut where most of the serotonin is made by bacteria that are more abundant in the neonatal gut.”

Dr. Melody Zeng, Assistant Professor and Study Senior Author, Department of Immunology, Weill Cornell Medicine

A human infant stool biobank that the Zeng lab built in partnership with the Neonatal Intensive Care Unit at the NewYork-Presbyterian Alexandra Cohen Hospital for Women and Newborns was also used to confirm this in infants. These samples were deidentified and collected with parental permission.

The study's findings imply that special gut bacteria may provide the neurotransmitters required for essential biological processes in the early stages of development before the newborn gut is developed enough to produce its own.

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 gut serotonin levels high.”

Dr. Katherine Sanidad, Postdoctoral Associate and Study Lead Author, Department of Pediatrics, Weill Cornell Medicine

By boosting the amount of Tregs, elevated serotonin levels alter the balance of immune cells and help keep the immune system from overreacting and attacking food antigens or gut flora.

Dr. Sanidad added, “The neonatal gut needs these serotonin-producing bacteria to keep the immune system in check.”

Healthy Immune System Helps Later in Life

According to Dr. Zeng, this research emphasizes how critical it is to have the correct kinds of probiotics in the body from the moment of birth. The availability of antibiotics for babies in industrialized nations is superior, and their clean environments expose them to fewer types of germs. Additionally, their diets may be poor, which could hurt the number of serotonin-producing bacteria in their intestines.

Consequently, these infants can have fewer Tregs and experience food allergies or immunological responses to their gut flora. This could be one of the reasons why children's food allergies are on the rise, especially in wealthy nations.

Dr. Zeng said, “If educated properly, the immune system in babies would recognize that things like peanuts and eggs are okay, and it does not have to attack them.” This may also have an impact on developing autoimmune diseases when the immune system attacks the body's healthy cells later in life.

Next, the group intends to examine microorganisms found in human newborn feces samples to quantify the amount of serotonin, other neurotransmitters, and chemicals that might be used to teach the immune system to avoid developing immunological-related disorders in the future, like allergies, infections, and cancer.

Dr. Sanidad said, “It is essential to understand how the immune system is trained during early life, but this is understudied in newborns and children. Further studies of these developmental periods may hopefully lead us to mitigation approaches to reduce the risk of inflammatory diseases like food allergies and inflammatory bowel disease later in life.”