The bacteria and other microbes living in the gut may influence how the immune system responds to food.

A small NIH-funded clinical trial has now found that transferring gut microbes from healthy donors to people with peanut allergy increased tolerance in some participants, offering an early clue that the microbiome itself could become a therapeutic target.

The study, published in Science Translational Medicine on 5 August 2026, involved just 15 people, so the findings are preliminary. Even so, the work provides both clinical and biological evidence that altering the gut microbiome may affect allergic responses.

How The Trial Worked

The research team, led by Dr Rima Rachid and Dr Talal Chatila of Boston Children’s Hospital, tested faecal microbiota transplantation in people with peanut allergy.

Instead of undergoing an invasive procedure, participants swallowed capsules containing gut microbes from donors without food allergies.

Ten participants received the microbiome transplant alone. Another five took antibiotics beforehand, with the aim of reducing their existing gut bacteria so the transplanted microbes might establish themselves more easily. No serious side effects were reported.

Four months later, researchers tested whether participants could tolerate a larger amount of peanut without developing an allergic reaction.

Three of the ten people who received the transplant without antibiotics showed increased tolerance. The same was true for three of the five participants who had received antibiotics first.

Immune Cells Changed Too

The researchers also looked beyond the food-challenge results to understand what might be happening biologically.

Among participants who responded to treatment, one type of regulatory T cell became more abundant. These cells produced a protein called RORγt and remained elevated for as long as a year after treatment.

Regulatory T cells help restrain inappropriate immune reactions, so their increase offered one possible explanation for why some participants became less reactive to peanut.

Another type of T cell initially declined during the first month after treatment before later recovering, suggesting that the immune system was undergoing a broader adjustment rather than a single isolated change.

Mice Helped Test The Mechanism

To investigate whether the altered gut microbiome itself was responsible for the effect, the team transferred microbes from human participants into mice that did not already have a microbiome.

Mice receiving microbes from people who had responded to treatment were better protected against food allergy. They also developed immune-cell changes similar to those observed in the human responders.

This does not prove that the same mechanism works identically in people, but it strengthened the argument that particular gut microbes were contributing to the change in allergic response.

Bacteroides Emerged As A Clue

The researchers then examined which bacteria might be involved.

Mice given microbiota from treatment responders had higher levels of bacteria from the genus Bacteroides. When researchers introduced these bacteria into microbiome-free mice, the animals were less likely to develop food allergy and showed higher levels of the same RORγt-producing regulatory T cells.

The team also identified a possible metabolic pathway involving bile acids.

Bile acids are produced in the liver and later modified by gut bacteria. Participants who benefited from the transplant showed increased blood levels of molecules produced through bile-acid breakdown.

When researchers tested Bacteroides strains unable to process bile acids, those bacteria no longer protected mice from food allergy or produced the same immune-cell changes.

Why The Findings Matter

The study points towards a possible chain of events: selected gut bacteria may alter bile-acid metabolism, which may then influence regulatory immune cells and improve tolerance to dietary antigens..

That idea is scientifically important because it could eventually lead to something more targeted than a full microbiome transplant. Instead of transferring an entire microbial community, researchers may be able to identify specific beneficial bacteria or bacterial products and develop them into defined probiotic treatments.

Rachid said larger studies are now needed both to confirm the findings and to identify which patients are most likely to benefit. She also highlighted the possibility of isolating useful bacterial strains for future therapies.

What The Study Cannot Yet Show

The trial was very small, with only 15 participants, and there was no large placebo-controlled comparison group. The response was also incomplete: not everyone became more tolerant, and the treatment did not eliminate peanut allergy.

That means microbiome transplantation is not ready to become a standard food-allergy treatment.

For now, the most important result is not that researchers have found a cure, but that they may have identified a biological pathway linking gut bacteria, bile acids and immune tolerance.

If larger trials confirm the effect, the microbiome could eventually become one more way to reduce allergic sensitivity — but the next step is to establish exactly which microbes matter, who benefits most and how durable the protection really is.