Which bacteria trigger the 'farm effect' that makes children less prone to allergies? Scientists have an answer now

Study identifies which bacteria trigger the 'farm effect' that makes children less prone to allergies

Children who grow up in a farm environment are less prone to allergies, asthma and hay fever than their classmates. This so-called farm effect has been identified in several observational studies worldwide. It is most likely attributable to the fact that various bacteria present in barn air prevent excessive inflammatory responses of the immune system that are characteristic of such diseases.

But exactly which bacteria? Now, an international team has answered this question. The researchers have shown for the first time which specific bacteria in barn air trigger the farm effect, which substances within those bacteria mediate the protection and which receptors in the body they bind to. Their findings have been published in NEJM Evidence.
Barn exposure–associated protection from asthma, hay fever, and atopic eczema is largely mediated by specific Gram-positive cow-derived bacteria, including Romboutsia timonensis and Glutamicibacter arilaitensis. Their metabolites (e.g., kynurenine, xanthine, α-linolenic and stearidonic acids) activate AhR and PPARγ in airway cells, attenuating excessive inflammatory responses and explaining most of the “farm effect.”
For years, we have been hearing about the hygiene hypothesis, which was first proposed in 1989. This came after three decades of dramatic increases in allergies, asthma and hay fever among children in Western industrialized countries. These are all conditions in which the immune system mounts an exaggerated inflammatory response and mistakenly attacks the body's own tissues.

According to the hygiene hypothesis, this happens because of understimulation in early childhood. The immune systems of children who encounter too few environmental microbes and common cold viruses are more likely to malfunction.

Girls and boys who grow up on farms and are exposed to a wider variety of microbes have the problem far less often.
As their immune systems constantly contend with bacterial 'sparring partners' from barn air, they are trained to avoid excessive inflammatory responses.

However, the hygiene hypothesis has not been definitively proven, as it is largely based on observational studies. This kind of research can only show more or less convincing correlation.
But with this new study, we can make a much stronger case, because we can identify the individual links in the proposed causal chain: the bacteria, the relevant microbial metabolic products and the human receptors.
The results
Researchers identified a small number of bacteria, which they can now pinpoint down to the species level - such as Romboutsia timonensis and Glutamicibacter arilaitensis. These Gram-positive bacteria together mediate two-thirds of the entire farm effect for asthma protection and half of the effect for hay fever and atopic eczema.
The bacteria originate in cows' digestive tracts, where they produce or metabolize substances like kynurenine, xanthine, alpha-linolenic acid and stearidonic acid. These compounds are easily inhaled and recognized by two receptors on human airway cells—AhR and PPARγ. These receptors have multiple functions, including in the immune system, where they appear to prevent excessive inflammatory responses.

Their role in the farm effect was previously unknown. For the first time, therefore, the complete biological chain is visible: cow → barn air → bacteria → metabolic products → human receptors → protection against asthma.
The new findings can now be used by laboratory researchers to decipher the molecular and cellular mechanisms of the farm effect. This opens up the prospect of developing a drug that mimics the farm effect—without the need for children to spend time in barns.

Giulia Pagani et al, Gram-Positive Bacteria and the Inverse Association between Farm Exposure and Childhood Asthma, NEJM Evidence (2026). DOI: 10.1056/evidoa2500271