The Mouth Bacteria That May Decide How Severe Your Peanut Allergy Is

Two people walk into an allergy clinic. Both have identical levels of peanut-specific antibodies in their blood. One reacts violently to a trace of peanut protein; the other tolerates a small handful without incident. Immunologists have puzzled over this for years. A study published March 3, 2026 in Cell Host and Microbe suggests the answer may be living in their mouths.

Bacteria That Dismantle Peanut Proteins

Researchers at McMaster University analyzed saliva and upper gut samples from healthy volunteers and found several bacterial species capable of breaking down major peanut allergens. One genus stood out: Rothia. These bacteria, common inhabitants of the oral cavity, were able to reduce peanut proteins ability to bind with antibodies - the molecular interaction that sets off allergic reactions.

The team then examined a cohort of participants with diagnosed peanut allergies. Those who had greater abundances of allergen-degrading bacteria in their microbiomes were able to tolerate higher amounts of peanuts before a reaction occurred. "We found evidence that some of these microbes may help break down peanut components in ways that could influence allergic responses," said co-senior author Alberto Caminero Fenandez, associate professor at McMaster Department of Medicine.

Validated in 120 Children, Then Tested in the Lab

To confirm the finding, the team checked an independent dataset of 120 children. Rothia species were significantly more abundant in children with higher peanut reaction thresholds - those who could eat more before symptoms appeared. The pattern held up across the external cohort, lending the finding more weight than a single-study association would carry on its own.

Pre-clinical experiments added mechanistic detail. Rothia reduced the amount of peanut allergens reaching the bloodstream, lowered activation of the immune cells responsible for anaphylaxis, and produced a significantly milder allergic response following exposure. The bacteria appear to work on multiple fronts: degrading allergens in the gut lumen before they are absorbed, and dampening the downstream immune cascade.

Why This Varies So Much Between People

Peanut allergy is the most common food allergy among Canadian children and the leading cause of allergy-related deaths in that age group. Strict avoidance is currently the only recommended management strategy. Despite that, accidental exposures are frequent - roughly one in three affected children experiences at least one per year.

"Peanut allergies can cause serious reactions like difficulty breathing, and in some cases can even be life threatening. However, some people with peanut allergies can still eat small amounts without having a reaction. We were curious about why this happens, and we discovered the answer while studying the microbes in our mouth," said lead researcher Liam Rondeau, a postdoctoral fellow at McMaster Farncombe Family Digestive Health Research Institute.

The new pathway - linking oral and gut microbiome composition to food allergy severity - helps explain individual variation that antibody levels alone cannot predict. It does not mean everyone can simply colonize themselves with Rothia and eat peanuts freely. These are early findings from a research study, and the clinical translation remains a distant goal.

A Possible New Target for Prevention and Treatment

The discovery opens several directions for future research. Microbial or probiotic therapies targeting allergen-degrading bacteria are one possibility. Improved oral immunotherapy strategies - which gradually expose allergic individuals to increasing allergen doses - might also benefit from accounting for microbiome composition. Could the right microbial profile make immunotherapy safer or more effective? That question is now on the table.

The study was multicentre, involving McMaster Schroeder Allergy and Immunology Research Institute and partners in Spain and the United States. Funding came from the New Frontiers in Research Fund, the Natural Sciences and Engineering Research Council of Canada, the Canadian Institutes of Health Research, and several European and Spanish government programs.