Cow's milk is the most common food allergen among children.
When scientists transplanted gut microbes, or microbiota, from healthy human babies into mice with no bacteria of their own, the animals did not have an allergic reaction on exposure to cow's milk.
In contrast, germ-free mice that received gut bacteria from human babies with cow's milk allergy did experience allergic reactions to cow's milk.
Allergy to cow's milk is the most common childhood food allergy.
The researchers, who report their findings in the journal Nature Medicine, also identified a bacterium that, when present in the gut, prevents allergic responses to food.
"This study," says senior study author Cathryn R. Nagler, Ph.D., a professor in food allergy at the University of Chicago in Illinois, "allows us to define a causal relationship and shows that the microbiota itself can dictate whether or not you get an allergic response."
She adds that the results "strongly suggest" that treatments that work by altering gut bacteria could help to reduce the "food allergy disease burden."
Food allergy and prevalence
Allergic reactions happen when the immune system responds in an extreme way to foreign substances, or allergens, that usually cause no harm in most people.
Some common substances that produce allergic reactions include pollen and certain types of food.
Although most reactions are not severe, when they are, they can be life-threatening because of the enormous stress that they place on circulation and breathing.
Cow's milk, eggs, peanuts, soy, wheat, and tree nuts are some of the foods that most often provoke allergic responses in children.
The foods that are most likely to cause allergic responses in adults include fish, shellfish, peanuts, and tree nuts.
In childhood, most food allergies develop in the first 2 years of life.
The prevalence of food allergy in those aged 0–17 years has been rising slowly in the United States. During 1997–1999 it was 3.4 percent, and it rose to 5.1 percent during 2009–2011.
Gut microbes, health, and disease
With an internal surface area of some 250–400 square meters, the gastrointestinal tract, or gut, is one of the largest interfaces between the human body and its environment.
Around 60 metric tons of food pass through the human gut in an average lifespan. This includes a huge quantity and variety of microorganisms that could be a considerable threat to gut health.
Over thousands of years, the human gut and the vast colonies of microbes that live in it — collectively termed the gut microbiota — have together evolved a complex relationship that benefits both sides.
As a result of this long association, gut microbes have come to play a key role in the health and disease of their human hosts. For example, they help to digest food, harvest energy, protect against pathogens, and control immunity.
However, imbalances in the composition of gut microbes can disrupt these vital functions. This may cause or contribute to illness or prompt failure to protect against it.
As tools for investigating and profiling gut bacteria have improved, so scientists have increasingly uncovered connections between gut microbes and diseases that affect not only the intestines but also other parts of the body.
Cow's milk allergy and gut bacteria differences
A few years ago, some researchers behind the new study found that the gut bacteria of healthy babies differed markedly from those of babies with cow's milk allergy.
This prompted them to wonder if the differences might help the allergy to develop.
To investigate this, they obtained fecal samples containing gut microbes from eight human babies. Four of the infants had cow's milk allergy, while the other four did not.
Using the fecal samples, the team transplanted gut microbes from the human babies with and without cow's milk allergy into mice that had been raised in a sterile environment and had no gut bacteria of their own.
The scientists fed the mice the same infant formula that the human babies received. This was to ensure that the bacteria had the same nutrients and colonized in the same way.
When they fed cow's milk to the germ-free mice that had received gut bacteria from infants with cow's milk allergy, the animals developed anaphylaxis, a life-threatening condition that arises during severe allergic responses.
The same severe response occurred when the team gave cow's milk to germ-free mice that had not received any bacteria (the controls).
However, the germ-free mice that had received gut bacteria from infants without cow's milk allergy showed no severe reactions on exposure to cow's milk. It appears that they were "completely protected."
The investigators then ran genetic comparisons of the gut microbes of the allergic germ-free mice with those that showed no allergic reaction.
The tests identified a particular bacterium by the name of Anaerostipes caccae. It seems that the presence of this species in the gut prevents allergic reactions to food.
One bacterium with a huge impact
A. caccae belongs to a class of bacteria called Clostridia. In earlier work, Prof. Nagler and her team had found that the presence of the bacterium in the gut protects against nut allergy.
The recent study suggests that this protection extends to other types of food allergy.
A. caccae produces a short-chain fatty acid called butyrate. This nutrient helps the gut to establish a bacterial composition that promotes health.
The researchers were surprised to find how big an impact this one bacterial species — out of the many that inhabit the gut — can have on the body's reaction to food.
"[This work] shows that we can use metabolic products of the healthy microbiome to develop drugs that protect against food allergy."
Prof. Cathryn R. Nagler, Ph.D.