Allergies and Asthma in Babies: The Unexpected Discovery of Intestinal Fungi

While we closely monitor the bacteria residing in our infants' bellies, science has uncovered an unexpected player in the development of childhood allergies. Two Canadian studies published in June 2026 reveal that a microscopic fungus, a yeast called Malassezia, directly influences the maturation of the immune system in young children. This major advancement highlights the impact of early antibiotics on this fragile ecosystem, paving the way for better prevention strategies.

The Intestinal Mycobiome Actively Participates in Immune Defenses

There is much talk about the gut microbiota and its billions of bacteria, but we often overlook that it also harbors a community of microscopic fungi known as the mycobiome. Far from being an anomaly, their presence in infants is entirely healthy. The real challenge lies in their balance during the first months of life, a critical period when the immune system learns to distinguish harmless elements from real dangers.

By analyzing over two thousand stool samples from the Canadian CHILD cohort, researchers demonstrated that this mycobiome evolves in a highly orchestrated manner. While certain families of fungi increase with age, the yeast Malassezia naturally decreases. However, when it remains present in abnormally high quantities around one year of age, the risk of developing atopic dermatitis or food allergies by the age of five rises. This fungal maturation delay thus acts as an early marker of immune vulnerability.

Antibiotics Temporarily Alter the Digestive Ecosystem

A second study conducted on infants under six months shows that antibiotic treatments profoundly disrupt this balance. By eliminating some bacteria, they free up an ecological niche that fungi quickly occupy. The overall fungal load then increases, and the yeast Malassezia proliferates to become dominant. The longer or more repeated the treatment, the more it takes up space.

No need for panic or guilt: antibiotics remain essential and save lives in cases of severe bacterial infection. This research simply sheds light on a previously overlooked collateral effect, offering a biological explanation for the frequent correlation between early antibiotic exposure and the later development of asthma.

The Proliferation of Malassezia Amplifies Inflammatory Reactions

To understand the direct impact of this imbalance, scientists conducted complementary laboratory experiments. In animal models colonized by this yeast, they observed a marked increase in inflammation in the intestine and respiratory tracts. In the face of common allergens like dust mites or during an infection with RSV, the bronchiolitis virus, the excessive presence of Malassezia significantly amplified allergic and pulmonary reactions.

This phenomenon can be explained by metabolic changes. This yeast, which is lipid-hungry, tends to consume protective and anti-inflammatory fatty acids like oleate, naturally present in milk. In doing so, it temporarily deprives the body of valuable molecules needed to regulate its immune defenses.

Fungal Profiles Open the Door to Personalized Prevention

Although these discoveries still require clinical confirmation in humans, they open exciting prospects for pediatrics. Ultimately, analyzing the fungal profile of babies may allow us to identify at-risk profiles well before the onset of the first symptoms and propose targeted preventive treatments. In the meantime, this research reminds us to take care of the overall health of our children's gut, a fascinating microscopic universe whose richness we are just beginning to measure.