Atopic dermatitis: protecting the skin from the Malassezia fungus

Dry and itchy skin, red plaques (especially in the folds) that appear in flares: atopic dermatitis (AD) is an inflammatory skin disease that affects nearly one in five infants. Although AD tends to improve with age, it is estimated that one in ten adults suffers from the disease in developed countries.

Why my child and why me? Many factors are involved, with both heredity (children of adults affected by AD are more likely to suffer from the disease) and the immune system playing important roles. Another key factor is the skin microbiota, the complex community of microorganisms (sidenote: Microorganisms Living organisms that are too small to be seen with the naked eye. They include bacteria, viruses, fungi, archaea and protozoa, and are commonly referred to as “microbes”. What is microbiology? Microbiology Society. ) (bacteria, fungi, parasites, and viruses) residing on our skin.

Malassezia depleted

In AD patients, some bacteria, such as Staphylococcus aureus, tend to be overrepresented, while others, such as Cutibacterium, are depleted. And what about microscopic fungi? There was no clear answer to this question until a recent study showed that the skin’s fungal flora differs in cases of severe dermatitis. Thus, while the Malassezia fungus predominates on healthy skin, it loses its prime position in AD patients. In its place, other fungi such as Candida or Debaryomyces make themselves at home. In other words, in cases of severe dermatitis, where Malassezia no longer dominates the skin fungi, there is greater fungal diversity.

Moreover, changes in the bacterial and fungal populations may be linked. For example, an increased abundance of S. aureus may favor the proliferation of Candida, as these two microorganisms have been shown to exhibit a synergistic activity.

Linked to AD severity

This skin microbiota imbalance (sidenote: Dysbiosis Generally defined as an alteration in the composition and function of the microbiota caused by a combination of environmental and individual-specific factors. Levy M, Kolodziejczyk AA, Thaiss CA, et al. Dysbiosis and the immune system. Nat Rev Immunol. 2017;17(4):219-232.   ) appears to be linked to the severity of AD. With a few exceptions, the skin microbiota of patients suffering from non-severe AD is similar overall to that of individuals with healthy skin. On the other hand, the researchers found a strong divide between patients suffering from severe AD and those with healthy skin or mild-to-moderate AD. One important reason for this is lower levels of Malassezia.