Atopic Dermatitis and Skin Fungal Microorganisms

A wide variety of bacteria and fungi are found on the human skin. Although some skin microorganisms produce antibacterial peptides that inhibit invasion by pathogens or promote the integrity of cutaneous defenses by eliciting host immune responses, the normal microbiome can also cause several skin diseases. Atopic dermatitis (AD) is a chronic disease that causes pruritus and involves cycles of remission and deterioration. AD is the result of dry hypersensitive skin. When the skin is dry, the protective barrier function of the cutaneous surface horny layer is compromised, and the skin readily develops dermatitis in response to various external stimuli, including skin microorganisms. Serum from almost all AD patients contains IgE antibodies against some skin microorganisms. For example, staphylococcal superantigen-specific IgE is present in the serum of AD patients, but not in the serum of healthy individuals. Normally, the weakly acidic condition of healthy skin prevents colonization by Staphylococcus aureus. However, in patients with AD, the skin pH is shifted toward neutrality, allowing S. aureus to grow and exacerbate AD. In the cutaneous fungal microbiome, lipophilic yeasts of the genus Malassezia are the predominant species on human skin. As Malassezia species require lipids for growth, they preferentially colonize sebum-rich areas such as the head, face, and neck, as opposed to the limbs or trunk. Specific IgE antibody against Malassezia species is found in the serum of AD patients. Antifungal therapy improves the symptoms of AD by decreasing the level of Malassezia colonization, suggesting that these microorganisms also exacerbate AD. Malassezia species, unlike S. aureus, colonize both AD patients and healthy subjects. Currently, the genus Malassezia consists of 14 species. Of these, M. globosa and M. restricta have been detected in almost all AD patients, suggesting that these two Malassezia species play a significant role in AD. The level of specific IgE antibody against both species is greater than that against other Malassezia species. This chapter discusses cutaneous fungi as an exacerbating factor in AD, focusing on: the fungal microbiome in patients with AD. immunological aspects of fungal colonization, and treatment with antifungal agents.