INTRODUCTION
Seborrheic dermatitis (SD) and dandruff are of a continuous spectrum of the same disease that affects the seborrheic areas of the body, with dandruff being milder and specifically localized to the scalp. It is characterized by a flaky, pruritic scalp and affects up to half the world's population post-puberty.1,2 Compared with dandruff, SD can affect the scalp as well as other seborrheic areas and is more severe when pruritus, flaking, scaling, and skin inflammation are involved. Environmental and host factors may alter the sebaceous gland activity, sebum composition, epidermal barrier function, and scalp microbiome balance. Various environmental (eg, pollution; stress) and intrinsic (eg, puberty; individual susceptibility) factors may affect the sebaceous gland activity and thus the sebum composition, scalp surface microbiome, and skin barrier function. Interactions between these factors, all contribute to the pathogenesis of SD and dandruff.3
Although the exact pathophysiology of dandruff is still not completely decoded, current theories highlight the role of the microbiome on the skin surface in the pathogenesis. Several scalp microbiome studies from different populations have revealed the association of dandruff with bacterial and fungal dysbiosis.4,5 Another study comparing the major bacterial-fungal populations colonizing dandruff scalps in China and France suggests that targeting one particular Malassezia species by antifungals instead of using broad-spectrum antifungals and rebalancing the dandruff scalp microbiota could be a common approach to improve dandruff condition.6,7 Overall scalp microbiome composition significantly differed between normal and dandruff groups linked with hyperproliferation of lipophilic yeasts of the genus Malassezia8 and Staphylococcus spp. Additional microbial markers such as Aspergillus and Pseudomonas have also been proposed.5,9
