Dandruff is associated with the conjoined interactions between host and microorganisms

Dandruff is a common scalp disorder that has occurred for centuries and has a prevalence of nearly 50% in the worldwide population1. The formation of dandruff has been studied for decades, but no coincident view has been widely accepted. Dawson and others have proposed that dandruff is the result of individual susceptibility, sebaceous secretion and Malassezia fungi2,3,4. Gaitanis has reported that some metabolic products of tryptophan produced by Malassezia, such as indole derivatives, cause dandruff5. Recently, another study has demonstrated that the disequilibrium in the proportion of the major bacterial and fungal populations are associated with dandruff6. For a long time, studies on dandruff predominantly focused on fungi, particularly the Malassezia species, which are major fungi colonizing the human scalp and the dominant members of the cutaneous fungal microbiome2,3. Of the 14 known cultured species of Malassezia, the most clinically significant species are M. restricta and M. globosa7,8. These species have been reported to be associated with skin diseases, including dandruff, seborrheic dermatitis, pityriasis dermatitis, and atopic dermatitis9. However, another microorganism community composed of bacteria also inhabits the human scalp and includes facultative anaerobic bacteria, such as P. acnes, and aerobic bacteria, such as Staphylococcus10,11. The scalp is covered with pilosebaceous units and sweat glands. Human sebum is a complex mixture of triglycerides, squalene, cholesterol esters, wax esters and cholesterols that are secreted from the scalp12,13. The secretion of sebum is controlled by sebaceous gland activity, which has been reported to have a strong correlation with scalp flaking disorders2. The sebum secretion rate increases throughout a person’s teenage years, reaches the highest in the 15- to 35-year-olds and appears to decline continuously throughout the adult age range12,13,14. Throughout the active period of sebum secretion, the secretion rate is higher in males than in females. However, these host physiological conditions could affect the microbial flora living on the scalp by affecting the scalp microenvironment. Sebum is an important food source for the growth of fungi and bacteria. Malassezia produces non-specific lipases that can degrade any available triglycerides15,16. As a result, the saturated fatty acids in sebum are consumed, and the abundant unsaturated fatty acids are left on the skin. Though recently had been challenged17, unsaturated fatty acids had long been considered to penetrate into the stratum corneum and lead to barrier disruption3,18. The bacterium P. acnes can release free fatty acids onto the skin and promote bacterial adherence by hydrolyzing triglycerides and can produce bacteriocins to kill other strains19,20. A person’s age and gender also contribute to the variability of the microbial flora resident on the skin10. Physiological and anatomical differences in the cutaneous environments (i.e., sweat, sebum and hormone production) partially account for the microbial differences between males and females21. Based on the results of previous studies, scalp dandruff could be affected by many factors, including not only the microorganism residents but also many host factors. However, the associations of the severity of dandruff with the composition structure of the microorganisms, host demographics, and host physiological conditions and their interaction have not been clarified to date. In this study, we constructed an association network based on pyrosequencing data of the bacterial and fungal communities on human scalp samples with slight to moderate dandruff (but not severe scaling such as seborrheic dermatitis) to attempt to systemically analyze their relationships.