633 Address all correspondence to Trefor Evans, tevans@triprinceton.org Preface TREFOR A. EVANS Director of Research and Institute Fellow, TRI Princeton, Princeton, New Jersey, USA (T.E.) Chairman of 9th International Conference on Applied Hair Science The International Conference on Applied Hair Science is organized by Textile Research Institute (TRI) Princeton every other year with the goal of bringing together hair scientists and researchers from around the globe to share findings, ideas, and discoveries. The ninth iteration of this event was originally scheduled for June 2020 in Red Bank, New Jersey, but the Covid health crisis led to postponement until June 7–11, 2021, and the implementation of a virtual format. This new medium allowed a considerably wider audience to be reached, and more than 1,000 hair scientists from all around the world registered for this online event. The program consisted of 24 presentations by speakers from seven different countries. A poster session attracted a further 17 submissions. Presentations were slotted into four sessions dedicated to Hair Biology, Insights into Hair Structure, Hair Insults and Protection, and New Methods. All lectures were recorded so they could be viewed “live” and/or later “on demand” by conference attendees. This special edition of Journal of Cosmetic Science features nine articles based on talks presented at this conference and includes submissions from each of the four sessions. These papers illustrate the diversity of the event by describing work in the areas of hair biology, hair structure, chemical composition, measurement science, and formulation science. Similarly, articles were penned by a variety of authors from academia, end-product companies, ingredient suppliers, and testing laboratories. TRI thanks everyone who helped make the 9th International Conference on Applied Hair Science a resounding success. This includes the organizing committee, sponsors, presenters, and participants. See you all in 2022! J. Cosmet. Sci., 72, 633 (November/December 2021)

634 Address all correspondence to Thomas Dawson, thomas_dawson@asrl.a-star.edu.sg The Role of the Scalp Microbiome in Health and Disease: Malassezia, Friend or Foe? THOMAS L. DAWSON, JR A*Star Skin Research Laboratories, Skin Research Institute, Singapore, Singapore Department of Drug Discovery, Medical University of South Carolina, USA (T.L.D.) Synopsis The human microbiome has soared to scientific celebrity status, while the skin microbiome has been somewhat dragging behind. We now understand that microbiomes are central to humanity, with the human body consisting of more microbes than human cells and the microbes both being essential for maintenance of life and regulating diverse functions such as digestion, obesity, blood pressure, skin disease, and even brain function, mood, and personality. While the skin microbiome has lagged in investigation and understanding, we now know it influences many facets of skin health, including acne, seborrheic dermatitis (dandruff), psoriasis, atopic dermatitis, and rarer disorders, including pityriasis versicolor and folliculitis. Bacteria rule the gut microbiome, but fungi play an important role in skin, particularly the dominant eukaryote Malassezia yeast. Malassezia are present on all humans (and all warm-blooded animals), including healthy and diseased skin. The Malassezia clade consists of 18 species with numerous functionally distinct subspecies and strains, and it remains unclear how the functional differences are involved in skin health. Historically, Malassezia have been considered “commensal” organisms, defined as those who derive a benefit from their host (humans, as their food source) but have no impact, either positive or negative. A “mutualistic” relationship is one in which both partners benefit, and a “pathogenic” one in which one benefits and one is harmed. Hence, commensalism is the default situation where there is limited data. Now, with more evidence, it is becoming clear that Malassezia likely have all three roles in human skin. INTRODUCTION Why would a talk on skin microbiome be important in a hair care conference? In this case, it results from the internal divisions of consumer care companies, with vertical sections based on product form. As dandruff is most commonly treated with shampoos, dandruff is a hair care issue, despite it being a disorder and treatment paradigm relating to the skin microbiome. It is obvious why we have skin—it keeps bad things out and necessary things, like water, in. Skin is our “space suit” protecting us from a hostile environment. But the skin is also completely different from a space suit, as it is a living entity replenished every 14 to 21 days and covered in a microbial “skin” of its own. As skin is complicated and this audience is focused on hair, skin warrants a bit of introduction. J. Cosmet. Sci., 72, 634–642 (November/December 2021)