There’s an unlikely culprit behind the itchy, flaky scalp condition better known as dandruff. It is caused by a yeast species known as Malassezia, the dominant member of an extensive community of fungi in human skin, also known as the mycobiome.
When resident scalp Malassezia feast on sebum, the oily substance produced by hair follicles, they break the sebum down and release free fatty acids. In some individuals, the fatty acids can trigger skin inflammation and hyperproliferation, leading to dandruff.
“Frequently, our normal, commensal fungal flora exist happily on most of us. In some people, however, they induce disorders like dandruff,” explained Thomas Dawson, a hair and scalp expert and Senior Principal Investigator at the Skin Research Institute of Singapore (SRIS), a tripartite collaboration involving A*STAR, NTU, and Singapore’s National Healthcare Group.
Thankfully, over-the-counter medicated shampoos have been soothing the itch of dandruff for decades. These formulations contain a range of active ingredients, most commonly zinc pyrithione. Exactly how these chemicals impact the mycobiome to alleviate dandruff has, until now, left scientists scratching their heads.
In collaboration with members of the international Malassezia Research Consortium, Dawson’s team tested shampoos containing various antifungal agents on the scalp mycobiomes of 35 study participants. They used the gold standard assay—the antifungal susceptibility test, or AFST—alongside fungal culture and genomic techniques to track changes in the growth dynamics of Malassezia for two weeks post-treatment.
Studying the scalp mycobiome proved to be a slippery task with the researchers facing several experimental hurdles. For one, oil and water don’t mix. Most antimicrobial assays such as the AFST are water-based, but Malassezia aren’t. “The big issue with antifungal susceptibility testing is that Malassezia are lipid-lovers. They live in sebum, a strongly hydrophobic environment, and most testing is done in water,” said Dawson, who was the corresponding author on the study.
There were also other logistical problems that made compliance difficult. For example, getting the participants to comply with the study’s strict shampooing regimens was a challenge, as some of them had to avoid washing their hair for up to two days.
Ultimately, zinc pyrithione was found to be superior to other antifungals for keeping dandruff-causing Malassezia at bay. “The optimal antifungal performance of zinc pyrithione shampoos is when they are used every other day,” recommends Dawson.
Follow-up studies will take a deep dive into how the mycobiome influences skin biology. “This should help us learn what Malassezia are doing on the skin and how to potentially intervene with non-antifungal technologies,” he said.
The A*STAR-affiliated researchers contributing to this research are from the Skin Research Institute of Singapore (SRIS).
