by Rachel Adams
There’s one thing that likely to unite every single person on the planet, and it’s only skin deep: Malassezia.
Malassezia is one of the few fungi that we know to grow on human skin; it is our special companion. Malassezia is a genus of yeasts with 14 species, all of which have been found on the skin of humans and other vertebrates. Different Malassezia species show preferences for different parts of the body. For example, M. globosa reigns on the scalp while M. restricta dominates that space in between your eyebrows and above your nose. (That part of your face actually has its own name: the glabella).
In most cases, Malassezia, which was first identified by the French human anatomist Louis-Charles Malassez in 1874, is a commensal microbe – meaning we, as host, provide it habitat and food but are not affected one way or another by its presence. What, you may be wondering, do we feed these fungi? Skin oils.
With only one exception, Malassezia species require a source of lipids, such as the oil you produce in your skin’s sebaceous glands, to grow. When we want it to survive on a petri dish in a laboratory, we need to add olive oil to our media. Presumably it would also thrive on canola or walnut oil, but I’ve never really tested its palate. M. furfur can even grow on castor oil.
Mostly Malassezia is just an unnoted passenger on our bodies, but not always.
Dandruff, or its more severe (and severe-sounding) form, seborrheic dermatitis, is caused by M. globosa. It’s a pretty simple ailment, but you wouldn’t know that from reading the medical journals. In medical-ese, the explanation might read: Upon consumption of scalp sebum by the lipophilic M. globosa, the fungus releases oleic acid, which can induce inflammation manifesting as excessive scalp desquamation. Translation: The fat-loving M. globosa eats the oil on your scalp and in the process leaves behind an irritant that makes your skin flake.
No doubt the former is the kind of talk you are likely to hear if you ever get the chance to attend the triennial Intercontinental Meeting of Hair Research Societies. Side-by-side with members of the European Hair Research Society, North American Hair Research Society, (the Japan-based) Society for Hair Science Research, and the Australasian Hair and Wool Research Society, you would hear cutting edge work by researchers who work on the science behind issues in hair care. For example, in 2007 Proctor & Gamble Beauty was bothered enough by Malassezia globosa to sequence its genome, each and every nucleotide. Once, the human genome project seemed impossibly difficult. Now there is a dandruff fungus genome project.
Ostensibly, Proctor & Gamble Beauty sequenced this fungus to try to figure out new ways to control it. The current dandruff-shampoos work by containing a general anti-fungal agent to keep M. globosa at bay.
The genome of M. globosa has not yet revealed a better treatment, or at least one that P&G is telling us about yet. What it has done, however, is to reveal some fascinating biology. M. globosa, it appears, has genetically lost its ability to make its own lipids; the same appears true for its other skin-dwelling kin. Meanwhile, the closest relative of the Malassezia clan, a corn smut (Ustilago maydis), has a lot more tools in its genomic toolbox (Technically, Malassezia are smuts too, such that it is biologically reasonable to describe someone you know who has dandruff as being affected by a hairy smut). Not only can Ustilago maydis synthesize lipids, it’s also good at breaking down carbohydrates – a skill necessary to infect corn but which Malassezia is ill equipped.
The inability to make your own fat is rare in nature, and the conventional wisdom is that Malassezia are specialized to live on animals – and exclusively so. In fact, the presence of Malassezia on skin unites the 7-billion humans on the planet with animals as diverse as dogs, cats, bears, ferrets, pigs, rhinoceros, and birds. But, recent work is showing that conventional wisdom may be wrong. Now, with our increased ability to detect organisms beyond those that will grow in culture, we are finding Malassezia and close-relatives in diverse places as Kansan tallgrass prairies, nematodes in Central Europe, Antarctic soils, deep-sea vents in the South China Sea, and associating with coral colonies in American Samoa.
Perhaps this dependence on something else’s fatty acid doesn’t specialize you to animal scalps after all. Fat is everywhere if you know how to look for it. This is probably not quite the lesson Proctor and Gamble was looking for, but it is one that will keep both them and Malassezia fungus in business for a long time.
About the Author
Rachel Adams is a post-doctoral fellow in the Department of Plant & Microbial Biology at the University of California at Berkeley. She studies the dispersal of fungi in homes and tweets at @Rachel_I_Adams.