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Malassezia is a monophyletic genus of fungi found on the skin of 7 billion humans and associated with a variety of conditions, including dandruff, atopic eczema (AE)/dermatitis, pityriasis versicolor, seborrheic dermatitis, and folliculitis ([1], [2]; Figure 1). In immunocompromised hosts Malassezia can also cause systemic infections. There are 14 currently recognized species of Malassezia, eight of which have been associated with humans, four of these commonly [3]. Malassezia spp. are Basidiomycetous fungi, as are most species of fungi readily seen on a walk through the forest. Among the Basidiomycota, only Malassezia and Cryptococcus are frequent human pathogens. However, their adaptations to humans are presumed to be independent: Malassezia's closest relatives are plant pathogens: the smuts and rusts, whereas the closest relatives for Cryptococcus pathogenic species are fungal saprotrophs associated with trees and insects. We summarize here a cellular and molecular description of some interactions of Malassezia with humans and speculate on properties (release of allergen-containing nanovesicles, mating) that may be critical to Malassezia virulence.    Figure 1  Malassezia phylogeny, impact on human skin, and mating type (MAT) locus.     What Is Known about the Gene Content of Malassezia? A genome sequence of Malassezia globosa reveals as small a genome size as any free-living fungus, with only 4,285 genes and spanning just ∼9 Mb [4]. This small genome size may reflect adaptation to the organisms' limited niche, the skin of warm-blooded vertebrates [5]. While many of the genes for biosynthetic enzymes are present, M. globosa is the only free-living fungus known to lack a fatty acid synthase gene [4]. With a plethora of lipase genes, M. globosa likely satisfies its lipid requirement by hydrolysis of sebum triglycerides. Within the genus, only Malassezia pachydermatis, isolated from dogs and other non-human animals [5], is known to grow in the absence of exogenous lipid [1]. It will be interesting to learn whether this atypical species contains a fatty acid synthase gene similar to that found in the close relative Ustilago maydis and whether the habitat requirements of M. pachydermatis are, as a consequence, less stringent by relieving the requirement for exogenous lipids. While it is possible to culture Malassezia species axenically under laboratory conditions by providing exogenous lipids that mimic those available on human skin, some species are still quite fastidious, suggesting in vitro culture conditions may not be optimized.

Malassezia Fungi Are Specialized to Live on Skin and Associated with Dandruff, Eczema, and Other Skin Diseases