Malassezia Yeasts: How Many Species Infect Humans and Animals?

Malassezia species are lipophilic yeasts that are members of the normal mycobiota of the skin and mucosal sites of a variety of homeothermic animals. They are also among the few basidiomycetous fungi, such as some Cryptococcus spp., Rhodotorula spp., and Trichosporon spp., that can produce disease in man and animals. However, in contrast with these other species, which are quite often involved in disseminated infections in immunosuppressed patients, Malassezia yeasts are associated mainly with certain skin diseases [1]. This special lipophilic group of yeasts is unique among the fungi. Phylogenetically, they form a well-defined cluster of skinliving yeasts, surrounded by plant pathogens and phylloplaneinhabiting fungi (e.g., Ustilago, Tilletiopsis). However, the taxonomic position of the genus Malassezia in the classes of the phylum Basidiomycota is not yet totally well defined. Moreover, the sexual form of these yeasts is still unknown. Recently, a region corresponding to the mating type locus (MAT) has been identified for these yeasts, and it has been suggested that if there is an extant sexual cycle for some of these yeasts that it is more likely to be bipolar, with just two mating types, rather than tetrapolar, with many mating types [2]. In the last higher-level fungal phylogenetic classification revision [3], the monophyletic genus Malassezia was the only genus included in the order Malasseziales, which has an uncertain taxonomic position in the subphylum Ustilagomycotina (e.g., smut fungi). Very recently, the class Malasseziomycetes has been proposed to accommodate these fungi (2013, provided from an anonymous reviewer; unreferenced). They are taxonomically distant to the orders which include the other commented pathogenic basidiomycetous yeasts of the genera Cryptococcus (Filobasidiales) and Trichosporon in Agaricomycotina (e.g., mushrooms) and of the polyphyletic genus Rhodotorula (Sporidiobolales and Cystobasidiales) in Pucciniomycotina (e.g., rust fungi).