Date Published: February 27, 2014
Publisher: Public Library of Science
Author(s): F. Javier Cabañes, Joseph Heitman.
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 .
At present, the genus Malassezia includes 14 species (; Table 1), all of which infect or colonize humans or animals. However, until the late 1980s, this genus remained limited to only to two species; one of these, M. furfur (sensu lato), was considered a heterogeneous group of lipid-dependent yeasts living on human skin and requiring long-chain fatty acids to grow, while the lipophilic but non–lipid-dependent species M. pachydermatis was restricted to animal skin. The latter is the only species in the genus that does not require lipid supplementation for development in culture medium. M. sympodialis, a lipid-dependent species isolated from human skin, was the third species accepted in the genus, a century after the description of M. furfur. Later, the genus Malassezia was revised on the basis of morphological, physiological, and rRNA gene sequencing studies, and four new lipid-dependent species were described . At the same time, different studies – confirmed that the skin of healthy animals could also be colonized by lipid-dependent species, in addition to the non–lipid-dependent species M. pachydermatis. These lipid-dependent species are the major component of the lipophilic mycobiota occurring on the skin of horses and various ruminants . Some of these yeasts isolated from animals were described subsequently as new species, such as M. nana, M. equina, or M. caprae. Nowadays, Malassezia yeasts have been isolated from almost all domestic animals, different wild animals held in captivity, and also from wildlife . Despite this, the occurrence of Malassezia yeasts on the skin of most animals remains unknown. The observed host specificity of some of these species made it possible to anticipate an increase in the number of new species in this genus, particularly if other animal species, mainly wild species, were studied.
The pathogenic role of Malassezia yeasts in skin diseases has always been a matter of controversy. Commensal Malassezia yeasts are clearly implicated in human skin diseases without the presence of inflammation but with heavy fungal load, such as pityriasis versicolor. They are also associated with other skin disorders with characteristic inflammation, such as seborrheic dermatitis, atopic dermatitis, folliculitis, and psoriasis, where their role in the pathogenesis is less clear and, in some cases, speculative . Emerging evidence demonstrates that the interaction of Malassezia yeasts with the skin is multifaceted and entails constituents of the fungal wall, enzymes, and metabolic products, as well as the cellular components of the epidermis. Some skin disorders can be exacerbated by the interactions between Malassezia yeasts and the host immune system .
The study of some Malassezia yeasts continues to be difficult, due mainly to their low viability and lack of suitable methods for their isolation and preservation. The majority of yeasts can be stored at temperatures between 4 and 12°C and subcultured at intervals of 6 to 8 months. However, Malassezia spp. do not fit this pattern. Freezing at −80°C is the only successful method to maintain viable all Malassezia spp., particularly M. globosa, M. restricta, and M. obtusa, which have been reported as difficult species to maintain in vitro .