Malassezia

Systematics

Malassezia is the sole genus in family Malasseziaceae, which is the only family in order Malasseziales, itself the single member of class Malasseziomycetes.

Due to progressive changes in their nomenclature, some confusion exists about the naming and classification of Malassezia yeast species. Work on these yeasts has been complicated because they require specific growth media and sometimes grow very slowly in laboratory culture.

Malassezia was originally identified by the French scientist Louis-Charles Malassez in the late nineteenth century; he associated it with the condition seborrhoeic dermatitis. Raymond Sabouraud identified a dandruff-causing organism in 1904 and called it Pityrosporum Malassezii, honoring Malassez, but at the species level as opposed to the genus level. When it was determined that the organisms were the same, the term "Malassezia" was judged to possess priority.

In the mid-twentieth century, it was reclassified into two species:

• Pityrosporum (Malassezia) ovale, which is lipid-dependent and found only on humans. P. ovale was later divided into two species, P. ovale and P. orbiculare, but current sources consider these terms to refer to a single species of fungus, with M. furfur the preferred name.
• Pityrosporum (Malassezia) pachydermatis, which is lipophilic but not lipid-dependent. It is found on the skin of most animals.

Malassezia is the sole genus in the family Malasseziaceae, which was validated by Cvetomir Denchev and Royall T. Moore in 2009. The order Malasseziales had been previously proposed by Moore in 1980, and later emended by Begerow and colleagues in 2000. At this time the order was classified as a member of unknown class placement in the subdivision Ustilaginomycotina. In 2014, Cvetomir and Teodor Denchev circumscribed the class Malasseziomycetes to contain the group.

Description

Malassezia grows rapidly, typically maturing within 5 days when incubated at temperatures ranging from 30–35 °C (86–95 °F). Growth is slower at 25 °C (77 °F), and certain species struggle at 37 °C (99 °F). These organisms can proliferate on media infused with cycloheximide. An essential factor for the growth of Malassezia is the presence of long-chain fatty acids, with M. pachydermatis being an exception. The most conventional cultivation method involves overlaying solid media with a layer of olive oil. However, for nurturing some clinically relevant species, such as the challenging-to-cultivate M. restricta, more intricate culture media may be required. For the most efficient recovery of Malassezia, it has been recommended to collect blood through a lipid infusion catheter and subsequently use lysis-centrifugation—a recommendation backed by multiple comparative studies.

The yeast-like cells of Malassezia, measuring between 1.5–4.5 μm by 3–7 μm, are characterised as phialides featuring tiny collarettes (a small, collar-like flange or lip at the mouth of a phialide from which spores or conidia are produced and released). These collarettes are challenging to identify using standard light microscopes. A defining characteristic of cells from this genus is their morphology: one end is round, while the other has a distinctly blunt termination. This latter end is where singular, broad-based bud-like structures emerge, although in certain species, these structures might be narrower. To effectively visualise the organism's shape, a staining technique involving safranin is recommended, followed by observation under oil immersion. Furthermore, Calcofluor-white staining provides an enhanced clarity of the cell wall and its unique contour. While Malassezia typically lacks hyphal elements, rudimentary forms can sporadically be present.

Species

The Index Fungorum lists 22 species of Malassezia. The following list gives the name, the taxonomic authority (those who first described the fungus, or who transferred it into Malassezia from another genus; standardized author abbreviations are used), and the name of the organism from which the fungus was isolated, if not human.

In the mid-1990s, scientists at the Pasteur Institute in Paris, France, discovered additional species.

• Malassezia arunalokei Honnavar, Rudramurthy, G.S.Prasad & Chakrabarti
• Malassezia brasiliensis F.J.Cabañes, S.D.A.Coutinho, M.R.Bragulat & G.Castellá – from lesions on the beak of turquoise-fronted amazon parrot
• Malassezia caprae J.Cabañes & Boekhout – from skin of goat
• Malassezia cuniculi J.Cabañes & G.Castellá – from healthy skin of external ear canal of rabbit
• Malassezia dermatis Sugita, M.Takash., A.Nishikawa & Shinoda
• Malassezia equi Nell, S.A.James, C.J.Bond, B.Hunt & Herrtage – from skin of horse
• Malassezia equina J.Cabañes & Boekhout – from skin of horse
• Malassezia furfur (C.P.Robin) Baill.
• Malassezia globosa Midgley, E.Guého & J.Guillot
• Malassezia japonica Sugita, M.Takash., M.Kodama, Tsuboi & A.Nishikawa
• Malassezia muris (Gluge & d'Ukedem ex Guég.) Escomel – skin of mouse
• Malassezia nana A.Hirai, R.Kano, Makimura, H.Yamag. & A.Haseg. – from discharge from ear of cat
• Malassezia obtusa Midgley, J.Guillot & E.Guého
• Malassezia ochoterenai Maecke
• Malassezia pachydermatis (Weidman) C.W.Dodge – from skin of Indian rhinoceros
• Malassezia psittaci F.J.Cabañes, S.D.A.Coutinho, M.R.Bragulat & G.Castellá – from lesions on the beak of blue-headed parrot
• Malassezia restricta E.Guého, J.Guillot & Midgley
• Malassezia slooffiae J.Guillot, Midgley & E.Guého – from skin of pig
• Malassezia sympodialis R.B.Simmons & E.Guého
• Malassezia tropica (Castell.) Schmitter
• Malassezia vespertilionis J.M.Lorch & Vanderwolf – from vesper bats in subfamily Myotinae
• Malassezia yamatoensis Sugita, M.Takash., Tajima, Tsuboi & A.Nishikawa