Malassezia

Lore & Background

Malassezia was originally identified by French scientist Louis-Charles Malassez in the late nineteenth century, who associated it with seborrhoeic dermatitis. Raymond Sabouraud identified a dandruff-causing organism in 1904 and called it Pityrosporum Malassezii, honoring Malassez at the species level. When the organisms were determined to be 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, and Pityrosporum (Malassezia) pachydermatis, which is lipophilic but not lipid-dependent and found on the skin of most animals. Current sources consider P. ovale and P. orbiculare to refer to a single species, with M. furfur the preferred name.

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 proposed by Moore in 1980 and emended by Begerow and colleagues in 2000, at which time it 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. The yeast-like cells of Malassezia, measuring between 1.5–4.5 μm by 3–7 μm, are characterised as phialides featuring tiny collarettes, which are challenging to identify using standard light microscopes. A defining characteristic is that one end is round while the other has a distinctly blunt termination, where singular, broad-based bud-like structures emerge.

Reader's Guide

Malassezia is significant primarily for its role in human diseases, including dandruff, seborrheic dermatitis, acne, tinea versicolor, and Malassezia folliculitis. When Malassezia grows too rapidly, the natural renewal of cells is disturbed, and dandruff can appear with itching. Molecular or DNA-based techniques have identified M. globosa as the species causing most skin disease in humans, and it is the most common cause of dandruff and seborrhoeic dermatitis, though M. restricta is also involved. There can be as many as ten million M. globosa organisms on a human head. A 2007 project sequenced the genome of dandruff-causing M. globosa and found it to have 4,285 genes, using eight different types of lipase and three phospholipases to break down scalp oils; any of these 11 proteins would be a suitable target for dandruff medications. Prescription and over-the-counter shampoos containing ketoconazole are commonly used to treat dandruff caused by Malassezia. In occasional opportunistic infections, some species can cause hypopigmentation or hyperpigmentation. Translocation of Malassezia species from the intestines into pancreatic neoplasms has been associated with pancreatic ductal adenocarcinoma, and M. restricta is linked to Crohn's disease and inflammatory bowel disease when found in the gut, especially in organisms with the N12 CARD9 allele. The genus requires specific growth media and sometimes grows very slowly in laboratory culture; an essential factor for growth is the presence of long-chain fatty acids, with M. pachydermatis being an exception.

Did You Know?

• M. globosa has been predicted to have the ability to reproduce sexually, but this has not been observed.
• Malassezia is the sole genus in the family Malasseziaceae, which is the only family in order Malasseziales.
• M. pachydermatis was first isolated from the skin of an Indian rhinoceros.