Malassezia

Malassezia (formerly known as Pityrosporum) is a genus of related fungi, classified as yeasts, naturally found on the skin surfaces of many animals and Man. It can cause hypopigmentation on the chest or back if it becomes an opportunistic infection.

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Changes in nomenclature

Some confusion exists about the naming and classification of Malassezia yeast species due to a series of changes in their nomenclature. Malassezia were originally identified by the French scientist Louis-Charles Malassez in the late 19th century, hence their proper current name. In the mid 20th century they were reclassified into two species: Pityrosporum ovale which is lipid dependent and found only on humans, and Pityrosporum pachydermatis, which is lipophilic but not lipid dependent and found on the skin of most animals. P. ovale was later divied into two classes, P. ovale and P. orbiculare, and later renamed again as Malassezia furfur. Work on these yeasts was complicated because they are extremely difficult to propagate in laboratory culture.

Later, in the mid 1990s, scientists at the Pasteur Institute in Paris, France (Evelyn Gueho and Jacques Guillot) discovered that there were indeed multiple species and reclassified and named the genus as Malassezia with several distinct species. Currently there are 10 recognized species: M. globosa, M. restricta, M. furfur, M. slooffiae, M. symposialis, M. nana, M. yamatoensis, M. dermatis, M. obtusa, and M. pachydermatis.

Role in human diseases

Recently, identification of Malassezia on skin has been aided by the application of molecular or DNA based techniques very similar to those used by forensic scientists to identify criminal suspects. These investigations show that in humans the species causing most skin disease, including the most common cause of dandruff and seborrhoeic dermatitis is M. globosa. The skin rash of tinea versicolor (pityriasis versicolor) is also due to infection by this fungus.

As the fungus requires fat to grow, it is most common in areas with many sebaceous glands: on the scalp,^[1] face, and upper part of the body. When the fungus grows too rapidly, the natural renewal of cells is disturbed and dandruff appears with itching (a similar process may also occur with other fungi or bacteria).

A project in 2007 has sequenced the genome of dandruff-causing Malassezia globosa and found it to have 4,285 genes. about 1/300th the size of the human genome. M. globosa uses eight different types of lipase, along with three phospholipases, to break down the oils on the scalp. Any of these 11 proteins would be a suitable target for dandruff medications.

Another surprising finding is M. globosa's ability to reproduce sexually, though this has not been seen in the laboratory or elsewhere.

Numbers

The number of specimens of M. globosa on a human head can be up to ten million.

Treatment of symptomatic scalp infections

Symptomatic scalp infections are often treated with selenium sulfide-containing shampoos. ^[2]

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