Cryptococcus (fungus)

''Cryptococcus'' is a genus of fungi in the family Cryptococcaceae that includes both yeasts and filamentous species. The filamentous, sexual forms or teleomorphs were formerly classified in the genus ''Filobasidiella'', while ''Cryptococcus'' was reserved for the yeasts. Most yeast species formerly referred to ''Cryptococcus'' have now been placed in different genera. The name ''Cryptococcus'' comes from the Greek for "hidden sphere" (literally "hidden berry"). Some ''Cryptococcus'' species cause a disease called cryptococcosis.

Taxonomy

The genus was described by French mycologist Jean Paul Vuillemin in 1901, when he failed to find ascospores characteristic of the genus ''Saccharomyces'' in the yeast previously known as ''Saccharomyces neoformans''. Over 300 additional names were subsequently added to the genus, almost all of which were later removed following molecular research based on cladistic analysis of DNA sequences. As a result, some ten species are currently recognized in ''Cryptococcus''.

The teleomorph was first described in 1975 by K.J. Kwon-Chung, who obtained cultures of the type species, ''Filobasidiella neoformans'', by crossing strains of the yeast ''Cryptococcus neoformans''. She was able to observe basidia similar to those of the genus '' Filobasidium'', hence the name ''Filobasidiella'' for the new genus. Following changes to the International Code of Nomenclature for algae, fungi, and plants, the practice of giving different names to teleomorph and anamorph forms of the same fungus was discontinued, meaning that ''Filobasidiella'' became a synonym of the earlier name ''Cryptococcus''.

General characteristics

The cells of species that produce yeasts are covered in a thin layer of glycoprotein capsular material that has a gelatin-like consistency, and that among other functions, serves to help extract nutrients from the soil. The ''C. neoformans'' capsule consists of several polysaccharides, of which the major one is the immunomodulatory polysaccharide called glucuronoxylomannan (GXM). GXM is made up of the monosaccharides glucuronic acid, xylose and mannose and can also contain O-acetyl groups. The capsule functions as the major virulence factor in cryptococcal infection and disease.

Some ''Cryptococcus'' species have a huge diversity at the infraspecific level with different molecular types based on their genetic differences, mainly due to their geographical distribution, molecular characteristics, and ecological niches.

''Cryptococcus'' species are not known to produce distinct, visible fruitbodies. All teleomorph forms appear to be parasites of other fungi. In teleomorphs the hyphae are colourless, are clamped or unclamped, and bear haustorial cells with filaments that attach to the hyphae of host fungi. The basidia are club-shaped and highly elongated. Spores arise in succession from four loci at the apex (which is sometimes partly septate). These spores are passively released and may remain on the basidium in chains, unless disturbed. In the type species, the spores germinate to form yeast cells, but yeast states are not known for all species.

Habitat, distribution and species

''Cryptococcus neoformans'' is cosmopolitan and is the most prominent medically important species. It is best known for causing a severe form of meningitis and meningoencephalitis in people with HIV/ AIDS. It may also infect organ-transplant recipients and people receiving certain cancer treatments. In its yeast state ''C. neoformans'' is found in the droppings of wild birds, often pigeons; when dust of the droppings is stirred up, it can infect humans or pets that inhale the dust. Infected humans and animals do not transmit their infection to others. The taxonomy of ''C. neoformans'' has been reviewed: it has now been divided into two species: ''Cryptococcus neoformans'' sensu stricto and ''Cryptococcus deneoformans''.

''Cryptococcus gattii'' (formerly ''C. neoformans'' var. ''gattii'') is endemic to tropical parts of the continent of Africa and Australia. It is capable of causing disease in non-immunocompromised people. In its yeast state it has been isolated from eucalyptus trees in Australia. The taxonomy of ''C. gattii'' has been reviewed; it has now been divided into five species: ''C. gattii'' sensu stricto, ''C. bacillisporus'', C. deuterogattii'', ''C. tetragattii'', and ''C. decagattii''.

''Cryptococcus depauperatus'' is parasitic on '' Lecanicillium lecanii'', an entomopathogenic fungus, and is known from Sri Lanka, England, the Netherlands, the Czech Republic, and Canada. It is not known to produce a yeast state.

''Cryptococcus luteus'' is parasitic on ''Granulobasidium vellereum'', a corticioid fungus, and is known from England and Italy. It too is not known to produce a yeast state.

''Cryptococcus amylolentus'' was originally isolated as a yeast from beetle tunnels in South African trees. It forms a basidia-bearing teleomorph in culture.

References

{{Taxonbar, from=Q309498

Tremellomycetes
Basidiomycota genera
Yeasts