Cryptococcus neoformans

Cryptococcus neoformans is a yeast that belongs to the Tremellomycetes class, has a basidiomycetous nature, and is surrounded by a capsule. It requires oxygen to survive and can inhabit both plants and animals. The fungus causes cryptococcosis, a disease that mainly affects people with weakened immune systems, such as those with HIV or cancer, though it can also infect apparently healthy individuals, particularly in developing countries. (Note: Infections in healthy individuals are more commonly reported in developing countries, while C. gattii is more associated with healthy individuals in developed regions.) Treatment is challenging because the organism displays high genomic plasticity and significant genetic variation across its strains.

Classification

The classification of this species has changed many times since it was first described in 1895. It originally included two varieties: C. neoformans var. neoformans and C. neoformans var. grubii. A third variety, C. neoformans var. gattii, was later recognized as a separate species, Cryptococcus gattii.

Under the most recent system, these varieties have been split into seven species. C. neoformans now refers specifically to C. neoformans var. grubii, while the former var. neoformans has been given the new name Cryptococcus deneoformans. C. gattii itself is divided into five species.

The sexual stage (teleomorph) of the fungus was first described in 1975 by K.J. Kwon-Chung. She obtained cultures of Filobasidiella neoformans by crossing different strains of the yeast C. neoformans and observed basidia similar to those in the genus Filobasidium, leading to the new genus name Filobasidiella. Because the International Code of Nomenclature for algae, fungi, and plants later stopped allowing separate names for teleomorph and anamorph forms of the same fungus, Filobasidiella neoformans became a synonym of the earlier name Cryptococcus neoformans.

Etymology

The genus name Cryptococcus comes from the Ancient Greek words kruptós (hidden) and kókkos (grain). The species name neoformans is derived from Latin neo- (new) and fōrmāns (shaping).

Characteristics

In its typical form, C. neoformans grows as a single-celled yeast and reproduces by budding. During mating, it produces hyphae and eventually forms basidiospores at the tips of those hyphae before releasing spores.