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Colonies are white to dark cream colored, smooth, mucoid, glistening to dull and generally grow slower than Candida. The degree of mucoidness is proportional to the amount of polysaccharide capsule that the fungus produces. Colonies that have small capsules may be wrinkled and granular in appearance. They grow well at 25 C and 37 C. Temperatures above 37 C slow the fungus growth. When grown on media containing diphenolic compounds colonies are brown to black in color.
Microscopically yeast cells are globose to ovoid, thick-walled, budding cells that do not produce hyphae or pseudophyphae. Polysaccharide capsules can be seen using India Ink preparation. The capsules can be up to twice the radius of the yeast cell.
This genus includes four serotypes A-D. Until recently serotypes A and D were included in Cryptococcus neoformans var. neoformans and serotypes B and C were included in Cryprococcus neoformans var. gattii. However a new variety, var. grubii has been proposed to contain serotype A.
The teleomorph of Cryptococcus neoformans var. neoformans is Filobasidiella neoformans var. neoformans and can be induced by placing two isolates of serotype D of opposite mating types in V-8 juice agar. The yeast cells will conjugate and produce long hyphae with clamp connections, haustria-like hyphae, and dolipore (nuclei cannot pass through) septa.
Terminal basidia form from the hyphae in about three weeks. The basidia are slender, subglobose to club-shaped, and non-septate.
Basidiospores are sessile, slightly rough, oval, pyriform, cylindrical to elliptical, uninucleate, and produced in chains by basipetal budding at the apex of the basidia, i.e., at the base of the chain.
a | The fungal polysaccharide capsule is produced in response to environmental cues, in this case after five days in the rabbit central nervous system. India ink (black) is excluded from the fungus by the capsule. b | The fungus produces melanin in the presence of diphenolic molecules (such as L-DOPA) to turn brown or black, in contrast to the common human pathogenic yeast Candida albicans, which forms white colonies on this medium. c | The fungus is a basidiomycete, able to produce filaments when two opposite mating partners (KN99 MAT and Bt63 MATa) are co-cultured, producing four chains of haploid basidiospores from a basidium (inset).
Reprinted by permission from Macmillan Publishers Ltd: [Nature Reviews Microbiology 3, 753-764] (Idnurm, 2005), copyright (2005)