Mycology

C neoformans has 2 varieties—neoformans and gattii. The species has 4 serotypes based on antigenic specificity of the capsular polysaccharide; these include serotypes A and D (C neoformans var neoformans) and serotypes B and C (C neoformans var gattii). The C neoformans var neoformans is the most common variety in the United States and other temperate climates throughout the world and is found in aged pigeon droppings. C neoformans var gattii develops in tropical and subtropical climates and is not associated with birds, but it grows in the litter around certain species of eucalyptus trees (ie, Eucalyptus camaldulensis, Eucalyptus tereticornis).

Worldwide, C neoformans var neoformans serotype A causes most cryptococcal infections in immunocompromised patients, including patients infected with HIV. For unknown reasons, C neoformans var gattii rarely infects persons with HIV infection and other immunosuppressed patients. Patients infected with C neoformans var gattii are usually immunocompetent, respond slowly to treatment, and are at risk for developing intracerebral mass lesions (eg, cryptococcomas).

C neoformans reproduces by budding and forms round yeastlike cells that are 3-6 ľm in diameter. Within the host and in certain culture media, a large polysaccharide capsule surrounds each cell. C neoformans forms smooth, convex, yellow or tan colonies on solid media at 20-37°C (68-98.6°F). This fungus is identified based on its microscopic appearance, biochemical test results, and ability to grow at 37°C (98.6°F); most nonpathogenic Cryptococcus strains do not grow at this temperature. In addition, C neoformans does not assimilate lactose and nitrates or produce pseudomycelia on cornmeal or rice-Tween agar.

Most strains of C neoformans can use creatinine as a nitrogen source, which may partially explain the growth of the organism in creatinine-rich avian feces. Another useful biochemical characteristic of C neoformans, which distinguishes it from nonpathogenic strains, is its ability to produce melanin. The fungal enzyme phenol oxidase acts on certain substrates (eg, dihydroxyphenylalanine, caffeic acid) to produce melanin.

C neoformans var gattii contains genotypes VGI and the more commonly identified VGIIa and VDIIb. Cryptococcus species can reproduce via same-sex mating, and VGIIa may have arisen from the same-sex mating of VGIIb and another strain that has yet to be identified.

In 1976, Kwon-Chung described the perfect (ie, sexual, teleomorphic) form of C neoformans, which was named Filobasidiella neoformans. Prior to the identification of F neoformans, which is mycelial, C neoformans was considered monomorphic yeast. F neoformans var neoformans results from the mating of suitable strains of serotypes A and D. The perfect state of C neoformans var gattii is Filobasidiella bacillisporus and results from the mating of serotypes B and C. Some strains of A and D can mate with strains of B and C.