Cryptococcus neoformans var. gattii
Cryptococcus gattii has two serotypes (B and C) and was reclassified as a separate species from C. neoformans in 2002 (Kwon-Chung et al. 2002). C. gattii generally has a more restricted geographical distribution than C. neoformans, causing human disease in climates ranging from temperate to tropical Australia, Papua New Guinea, parts of Africa, India, Southeast Asia, Mexico, Brazil, Paraguay and Southern California, although recent infections have also been reported from Vancouver Island, Canada and in the Pacific Northwest, USA (Pfaller & Diekema, 2010, Espinel-Ingroff and Kidd, 2015). C. gattii has a specific ecological association with numerous species of Eucalyptus trees, although the Canadian isolates are associated with a range of native non-Eucalyptus species (Kidd et al. 2007). Historically considered a pathogen in immunocompetent hosts, a recent review in Australia noted an increase in C. gattii infections in HIV-negative immunocompromised patients (Chen et al. 2012). Cryptococcosis caused by C. gattii is often associated with large mass lesions (cryptococcomas) in the lung and/or brain (Sorrell, 2001).
Canavanine glycine bromothymol blue (CGB) agar (Kwon-Chung et al. 1982) is the media of choice to differentiate C. gattii from C. neoformans. This simple biotype test is based on the ability of C. gattii isolates to grow in the presence of L-canavanine and to assimilate glycine as a sole carbon source. A heavy inoculum is important.
Cryptococcus gattii turns CGB agar blue within 2-5 days;
Cryptococcus neoformans does not grow on this medium
Culture: Colonies (SDA) cream-coloured smooth, mucoid, yeast-like colonies.
Microscopy: Globose to ovoid budding yeast-like cells 3.0-7.0 x 3.3- 7.9 µm.
India Ink Preparation: Positive - distinct, wide gelatinous capsules are present. Some strains may not produce apparent capsules from culture.
Dalmau Plate Culture: Budding yeast cells only. No pseudohyphae present.
Bird Seed Agar: Colonies turn dark brown in colour as colonies selectively absorb a brown pigment from this media. Colonies are often more mucoid when compared with C. neoformans (Staib, 1987).
Canavanine Glycine Bromothymol Blue (CGB) Agar: Turns blue within 2-5 days.
Key Features: Encapsulated yeast; absence of pseudohyphae; growth at 37C; positive hydrolysis of urea; negative fermentation of sugars and positive assimilation of glucose, maltose, sucrose, galactose, trehalose, raffinose, inositol, cellobiose, rhamnose, arabinose, melezitose and xylose, and negative assimilation of nitrate, lactose, melibiose, erythritol and soluble starch; growth on bird seed (Guizotia abyssinica seed) or caffeic acid agar - colonies turn a dark brown colour; growth on CGB agar turning it blue within 2-5 days.