Cryptococcus

Synonymy: 
Filobasidiella bacillispora; Cryptococcus neoformans var. gattii

RG-2 organism.

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.

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.

Synonymy:
Filobasidiella neoformans; Cryptococcus neoformans var. neoformans.

RG-2 organism.

This species comprises two varieties: C. neoformans var. grubii (serotype A) and C. neoformans var. neoformans (serotype D).

C. neoformans var. grubii:
Has a worldwide distribution, causing 95% of all C. neoformans infections. It has been isolated from various sources in nature and is noted for its association with accumulations of avian guano, especially with pigeon excreta. The fungus has also been isolated from the dung of caged birds including canaries, parrots and budgerigars. Other environmental isolations of C. neoformans var. grubii include rotting vegetables, fruits and fruit juices, wood, dairy products and soil.

C. neoformans var. neoformans:
Has a more restricted distribution with infections being more prevalent in Europe, including France, Italy and Denmark, where it accounts for 30% of isolates. Moreover, C. neoformans var. neoformans infections are more strongly correlated with older patients, the skin, and the use of corticosteroids (Franzot et al. 1999).

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 on direct microscopy. 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 (Staib, 1987).

Canavanine glycine bromothymol blue (CGB) agar:
No growth or colour change.

Creatinine dextrose bromothymol blue thymine (CDBT) agar:
May be used to differentiate C. neoformans var. neoformans and C. neoformans var. grubii. C. neoformans var. neoformans grows as bright red colonies, turning the medium a bright orange after 5 days while no colour change is observed for C. neoformans var. grubii (Irokanulo et al. 1994).

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; does not grow on CGB agar (no colour change).