Cryptococcus gattii is an environmental encapsulated yeast, associated with trees/decaying organic matter. (C. neoformans yeast is associated with pigeon droppings.) This yeast has been around since the 1960s but was not identified as a separate strain until 2004.6
Several phenotypically indistinguishable species comprise the Cryptococcus gattii species complex; several outbreaks have been identified in the past few years. The inhalation of the basidiospores may cause pneumonia and meningitis in compromised and non-compromised patients.
According to Westblade7,8:
Characteristics
- Cryptococcal antigen is negative
- Rapid growth (with 5 days); cream color colonies
- Spherical cells with narrow-based budding
- Urease-positive, melanin-positive
- Brown-colored colonies on birdseed agar
- Grows on CGB (L-canavanine-glycine-bromothymol blue) agar, turning agar from green/yellow to a cobalt blue color
- Cannot distinguish between C. neoformans and C. gattii in tissue. Both produce capsules when stained with India Ink.
DiagnosisSerum and CSF for LA (latex antigen) and LFA (lateral flow immunoassay) detect the capule polysaccharide. However, a diagnosis cannot be made between C. neoformans and C. gattii.
Automated systems frequently misidentify C. gattii as C. neoformans.
Important! Cryptococcus neoformans and Cryptococcus gattii can only be differentiated by:- MALDI-TOF MS
- Nucleic acid methods
- CGB agar: Test only melanin-positive yeasts (birdseed agar or caffeic acid disk positive); C. gattii grows and produces a color change from yellow/green to cobalt blue; neoformans does not grow (and if it does, it does not produce a color change).
Testing note: If Cryptococcosis is suspected (but LA and LFA tests are negative), dilute the specimen and retest. This scenario is suspect of postzone effect (not prozone).
Reporting note: If identification of C. gattii is determined by definitive testing, report as: "Cryptococcus gattii species complex".
8. Westblade LF, Burd EM, Lockhart SR, Procop GW. Larone's Medically Important Fungi: A Guide to Identification, 7th Edition. ASM Press; August 2023.
