Cryptococcus neoformans is an encapsulated yeast that can cause both pulmonary infections and meningitis, especially in immunocompromised hosts.
- Upon observing mucoid appearing colonies on a primary isolation culture medium, perform a rapid urease test and set up a cornmeal agar preparation.
- If the urease test is rapidly positive, observe the cornmeal agar morphology. If pseudohyphae are absent, and particularly if the yeast cells are spherical, irregular in size and widely separated (presence of capsular material), inoculate the surface of a bird seed agar plate with a small portion of the unknown colony.
- C neoformans colonies appear smooth and have a distinct reddish-brown pigmentation on the bird seed agar, as shown in the image on the right. The active ingredient in bird seed (Guizotia abyssinica) agar is caffeic acid, which is extracted and placed in an agar containing 1% glucose. Of the cryptococci, and other species of yeasts, Cryptococcus neoformans selectively produces the enzyme phenoloxidase, which oxidizes the caffeic acid in the medium to melanin, producing the red-brown pigmentation.
Other yeast species that also do not produce pseudohyphae and would not be ruled out after observation on cornmeal agar include Cryptococcus laurentii and Saccharomyces cerevisiae. However, these organisms do not possess phenoloxidase activity and therefore remain non-pigmented when grown on bird seed agar.