An apparently immunocompetent man with heavy occupational exposure to eucalyptus trees develops cryptococcal meningitis. The CSF isolate is subcultured on canavanine-glycine-bromothymol blue (CGB) agar, and the agar turns bright blue around the growth. This result identifies the organism as:

• A Cryptococcus neoformans var. grubii
• B Cryptococcus laurentii
• C Cryptococcus albidus
• D Cryptococcus gattii ✓

Explanation

On CGB agar, Cryptococcus gattii assimilates glycine and uses canavanine as a nitrogen source, alkalinising the medium so bromothymol blue turns blue. C. neoformans varieties grubii and neoformans cannot utilise canavanine-glycine and leave the agar unchanged. This distinction matters clinically because gattii affects immunocompetent hosts, is classically linked to eucalyptus and certain trees, and forms large toruloid masses with a poorer response to flucytosine-based regimens.

Reference: Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.