Sabouraud dextrose agar containing cycloheximide and chloramphenicol is inoculated for primary isolation from a respiratory specimen suspected to contain pathogenic fungi. Growth of which organism will be suppressed by the cycloheximide in this medium?

• A Histoplasma capsulatum
• B Sporothrix schenckii
• C Cryptococcus neoformans ✓
• D Microsporum canis

Explanation

Cycloheximide suppresses rapidly growing saprophytic moulds while permitting slow-growing pathogens such as Histoplasma, Sporothrix and dermatophytes, which is exactly why it is added to primary isolation media. However, Cryptococcus neoformans is inherently susceptible to cycloheximide and will fail to grow on such plates. This is the reason birdseed (niger seed) agar, which lacks cycloheximide, is preferred when Cryptococcus isolation is intended, and it kills the tempting distractor Histoplasma, which grows well on cycloheximide-containing media.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 12th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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