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Bird Seed Agar: A Selective Medium for Cryptococcus neoformans
Bird Seed Agar (also known as Niger Seed Agar or Staib Agar) is a specialized fungal culture medium primarily used for the selective isolation and identification of Cryptococcus neoformans, a pathogenic yeast that can cause cryptococcosis, particularly in immunocompromised individuals. The medium contains Guizotia abyssinica (niger seed) extract, which serves as a substrate for phenoloxidase, an enzyme produced by C. neoformans. This enzyme converts the seed’s caffeic acid into melanin, resulting in brown to black colonies—a key diagnostic feature that differentiates Cryptococcus from other yeasts.
Composition and Mechanism of Bird Seed Agar
The Bird Seed Agar recipe typically includes:
- Niger seed extract (provides caffeic acid for melanin production)
- Glucose (carbon source for fungal growth)
- Agar (solidifying agent)
- Chloramphenicol or gentamicin (to inhibit bacterial contamination)
Unlike general fungal media like Sabouraud Dextrose Agar (SDA), Bird Seed Agar is highly selective for C. neoformans and C. gattii due to their unique ability to produce melanin. Other yeasts, such as Candida albicans, grow poorly or fail to produce pigment on this medium.
Applications and Interpretation of Results
Bird Seed Agar is commonly used in:
- Clinical microbiology labs to diagnose cryptococcal infections from cerebrospinal fluid (CSF), blood, or respiratory samples.
- Environmental studies to detect Cryptococcus in pigeon droppings or soil.
- Research settings to study melanin production in pathogenic fungi.
Expected Results:
- Cryptococcus neoformans: Dark brown to black colonies (due to melanin).
- Other yeasts (e.g., Candida): Cream-colored or non-pigmented colonies (no melanin production).
- Bacteria: Inhibited by antibiotics in the medium.
Comparison with Other Fungal Media
| Feature | Bird Seed Agar | Sabouraud Dextrose Agar (SDA) | CHROMagar Candida |
|---|---|---|---|
| Selectivity | High (for Cryptococcus) | Low (general fungi) | Moderate (for Candida spp.) |
| Differential Property | Melanin production (black colonies) | None | Color-based species differentiation |
| Common Use | C. neoformans identification | Broad fungal isolation | Candida speciation |
