Bird Seed Agar- Composition, Principle, Preparation, Results, Uses, Limitations


Bird seed agar is a type of culture medium that is used in microbiology to isolate and identify a specific fungus called Cryptococcus neoformans. This fungus can cause a serious infection called cryptococcosis, which affects the lungs and the central nervous system, especially in people with weakened immune systems. Cryptococcosis is one of the most common opportunistic infections in people living with HIV/AIDS.

Bird seed agar was developed by Staib in 1962, who used ground seeds of Guizotia abyssinica, also known as niger seeds or ramtil seeds, as the main ingredient. These seeds contain a compound called caffeic acid, which reacts with an enzyme produced by C. neoformans called phenoloxidase. This reaction leads to the formation of a brown pigment called melanin, which is incorporated into the cell wall of the fungus. The melanin production gives C. neoformans a distinctive appearance on bird seed agar, making it easy to differentiate from other fungi that may grow on the same medium.

Bird seed agar also contains other ingredients such as creatinine, glucose, and antibiotics that provide nutrients and selective conditions for the growth of C. neoformans. Creatinine enhances the melanization of some strains of C. neoformans, while glucose serves as an energy source. Antibiotics such as penicillin and gentamicin inhibit the growth of bacteria and other molds that may contaminate the samples or overgrow the slow-growing C. neoformans.

Bird seed agar is a simple, inexpensive, and reliable method for the diagnosis of cryptococcosis. It can be used to culture specimens from various sources such as cerebrospinal fluid, blood, urine, sputum, skin lesions, and environmental samples. It can also be used to test the susceptibility of C. neoformans to antifungal drugs.