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.
Bird seed agar is a selective and differential medium that contains the following ingredients:
- Guizotia abyssinica seeds: These are also known as niger seeds or ramtil seeds. They are ground and boiled to extract their oil and other components. The extract contains caffeic acid, which is a substrate for the enzyme phenoloxidase produced by Cryptococcus neoformans. Phenoloxidase catalyzes the oxidation of caffeic acid to melanin, which gives a brown color to the colonies of C. neoformans.
- Creatinine: This is a nitrogenous waste product that enhances the melanization of some strains of C. neoformans. It also serves as a nitrogen source for the growth of the fungus.
- Dextrose: This is a simple sugar that provides energy for the metabolism of the fungus.
- Agar: This is a polysaccharide derived from seaweed that solidifies the medium and provides a surface for the growth of the fungus.
- Antibiotics: These are added to inhibit the growth of bacteria and other fungi that may contaminate the samples or overgrow C. neoformans. Penicillin G and gentamicin are commonly used antibiotics in bird seed agar. Penicillin G inhibits the cell wall synthesis of gram-positive bacteria, while gentamicin interferes with the protein synthesis of gram-negative bacteria and some molds.
The final pH of bird seed agar is 6.5 +/- 0.3 at 25ºC. The medium is sterilized by autoclaving at 110°C for 15-20 minutes and then cooled to 48°C before adding the antibiotics. The medium is then poured into petri dishes and allowed to solidify.
Bird seed agar is a selective and differential medium that allows the isolation and identification of Cryptococcus neoformans, a pathogenic yeast that can cause cryptococcosis, a life-threatening infection of the central nervous system. Cryptococcus neoformans has a unique ability to produce melanin, a dark pigment that protects it from environmental stress and host immune responses. Melanin is synthesized by the enzyme phenoloxidase, which catalyzes the oxidation of phenolic compounds to quinones.
Bird seed agar contains Guizotia abyssinica seeds, also known as niger seeds, which are rich in caffeic acid, a phenolic compound that serves as a substrate for phenoloxidase. When Cryptococcus neoformans grows on bird seed agar, it produces phenoloxidase and converts caffeic acid to melanin, which accumulates in the cell wall and gives the colonies a tan to reddish-brown color. This color change is a distinctive characteristic of Cryptococcus neoformans and helps to differentiate it from other yeasts and fungi that may grow on the medium.
Bird seed agar also contains creatinine and glucose, which provide nutrients for the growth of Cryptococcus neoformans. Creatinine enhances the melanization of some strains of Cryptococcus neoformans by increasing the availability of ammonia, which is required for phenoloxidase activity. Glucose is the main energy source for the yeast cells.
Bird seed agar is supplemented with antibiotics, such as penicillin G and gentamicin, to inhibit the growth of bacteria and rapidly growing molds that may contaminate the samples or overgrow the slow-growing Cryptococcus neoformans. Agar is the solidifying agent that allows the formation of discrete colonies on the surface of the medium.
To prepare bird seed agar, you will need the following ingredients and equipment:
- Guizotia abyssinica seeds (also known as niger seeds or nyjer seeds)
- Distilled water
- Penicillin G
- A blender or a mortar and pestle
- A measuring scale
- A measuring cylinder
- A saucepan
- A filter paper and a funnel
- A flask
- An autoclave
- A thermometer
- Petri dishes
The steps for preparing bird seed agar are as follows:
- Grind the Guizotia abyssinica seeds using a blender or a mortar and pestle until they are finely powdered. You will need about 40 grams of ground seeds for 1000 ml of distilled water.
- Transfer the ground seeds to a saucepan and add 1000 ml of distilled water. Bring the mixture to a boil and simmer for 30 minutes, stirring occasionally.
- Filter the mixture through a filter paper and a funnel into a flask. Adjust the volume to 1000 ml by adding more distilled water if needed.
- Add 10 grams of dextrose, 1 gram of creatinine, and 15 grams of agar to the filtrate and dissolve them by heating and stirring. The final pH of the medium should be around 6.5.
- Sterilize the medium by autoclaving at 110°C for 15 to 20 minutes.
- Cool the medium to 48°C and add 0.5 ml of penicillin G (20 units/ml) and 0.5 ml of gentamicin (40 mg/ml) to each 500 ml of bird seed agar. These antibiotics will inhibit the growth of bacteria and other fungi that may contaminate the samples.
- Mix the medium gently and pour it into sterile petri dishes. Allow the medium to solidify and store it in a refrigerator until use.
You have now prepared bird seed agar that can be used for isolating and differentiating Cryptococcus neoformans from other yeasts.
The growth and pigmentation of colonies on bird seed agar can be used to identify and differentiate Cryptococcus neoformans from other yeasts and fungi. C. neoformans is a pathogenic yeast that can cause cryptococcosis, a life-threatening infection of the central nervous system, especially in immunocompromised patients.
C. neoformans produces a brown to black pigment called melanin when it grows on bird seed agar. This is because the medium contains caffeic acid, which is a substrate for the enzyme phenoloxidase that is secreted by C. neoformans. Phenoloxidase catalyzes the oxidation of caffeic acid to melanin, which accumulates in the cell wall of C. neoformans and gives it a distinctive color.
Other yeasts and fungi may also grow on bird seed agar, but they do not produce melanin or produce it in a different way. For example, other Cryptococcus species may grow as white or cream-colored colonies, or may produce a yellowish pigment that is not derived from caffeic acid. Some molds, such as Aureobasidium, Sporothrix, Wangiella, and Phialophora may produce dark brown colonies, but their pigment is not enzymatic and does not depend on caffeic acid.
To interpret the results of bird seed agar, the following steps can be followed:
- Observe the growth and color of colonies after 24 to 72 hours of incubation at 25°C to 30°C.
- Compare the colonies on bird seed agar with those on another medium, such as Sabouraud dextrose agar (SDA), to check if the pigment is specific to bird seed agar or not.
- Perform additional tests, such as microscopic examination, biochemical tests, or molecular methods, to confirm the identity of C. neoformans or other yeasts and fungi.
The table below summarizes the typical results of bird seed agar for some common yeasts and fungi:
|Organism||Growth on Bird Seed Agar||Pigment on Bird Seed Agar||Growth on SDA||Pigment on SDA|
|C. neoformans||Good||Brown to black||Good||White to cream|
|Other Cryptococcus spp.||Good||White to cream or yellowish||Good||White to cream or yellowish|
|Candida albicans||Poor to none||None||Good||White|
|Candida tropicalis||Poor to none||None||Good||White|
|Aureobasidium pullulans||Good||Dark brown||Good||Dark brown|
|Sporothrix schenckii||Good||Dark brown||Good||White|
|Wangiella dermatitidis||Good||Dark brown||Good||Black|
|Phialophora verrucosa||Good||Dark brown||Good||Olive-brown|
Bird seed agar is a selective and differential medium that is used for the isolation and identification of Cryptococcus neoformans, a pathogenic yeast that can cause cryptococcosis, a life-threatening infection of the central nervous system, lungs, and skin. Cryptococcosis is especially common in immunocompromised individuals, such as those with HIV/AIDS, organ transplant recipients, or cancer patients.
Bird seed agar is useful for detecting C. neoformans from various clinical and environmental samples, such as cerebrospinal fluid, blood, sputum, urine, feces, soil, and pigeon droppings. The medium contains Guizotia abyssinica seeds (also known as niger seeds), which are rich in caffeic acid, a substrate for the enzyme phenoloxidase that is produced by C. neoformans. Phenoloxidase catalyzes the oxidation of caffeic acid to melanin, a dark brown pigment that accumulates in the cell wall of C. neoformans. This results in the formation of tan to reddish-brown colonies on bird seed agar, which can be easily distinguished from other yeasts or fungi that may grow on the medium.
Bird seed agar also contains creatinine, which enhances the melanization of some strains of C. neoformans, and glucose, which provides energy for the growth of the yeast. In addition, bird seed agar contains antibiotics (penicillin G and gentamicin) that inhibit the growth of bacteria and rapidly growing molds that may overgrow or contaminate the medium.
Bird seed agar is a simple and inexpensive method for the presumptive identification of C. neoformans, which can be confirmed by other tests, such as India ink staining, urease test, capsule formation, or molecular methods. Bird seed agar is also useful for studying the phenotypic variation and virulence factors of C. neoformans, such as capsule production, melanin synthesis, and antifungal susceptibility. Bird seed agar is therefore an important tool for the diagnosis and research of cryptococcosis and its causative agent.
Bird seed agar is a useful medium for the isolation and differentiation of Cryptococcus neoformans, but it also has some limitations that should be considered:
- Rare strains of C. neoformans may not produce pigmented colonies on bird seed agar, which can lead to false-negative results. Therefore, other methods such as biochemical tests or molecular identification should be used to confirm the presence or absence of C. neoformans in the samples.
- Specimens heavily contaminated with bacteria may obscure the growth and/or pigmentation of C. neoformans on bird seed agar, which can lead to false-positive or false-negative results. Therefore, specimens should be properly collected, stored and processed to minimize bacterial contamination. Alternatively, other selective media such as Niger seed agar or CHROMagar Cryptococcus can be used to enhance the isolation of C. neoformans from mixed cultures.
- Some other fungi such as Aureobasidium, Sporothrix, Wangiella, and Phialophora may produce dark brown colonies on bird seed agar, which can lead to false-positive results. However, these fungi produce pigmentation due to their natural melanin synthesis, not due to the enzymatic activity of phenoloxidase on caffeic acid. Therefore, the pigmentation of these fungi will not be specific to bird seed agar and will also appear on other media such as Sabouraud dextrose agar. A comparison of the growth and pigmentation of the same isolate on different media can help to differentiate these fungi from C. neoformans.
- Bird seed agar may not support the growth of some rare or emerging species of Cryptococcus such as C. gattii, C. albidus, C. laurentii, and C. uniguttulatus, which can lead to false-negative results. Therefore, other media such as Sabouraud dextrose agar or cornmeal agar should be used to cultivate these species and perform further identification tests.
In summary, bird seed agar is a valuable medium for the selective isolation and differentiation of C. neoformans from other yeasts, but it also has some limitations that require careful interpretation of the results and confirmation by other methods.
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