Negro seed agar
The negro seed agar (also Cryptococcus agar according to Staib or Staib agar) is a microbiological selective and differential medium for cultivating the disease-causing fungus Cryptococcus neoformans . As the most important component it contains negro seeds, the seeds of the ramtill herb ( Guizotia abyssinica ).
Friedrich Staib developed this nutrient medium in 1962 from the observation that negro seed is often found in feed mixes for birds and that the pathogen Cryptococcus neoformans is often found in bird droppings . The negro seed extract used contains substrates which a phenol oxidase from the fungus which is located in the cell wall can enzymatically convert it, creating brown-black melanin and melanin-like pigments. The addition of 1% creatinine to the agar promotes pigment synthesis, about 1% glucoseand an acidic pH of around 5.5 supports fungal growth. The pathogen can be recognized selectively by the brown-blackish to beige color of the colonies. Cryptococcus albidus is able to grow on the culture medium, but it lacks the ability to produce pigment.
The addition of penicillin and streptomycin suppresses the growth of bacteria and makes the negro seed agar largely selective. Due to the favorable nutrient supply, it is highly sensitive to C. neofomans . The agar is incubated at 25 ° C and checked for the first time after 48 hours; it should be incubated and observed for a further two weeks.
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- ^ F. Staib: Cryptococcus neoformans and Guizotia abyssinica (Syn. G. oleifera, DC). Color reaction for Cryptococcus neoformans . Zeitschrift für Hygiene (1962) 148: pp. 466-475
Web links
- Diagnostics of C. neoformans with an image of a black seed agar ( bird seed agar )