Aspergillus niger (A. niger) is an extremely common fungus, found in soil and decaying vegetation all over the world. It is used in the food industry to produce a number of enzymes and acids for use in processed foods. It also has a traditional use in the fermentation of Pu-erh tea. Aspergillus niger is sometimes confused with Stachybotrys fungi, known as "black mold" and associated with indoor health problems, but A. niger is rarely associated with human health problems and is not considered a significant pathogen.
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Things you need
- Aspergillus niger stock culture
- Dehydrated potato dextrose agar
- Glass rod
- Autoclave (optional)
- Sterile petri plates
Obtain an Aspergillus niger culture. The U.S. Department of Agriculture's Agricultural Research Service Culture Collection maintains several strains of Aspergillus niger and distributes them for research purposes at no charge (see Resources).
Combine 39 grams of dehydrated potato dextrose agar and 1 litre of water in a clean beaker. Heat the mixture until the agar is completely dissolved, stirring occasionally with a sterile glass rod. If an autoclave is available, sterilise the mixture for 15 minutes at 15 PSI.
Remove from heat. When the agar has cooled to approximately 45 degrees C, pour it into sterile petri plates and allow to set. Once the plates have set, they can be used or refrigerated for future use. If refrigerating, store the prepared petri plates upside down with the agar on the upper half of the dish. This prevents condensation from dripping down and contaminating the growing surface.
Inoculate the agar with the Aspergillus by wiping a swab containing the culture across the surface of the agar. Store the inoculated petri plate upside down at 25 degrees C with the agar on the upper surface. A substantial colony of fungus will take 5 to 7 days to develop.
Tips and warnings
- Potato dextrose agar is readily available in dehydrated form, but you can also make your own. Boil 300 grams of clean, sliced, unpeeled potatoes for 30 minutes. Strain the resulting potato broth through cheesecloth and add distilled water to produce a total volume of 1 litre of broth. Add 20 grams each of dextrose and agar powder and heat as described above. Already-prepared potato dextrose agar petri plates are also available if lab facilities are limited, but prepared plates are more expensive.
- Most strains of Aspergillus niger, especially those used in the food industry, have a history of safe use. However, a few specific strains may elicit allergic responses from some people. Be aware of the history of the specific strain you're working with.
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