Potato Dextrose Agar (PDA) is a widely used medium in microbiology for the identification, cultivation, and enumeration of yeasts and molds from foodstuffs and other materials. The medium’s formulation adheres to the guidelines of the American Public Health Association and the Pharmacopoeial harmonized method for Aspergillus brasiliensis test strains. This medium is pivotal in various microbiological examinations, including quality control in the food industry and clinical diagnostics.
Composition and Principle
The typical formula for Potato Dextrose Agar consists of Potato Infusion (from 200 g of potatoes), Dextrose, and Agar. The Potato Infusion and Dextrose offer an excellent growth environment, encouraging mold sporulation and pigment production, while Agar serves as the solidifying agent. The final pH of the medium is approximately 5.6 ± 0.2 at 25°C, creating an optimal acidic environment for fungal growth.
To prepare the medium, 42 g of the dehydrated powder is suspended in 1 liter of distilled or deionized water. The solution is heated to boiling, with frequent shaking, until fully dissolved, and then sterilized in an autoclave at 121°C for 15 minutes. For fungal counts, it’s sometimes desirable to suppress bacterial growth by adding 1 ml of lactic acid 10% to each 100 ml of sterilized medium at 50°C, acidifying the pH to approximately 3.5.
The medium can be inoculated using the pour-plate method or spread/streak technique. Incubation is carried out aerobically at 20-25°C for 5-7 days or at 25-30°C for 18-48 hours. Tubed slants, primarily used for the cultivation and maintenance of pure cultures, should be inoculated with an inoculating loop and incubated under the same conditions as plated media.
The medium is examined for fungal growth and colony morphology. The growth characteristics of fungi on PDA are often distinctive, aiding in the preliminary identification of various yeast and mold species.
The dehydrated medium appears as a free-flowing, homogeneous, light beige powder, while the prepared medium is slightly opalescent and light amber.
The dehydrated medium is highly hygroscopic and should be stored at 10-30°C in a dry environment in its original container, tightly closed. Prepared bottles, tubes, and plates should be stored at 10-25°C, away from light.
Shelf Life and Quality Control
The shelf life of the dehydrated medium is 4 years, and for medium in bottles, it’s 2 years. Quality control involves inoculation with microbial strains like Aspergillus brasiliensis (ATCC 16404), Candida albicans (ATCC 10231), and Saccharomyces cerevisiae (ATCC 9763) with expected good growth results.
Warnings and Precautions
Although not classified as hazardous, it’s recommended to consult the safety data sheet for correct use. The product is intended for professional use only and must be used by properly trained operators.
Disposal of Waste
Disposal of waste materials must be carried out according to national and local regulations.
In conclusion, Potato Dextrose Agar is an essential medium in microbiological laboratories for the cultivation and enumeration of fungi. Its specific composition, ease of preparation, and ability to provide a conducive environment for fungal growth make it an indispensable tool in food safety testing, environmental monitoring, and clinical diagnostics.