Potato Dextrose Agar (PDA): Composition, Preparation, Uses, and Fungal Colony Characteristics

A laboratory receives a skin scraping from a patient with a chronic nail infection. Culture on Sabouraud dextrose agar grows a slow-growing white mould after three weeks — but the hyphae are sterile: no conidia, no identifiable sporulation structure. A subculture onto potato dextrose agar produces rich sporulation after one week, revealing the characteristic pencil-shaped macroconidia and the red reverse pigment of Trichophyton rubrum.

Potato dextrose agar was developed specifically for this purpose — stimulating sporulation in fungi that fail to produce identifying structures on nutritionally richer media. It is an essential supplementary medium in dermatophyte identification.

Potato dextrose agar (PDA) is a general-purpose basal medium for identifying, cultivating and enumerating yeast and molds in foods and dairy products. It may also be used to cultivate yeasts and molds from clinical specimens. Since it stimulates sporulation and pigmentation, it also aids in cultivating and differentiating pathogenic and non-pathogenic fungi.

Figure: Fungal colony in PDA
Source: Rachel Brown, University of Florida, Bugwood.org

PDA is also useful for maintaining stock cultures of certain dermatophytes. Certain antibiotics or acids like chloramphenicol, tartaric acid and chlortetracycline can be added as selective agents.

Potato dextrose agar with TA (tartaric acid) is recommended for the microbial examination of food and dairy products. Addition of chlortetracycline is recommended for the microbial enumeration of yeast and mold from cosmetics. Potato dextrose agar with chloramphenicol is recommended for the selective cultivation of fungi from mixed samples.

Principle

Potato dextrose agar (PDA) contains dextrose as a carbohydrate source which serves as a growth stimulant and potato infusion provides a nutrient base for the luxuriant growth of most fungi. Agar is added as the solidifying agent. A specified amount of sterile tartaric acid (10%) may be incorporated to lower the pH of the medium to 3.5 so that bacterial growth is inhibited.

Care should be taken not to reheat the acidified medium; heating in the acid state will hydrolyze the agar which can render the agar unable to solidify.

Composition of PDA

*4.0gm of potato extract is equivalent to 200gm of potato infusion

If supplement added: tartaric acid – 1.4 gm (pH-3.5 +/- 0.3 at 25°C)

Chloramphenicol   – 25 mg ( pH-5.6 +/- 0.2 at 25°C)

Chlortetracycline    –  40 mg

Why potato infusion induces sporulation: The potato infusion in PDA provides a nutritionally restricted environment — lower in amino acids and complex growth factors than Sabouraud agar. Under nutritional stress, fungi are triggered to reproduce sexually or asexually, producing spores (conidia) as a survival mechanism. This is the reverse of what happens on rich media (blood agar, BHI), where organisms grow vegetatively without sporulating. The 2% dextrose provides sufficient carbon for sustained growth, while the restricted nitrogen from potato infusion creates the stress signal that triggers sporulation.

Procedure for Preparation of media

1. Suspend 39 grams of dehydrated media (supplied by commercial suppliers) in 1000 ml of distilled water. Heat to boiling to dissolve the medium completely.
2. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes. Mix well before dispensing.
3. In specific work, when pH 3.5 is required, the medium should be acidified with sterile 10% tartaric acid. The amount of acid required for 100 ml. of sterile, cooled medium is approximately 1 ml. Do not heat the medium after the addition of the acid.
4. To process the specimen, streak the specimen onto the medium with a sterile inoculating loop to obtain isolated colonies.
5. Incubate the plates at 25 – 30°C in an inverted position (agar side up) with increased humidity.
6. Cultures should be examined weekly for fungal growth and held for 4 – 6  weeks before being reported as negative.

Result

Yeasts will grow as creamy to white colonies. Molds will grow as filamentous colonies of various colors.

Typical colony morphology of some fungi

Count the number of colonies and consider the dilution factor (if the test sample was diluted) in determining the yeast and/or mold counts per gram or milliliter of material.

PDA vs Sabouraud Dextrose Agar (SDA)

Practical rule: Use SDA for primary isolation of all fungi. Subculture onto PDA when dermatophyte identification requires better sporulation or pigment development.

Key Exam Facts in One Table

References and further readings

1. Acharya T., Hare J. (2022) Sabouraud Agar and Other Fungal Growth Media. In: Gupta V.K., Tuohy M. (eds) Laboratory Protocols in Fungal Biology. Fungal Biology. Springer, Cham. https://doi.org/10.1007/978-3-030-83749-5_2
2. Larone DH. Larone's Medically Important Fungi: A Guide to Identification. 6th ed. ASM Press; 2018.
3. Chander J. Textbook of Medical Mycology. 4th ed. Jaypee Brothers Medical Publishers; 2018.
4. Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2020.
5. Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 14th ed. Elsevier; 2023.
6. Koneman EW, Allen SD, Janda WM, Schreckenberger PC, Winn WC. Koneman's Color Atlas and Textbook of Diagnostic Microbiology. 6th ed. Lippincott Williams & Wilkins; 2006.