Introduction of Potato Dextrose Agar (PDA)
Table of Contents
Potato Dextrose Agar ( PDA) is a fungal medium and its ingredients are clear from its name it contains potato ( vegetable), dextrose ( sugar), and agar ( solidifying agent). PDA is recommended for the isolation and enumeration of fungi (yeasts and molds) from water, dairy, other food products, and even clinical specimens ( skin scrappings). The nutritionally rich base, potato infusion encourages mold sporulation and pigment production in some dermatophytes (e.g.Trichophyton rubrum).
Principle of Potato Dextrose Agar (PDA)
Potato dextrose agar (PDA) contains dehydrated Potato infusions and Dextrose. Potato infusion provides a nutrient base for the luxuriant growth of most fungi whereas dextrose serves as a growth stimulant. Agar in the medium acts as the solidifying agent. PDA has been further modified by incorporating agents like tartaric acid, chloramphenicol, and chlortetracycline. The incorporation of tartaric acid (TA) in the medium lowers the pH to 3.5 which inhibits bacterial growth. Chloramphenicol acts as a selective agent to inhibit the bacterial overgrowth of competing microorganisms from mixed specimens while permitting the selective isolation of fungi. The application of chlortetracycline in PDA is for the evaluation of yeast and mold from cosmetic products.
Composition of Potato Dextrose Agar (PDA)
Potato dextrose agar ( PDA) ingredients and their amounts are as follows-
|Ingredients||Gms / Litre|
|Potatoes, infusion form||200.0|
|Distilled water(D/W)||1000 ml|
|Final pH ( at 25°C)||5.6±0.2|
Preparation of Potato Dextrose Agar ( PDA)
- Suspend 39.0 grams of potato dextrose agar ( PDA) in 1 liter of purified/distilled or deionized water.
- Heat to boiling to dissolve the medium completely.
- Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
- After autoclaving, leave for cooling to 45-50°C.
- Note: In specific work, when pH 3.5 is required, acidify the medium with sterile 10% tartaric acid. The amount of acid required for 100 ml of sterile, cooled medium is approximately 1 ml, and do not heat the medium again after the addition of the acid.
- Mix well before dispensing.
- Pour Potato Dextrose Agar into each plate and leave plates on the sterile surface until the agar has solidified.
- Store the plates in a refrigerator at 2-8°C.
Storage and Shelf life of Potato Dextrose Agar (PDA)
- Store at 2-8ºC and away from direct light.
- Media should not be used if there are any signs of deterioration (shrinking, cracking, or discoloration), or contamination.
- The product is light and temperature-sensitive; protects from light, excessive heat, moisture, and freezing.
Test Requirements for PDA
- Test specimens (samples or fungal growth)
- Inoculating loop
- Bunsen burner
- Control strains (Candida albicans ATCC ® 10231 and Trichophyton rubrum
ATCC ® 28188)
Test procedure (specimen/organism inoculation)
- Allow the plates to warm at 37°C or to room temperature, and the agar surface to dry before inoculating.
- Take two plates for one specimen.
- Inoculate and streak the specimen as soon as possible after collection.
- If the specimen to be cultured is on a swab, roll the swab over a small area of the agar surface.
- Streak for isolation with a sterile loop.
- Incubate one plate aerobically at room temperature ( 20-25ºC) and another 35-37ºC for up to 4 weeks ( depending on the type of organisms suspected)
- Examine colony characteristics.
Result and Interpretation of PDA
- Control strains i.e. Candida albicans ATCC ® 10231 ( rapid grower) and Trichophyton rubrum ( slow grower) ATCC ® 28188): Presence of growth
- Presence of fungi in specimen: Presence of growth on PDA
Colony Characteristics of various organisms in PDA
Candida albicans: Growth; smooth white colonies after 24-48 hours
Candida tropicalis on SDA, PDA, CMA, CHROMagar, AFST
Trichophyton rubrum: Growth is seen in 7 days and it may take 3-4 weeks for the red color on the reverse side of the colony to be visible.
Penicillium colony characteristics on Potato Dextrose Agar (PDA)
Aspergillus niger colony morphology on PDA
Aspergillus fumigatus growth on Potato Dextrose Agar (PDA)
Paecilomyces growth on Potato dextrose agar (PDA)
Modification of PDA
- Potato Dextrose Agar with TA (Tartaric Acid): It uses for the microbial examination of food and dairy products.
- Potato Dextrose Agar with Chlortetracycline: It uses is for the microbial enumeration of yeast and mold from cosmetics.
- Potato Dextrose Agar with Chloramphenicol: It uses for the selective cultivation of fungi from mixed samples.
Uses of Potato Dextrose Agar (PDA)
- PDA uses for the detection of fungi from dairy products, prepared foods as well as water samples.
- It also uses for the cultivation of fungi (yeasts and molds) from clinical specimens.
- This medium is also applicable for pigment expression as good sporulation.
- Various modified PDAs and their applications are as follows-PDA TA is used for the microbial examination of food and dairy products while PDA with chlortetracycline uses are for the microbial enumeration of yeast and mold from cosmetics. PDA with chloramphenicol uses for the selective cultivation of fungi from mixed samples.
Keynotes on PDA
- Heating the medium after acidification (incorporation of tartaric acid) should be avoided as it may hydrolyze the agar which can render the agar unable to solidify.
- 4.0gm of potato extract is equivalent to 200 gm of potato infusion.
- The original potato dextrose agar not only supports the growth of fungi but also some acidic bacteria and thus its modifications are preferred.
- The amount of chlortetracycline 40.0 mg, chloramphenicol 25.0 mg, and tartaric acid 1.4 gm can use in 1000 ml of PDA in modified forms additionally.
Aspergillus fumigatus growth on PDA
Further Readings on PDA
- Medical Mycology. Editors: Emmons and Binford, 2nd ed 1970, Publisher Lea and Febiger, Philadelphia.
- Rippon’s JW: Medical Microbiology. The pathogenic fungi and the Pathogenic Actinomycetes. 3rd ed 1988 Publisher WB Saunder co, Philadelphia.
- Clinical Microbiology Procedure Handbook, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
- A Text-Book of Medical Mycology. Editor: Jagdish Chander. Publication Mehata, India.
- Practical Laboratory Mycology. Editors: Koneman E.W. and G.D. Roberts, 3rd ed 1985, Publisher Williams and Wilkins, Baltimore.