Potato Dextrose Agar (PDA): Principle, Composition, Colony Characteristics

Last updated on June 17th, 2021

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

Fungal colony in PDA Source:Rachel Brown, University of Florida, Bugwood.org
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 that provides a nutrient base for 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

IngredientsGm/L
Dextrose20 g
Potato extract4 g*
Agar15 g

*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

Procedure for Preparation of media

  1. Suspend 39 grams of dehydrated media (supplied by commercial suppliers) in 1000 ml 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 addition of the acid.
  4. For processing of specimen, streak the specimen onto the medium with a sterile inoculating loop in order 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 at least weekly for fungal growth and should be 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

FungiColony Characteristics 
 TextureSurface colourReverse colourZonationSporulation
A.candidusVelvety thickCreamish whiteSlightly creamishRadially furrowed on the reverseModerate
A.nigerVelvetyWhite with typical black sporesYellowHeavily furrowed on the reverseHeavy
A.sulphureusVelvetyDirty white with yellow spores at the centreOrange to chocolate colourSlightly radially furrowedModerate
A. versicolorFloccoseWhite to orange-cream with green spores at the centreBright orangeHeavily wrinkled on reverseModerate
Penicillium corylophilumVelvetyDark greenColourless to CreamishWith shallow centre and radially furrowed raised marginModerate
P. expansumVelvetyDark green with clear exudates and distinct sterile white marginYellowRadially furrowedHeavy
Penicillium sppPowderyOlivaceous green with sterile white marginOrange to red, wrinkledRadially furrowedHeavy
Fusarium oxysporumFloccoseMagenta pinkMagenta-red turning violetWith concentric zones of dark and light reddish colourationPoor

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.

About Nisha Rijal 46 Articles
I am working as Microbiologist in National Public Health Laboratory (NPHL), government national reference laboratory under the Department of health services (DoHS), Nepal. Key areas of my work lies in Bacteriology, especially in Antimicrobial resistance.

7 Comments

  1. Hi,
    I am working to identify a pathogen in the lab. I cultured the samples (leaves) on PDA and there was a fungi growth but some of the plates appears to contain bacteria. Please I want to know if it is possible for bacteria to grow on PDA, if not contamination.
    Thank you.

    • i do think there was the contamination of your plate another posibility is that u did not use tartaric acid to inhibit bacterial growth.

  2. Pls to check bacteria growth, in raising a pure culture of fungi,
    i will like to know the quantity of antibiotic prepared and added to each plate of pda

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