Potato Dextrose Agar (PDA) – Preparation, Principle, Composition and Uses

What is a Potato dextrose agar?

It is a special type of agar used to cultivate fungi. It is a general purpose medium used to cultivate yeast and mold. To inhibit growth a certain type of acid or antibiotic is used. It is used for different purposes such as:

• Used to detect the presence of mold and yeast in the prepared foods and dairy products.
• Used to cultivate yeast and mold from a clinical specimen.
• PDA, when mixed with tartaric acid, is useful for microbial examination of dairy and food products.
• PDA, when used with chloramphenicol is used for selective cultivation of fungi from a mixed sample.
• PDA, when mixed with chlortetracycline is helpful in the microbial enumeration of mold and yeast from cosmetic products.
• Potato Dextrose Agar with Chlortetracycline is recommended for the microbial enumeration of yeast and mold from cosmetics.
• It aids in the cultivation and differentiation of pathogenic and non-pathogenic fungi. ^{(1, 2, 3, 4, and 5)}

Image 1: The image on the left shows A. flavus growth on potato dextrose agar while the one on the right is P.chrysogenum.

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Image 2: The photo above is the standard potato dextrose agar plate.

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What is the principle of potato dextrose agar?

It consists of dextrose and dehydrated potato infusion, which encourage luxuriant fungal growth. The solidifying agent is agar. To inhibit the growth of bacteria, the pH level of the medium is lowered using a specific amount of 10% sterile tartaric acid.

To inhibit the overgrowth of competing microorganisms from the mixed specimen, a selective agent is used in the form of chloramphenicol. It inhibits overgrowth and at the same time permit the isolation of fungi. The acidified medium should not be reheated as doing so will hydrolyze the agar making the agar fail to solidify. ^{(5, 6)}

What are the components of potato dextrose agar?

• Dextrose
• Potato extract
• Agar
• Supplement can be added in the form of tartaric acid, chlortetracycline, and chloramphenicol. ^{(5, 6, and 7)} 

How do you make potato dextrose agar media?

1. Potato infusion – an unpeeled and sliced potatoes should bring to boil in a liter of distilled water for about 30 minutes.
2. Using a cheesecloth, filter the potato infusion and save the effluent.
3. Mix the infusion with agar, dextrose, and water and bring to boil to dissolve.
4. Autoclave at a temperature of 121 degrees Celsius for 15 minutes.
5. Dispense about 25 ml into the sterile petri dish.
6. The final pH is 5.6 ± 0.2.
7. If you are going to use a commercial medium powder, you should add 39 grams of potato dextrose agar powder to a liter of distilled water. Bring to boil and mix continuously to dissolve the powder. Autoclave at the same temperature and timeframe.
8. If supplements are added, they should be in a controlled amount – chlortetracycline is 40 mg, chloramphenicol is 25 mg, and tartaric acid is 1.4 mg.
9. To process the specimen, stream the specimen being studied on the medium using a sterile inoculating loop.
10. The plate should be incubated at 25 to 30 degree Celsius in an inverted position.
11. The culture should be examined every week for fungal growth. The culture should be kept for at least six weeks before a negative result is made. ^{(1, 5, 9, and 10)}

Image 3:An Aspergillus growth on potato dextrose agar.

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Characteristics of colony on potato dextrose agar.

• Yellow-green spores that appear to be powdery are visible on the upper surface while the lower surface is reddish-gold. – Aspergillus flavus.
• The upper surface appears olive-green with a sterile white margin. The lower surface appears wrinkled with a distinct orange to red color. – Penicillium chrysogenum.
• The colony appears thick and velvety. It appears cream white on the surface and radially furrowed on the lower surface. – A candidus.
• The colony appears velvety with white and black spores on the surface. The lower surface is yellow and heavily furrowed. – A niger.
• The colony is dirty white on the surface with yellowish pores at the center. It looks velvety and the lower surface has a distinct orange to a chocolate color. – A sulphureus.
• It has a floccose texture with a surface color of white to orange-cream with green. The lower surface looks bright orange and heavily wrinkled. – A versicolor.
• The surface color is dark green with a velvety texture. The lower surface is colorless to cream in color with a shallow center and a furrowed raised margin. – Penicillium corylophilum.
• It has a velvety texture with a dark green surface. The lower surface appears yellow and radially furrowed. – P. expansum.
• The colony has a floccose texture and appears pink on the surface. The lower surface is magenta-red to violet. – Fusarium oxysporum. ^{(2, 4, 7, and 10)}

Is potato dextrose agar selective or differential?

A potato dextrose agar is both a selective medium. However, there are a few limitations. For complete identification, a biochemical, molecular, mass spectrometry, and immunological testing should be done on colonies from pure culture first.

References

1. https://microbiologyinfo.com/potato-dextrose-agar-pda-principle-uses-composition-procedure-and-colony-characteristics/
2. https://microbeonline.com/potato-dextrose-agar-pda-principle-composition-colony-characteristics/
3. https://www.sciencedirect.com/topics/immunology-and-microbiology/potato-dextrose-agar
4. https://www.mycrobe.org/blog/2018/7/6/potato-dextrose-agar-pda
5. https://foodsafety.neogen.com/en/ncm-potato-dextrose-agar
6. https://microbenotes.com/potato-dextrose-agar-pda/
7. https://catalog.hardydiagnostics.com/cp_prod/Content/hugo/PotatoDextroseAgar.htm
8. https://www.microbiologyresearch.org/docserver/fulltext/micro/60/2/mic-60-2-273.pdf?expires=1571157564&id=id&accname=guest&checksum=2B5C1821A5D5E7C02DCEFFD9A2D12013
9. https://www.hunker.com/13406994/how-to-make-potato-dextrose-agar
10. https://www.usbio.net/media/P5200