Potato Dextrose Agar (PDA)
Introduction of Potato Dextrose Agar
Potato dextrose agar is common microbiological basal growth media made from potato infusion, and dextrose used for the growth of the fungi. PDA is recommended by APHA and FDA for plate counts of yeasts and molds in the examination of foods and dairy products. It is a general purpose medium for yeasts and molds that can be supplemented with acid or antibiotics like Chloramphenicol, Tartaric Acid and Chlortetracycline can be added as selective agents and to inhibit bacterial growth. PDA can be used for growing clinically significant yeast and molds. PDA is also useful for maintaining stock cultures of certain dermatophytes.
Composition of Potato Dextrose Agar
|Potatoes, infusion from||200.0|
Final pH ( at 25°C) 5.6±0.2
Principle of Potato Dextrose Agar
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.
Preparation of Potato Dextrose Agar
- Suspend 39 grams in 1000 ml distilled water. Heat to boiling to dissolve the medium completely.
- Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes. Mix well before dispensing.
- 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 1ml.
- Do not heat the medium after addition of the acid.
Result Interpretation of Potato Dextrose Agar
Yeasts will grow as creamy to white colonies.
Molds will grow as filamentous colonies of various colors.
Colony morphology of some fungi on Potato Dextrose Agar
|Fungi||Texture||Surface colour||Reverse colour||Zonation||Sporulation|
|Aspergillus candidus||Velvety thick||Creamy white||Slightly creamy||Radially furrowed on the reverse||Moderate|
|Aspergillus niger||Velvety||White with typical black spores||yellow||Heavily furrowed on the reverse||Heavy|
|Aspergillus sulphureus||Velvety||Dirty white with yellow spores at the centre||Orange to chocolate colour||Slighty radially furrowed||Moderate|
|Penicillium corylophilum||Velvety||Dark green||Colourless to Creamy||With shallow centre and radially furrowed raised margin||Moderate|
|P. expansum||Velvety||Dark green with clear exudates and distinct sterile white margin||Yellow||Radially furrowed||Heavy|
|Fusarium oxysporum||Floccose||Magenta pink||Magenta-red turning violet||With concentric zones of dark and light reddish colouration||Poor|
Uses of Potato Dextrose Agar
- Potato Dextrose Agar is used for the detection of yeasts and molds in dairy products and prepared foods.
- It may also be used for the cultivation of yeasts and molds from clinical specimens.
- Potato Dextrose Agar with TA (Tartaric Acid) is recommended for the microbial examination of food and dairy products.
- Potato Dextrose Agar with 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.
Limitations of Potato Dextrose Agar
- It is recommended that biochemical, immunological, molecular, or mass spectrometry testing be performed on colonies from pure culture for complete identification.