Potato dextrose agar is a fungi growth media recommended for the isolation and counting of yeasts and moulds from dairy products and other foodstuffs.
It is also used for stimulating sporulation,maintaining stock cultures of certain dermatophytes and for the differentiation of typical varieties of dermatophytes on the basis of pigment production.
Principle of Potato Dextrose Agar
- Potato infusion and dextrose promote abundant fungal growth.
- Adjustment of the pH value of the medium by tartaric acid to 3.5, inhibits bacterial growth.
- Heating of the medium after acidification should be avoided as it may hydrolyse the agar, which may prevent the agar from solidifying.
Composition of Potato Dextrose Agar
The following ingredients are used for the composition potato dextrose agar
|Ingredients||Grams / Litre|
|Potatoes, infusion from||200.000|
|Final pH at temperature 25°C||5.6±0.2|
How to Prepare Potato Dextrose Agar
- Start by suspending 39 grams in 1000 ml distilled water.
- Now heat to boiling in order to dissolve the medium completely.
- Sterilize the medium by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
- Then mix well before dispensing. In specific work, when pH 3.5 is required, acidify the medium with sterile 10% tartaric acid or lactic acid.
- The amount of acid required for 100 ml of sterile, cooled medium is approximately 1 ml.
- Take note not to heat the medium after addition of the acid.
NB: It is also advisable to incubate a petri dish for 24 hour for quality control in order to avoid wastage of resources and ease early detection of contamination after of the batch of culture media prepared.
Appearance of Medium after preparation
It appears light amber coloured clear to slightly opalescent gel forms when poured in Petri dishes
Type of Inoculums
- dairy products
- other foodstuffs
- Biological specimens
Plate reading and Colonies morphology
These cultural characteristics are observed after incubation of the inoculated medium at a temperature of 20-25 °C for 2-5 days.
|Organism||Growth||Incubation temperature||Incubation period|
|Candida albicans||luxuriant||20 -25 °C||2 -3 days|
|Aspergillus brasiliensis||luxuriant||20 -25 °C||5 -7 days|
|Aspergillus fumigatus||luxuriant||20 -25 °C||5 -7 days|
|Saccharomyces cerevisiae||luxuriant||30 -35 °C||2 -5 days|
|Rhodotorula mucilaginosa||luxuriant||20 -25 °C||3 -5 days|
|Geotrichum candidum||good- luxuriant||25 -30 °C||3 -5 days|
|Penicillium commune||fair -good||25 -30 °C||3 -5 days|
|Trichophyton ajelloi||fair-good||25 -30 °C||3 -7 d|
|Fusarium solani||luxuriant||20 -25 °C||3 -5 d|
- Downes F. P. and Ito K., (Eds.), 2001, Compendium of Methods for the Microbiological Examination of Foods, 4th Ed., APHA, Washington, D.C.
- FDA Bacteriological Analytical Manual, 2005, 18th Ed., AOAC, Washington, DC.
- Wehr H. M. and Frank J. H., 2004, Standard Methods for the Microbiological Examination of Dairy Products, 17th Ed., APHA Inc., Washington, D.C.
- MacFaddin J. F., 1985, Media for the Isolation-Cultivation-Identification-Maintenance of Medical Bacteria, Vol.1, Williams and Wilkins, Baltimore
- The United States Pharmacopoeia, 2016, The United States Pharmacopoeial Convention. Rockville, MD.