Potato dextrose agar media is primarily exercised for the growth and isolation of different yeasts and moulds. It can be practised as both liquid broth and solid culture media. PDA media functions as an ordinary growth medium for the yeasts and moulds, but it can be made selective by the addition of selective agents like tartaric acid, chloramphenicol etc.
PDA media comprises potato infusion and dextrose sugar that gives a nutrient base and the carbohydrate source, respectively for the fungal growth.
Content: Potato Dextrose Agar
Principle of Potato Dextrose Agar Media
Potato dextrose agar media principally works as the ordinary or selective medium, which determines the growth or presence of different yeasts and fungi from the different samples that are important for the sanitary purpose, especially for the food products.
It can be supplemented with selective agents like acids and antibiotics (chloramphenicol, tartaric acid and chlortetracycline) that restricts the bacterial growth that may interfere with the fungal growth. Potato dextrose agar media is clinically significant for the culturing, isolation and maintenance of fungi (like yeasts and moulds).
In dehydrated form: The potato dextrose broth appears as a homogenous solution and light amber in colour.
The ingredients of the Potato dextrose agar media in 1 L of distilled water, are as follows:
- Potato: 200 g
- Dextrose: 20 g
- Agar: 20 g
Besides, Potato Dextrose Agar is sometimes supplemented with some selective agents, which quantity for the 1 L of deionized water are as follows:
- Chlortetracycline: 40.0 mg
- Chloramphenicol: 25.0 mg
- Tartaric acid: 1.4 g
Role of media components
The potato infused in PDA medium supplies the nutrient base for the development of yeasts and moulds. Dextrose is a fermentable carbohydrate that supplies a sole C-source to promote fungal growth. Agar acts as a solidifying agent of the media. The selective antibiotics and organic acids can be added to inhibit bacterial growth.
TA (Tartaric acid) added in the PDA medium to enumerate the presence of microbes in food and dairy products. Sometimes, antibiotic like chlortetracycline is added as a selective agent that enumerate the microbial population from cosmetics. Chloramphenicol is also an antibiotic that is commonly used to cultivate fungi from the mixed samples.
Preparation of Potato Dextrose Agar Media
The preparation of potato dextrose agar media follows the given protocol:
To infuse 200 grams of potato in the PDA media, we need to follow the following steps:
- First, wash the potatoes, and followed by peeling and weighing, cut the potatoes into small pieces.
- Then, cook the chopped potatoes in 500 ml of distilled water for 20 minutes in a pressure cooker.
- Collect the potato extract into the conical flask through a muslin cloth or net filter.
Melting of solid agar
- After this step, 20 grams of agar is added into the pressure cooker along with 500 ml of distilled water for about 30 minutes.
Mixing of media components
- Weigh 20 grams of dextrose accurately and add into a sterilized conical flask having potato extract.
- Finally, thoroughly mix the molten agar with the potato dextrose mixture and make the volume to 1 litre with distilled water.
- Then put a cotton plug into the mouth of the conical flask to avoid any contamination.
Autoclaving or sterilization
- After getting a homogenous solution, place the flask and sterilize the contents in an autoclave at 15 lbs pressure for 20 minutes.
- Then, take out the flask from the autoclave directly to the laminar airflow chamber for the plating and the remaining media can be kept in a refrigerator for further use.
Checking of pH
Before plating, the pH of the media can be checked by dipping one end of a small piece of litmus paper into the medium. Then take out the litmus paper and compare the colour formed accordingly with the colour chart of the litmus paper book.
- In case the pH is above 7, we need to add a few drops of 0.1N HCl to make the pH of the media into neutral.
- In case the pH is below 7, we need to add a few drops of 0.1 N NaOH to make the pH of the media to 7 or neutral.
Ideally, the PDA media must be kept under refrigeration (at 2-8 degrees Celsius). If there is cracking, discolouration or any other signs of contamination, then the media must be discarded. The media should be kept away from the direct light.
The result can be indicated by the growth of fungi in the culture media.
Yeasts: These develop as a creamy white colony.
- Potato Dextrose Agar detects the presence of fungal biomass in the dairy and prepared food products.
- Potato dextrose agar with tartaric acid examines microbes attacking food and dairy products.
- PDA media with chlortetracycline enumerates yeast and mould counts from cosmetics.
- Potato dextrose agar with chloramphenicol selectively cultivates fungi from the mixed samples.
PDA media only detects the presence or absence of multicellular fungi, but the isolated fungi require biochemical, immunological, molecular, or mass spectrometry testing for the confirm identification.