Potato Dextrose Agar (PDA): Introduction, Principle, Composition, Test Procedure, Colony Characteristics and Uses

Potato Dextrose Agar contains dehydrated Potato Infusion and Dextrose. Potato infusion provides a nutrient base for luxuriant growth of most fungi where as Dextrose serves as a growth stimulant. Agar in the medium acts the solidifying agent. PDA has further modified incorporating agents like tartaric acid, Chloramphenicol  and Chlortetracycline. The incorporation of tartaric acid (TA) in the medium lowers the pH to 3.5 which inhibits bacterial growth. Chloramphenicol acts as a selective agent to inhibit bacterial overgrowth of competing microorganisms from mixed specimens, while permitting the selective isolation of fungi. Application of Chlortetracycline in PDA is  for the evaluation of yeast and mold from cosmetic products.

Introduction of Potato Dextrose Agar (PDA)

Potato Dextrose Agar ( PDA) is a fungal medium and its ingredients are clear from its name which contain potato ( vegetable), dextrose ( sugar) and agar ( solidyfing agent). PDA  is recommended for the isolation and enumeration of  fungi (yeasts and moulds from ) from  water, dairy, other food products and  even clinical specimens ( skin scrappings). The nutritionally rich base, potato infusion encourages mold sporulation and pigment production in some dermatophytes (e.g.Trichophyton rubrum).

Principle of Potato Dextrose Agar (PDA)

Potato dextrose agar (PDA) contains dehydrated Potato Infusion and Dextrose. Potato infusion provides a nutrient base for luxuriant growth of most fungi where as dextrose serves as a growth stimulant. Agar in the medium acts the solidifying agent. PDA has further modified incorporating agents like tartaric acid, chloramphenicol  and chlortetracycline. The incorporation of tartaric acid (TA) in the medium lowers the pH to 3.5 which inhibits bacterial growth. Chloramphenicol acts as a selective agent to inhibit bacterial overgrowth of competing microorganisms from mixed specimens, while permitting the selective isolation of fungi. Application of chlortetracycline in PDA is  for the evaluation of yeast and mold from cosmetic products.

Composition of Potato Dextrose Agar (PDA)

Potato dextrose agar ( PDA) ingredients and their amounts are as follows-

Ingredients                        Gms / Litre

  • Potatoes, infusion from:  200.0
  • Dextrose: 20.0
  • Agar: 15.0
  • Distilled water(D/W): 1000 ml
    Final pH ( at 25°C) 5.6±0.2

Preparation of Potato Dextrose Agar ( PDA) 

  1. Suspend 39.0 grams potato dextrose agar ( PDA) in 1 liter purified / distilled or deionized water.
  2. Heat to boiling to dissolve the medium completely.
  3. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
  4. After autoclaving,  leave for cooling to 45-50°C.
  5. Note: In specific work, when pH 3.5 is required, acidify the medium with sterile 10% tartaric acid. The amount of acid required for 100 ml of sterile, cooled
    medium is approximately 1 ml and do not heat the medium  again after addition of the acid.
  6. Mix well before dispensing.
  7. Pour Potato Dextrose Agar into each plate and leave plates on the sterile surface until the agar has solidified.
  8. Store the plates in a refrigerator at 2-8°C.

Storage and Shelf life of Potato Dextrose Agar ( PDA) 

  • Store at 2-8ºC  and away from direct light.
  • Media should not be used if there are any signs of deterioration (shrinking, cracking, or discoloration), contamination.
  • Product is light and temperature sensitive; protect from light, excessive heat, moisture, and freezing.

Test Requirements

  • Test specimens ( samples or fungal growth)
  • Inoculating loop
  • Bunsen burner
  • Incubator
  • Control strains (Candida albicans ATCC ® 10231 and Trichophyton rubrum
    ATCC ® 28188)

Test procedure (specimen/organism inoculation)

  1. Allow the plates to warm at 37°C or  to room temperature, and the agar surface to dry before inoculating.
  2. Take two plates for one specimen.
  3. Inoculate and streak the specimen as soon as possible after collection.
  4. If the specimen to be cultured is on a swab, roll the swab over a small area of the agar surface.
  5. Streak for isolation with a sterile loop.
  6. Incubate one plates aerobically at room  temperature ( 20-25ºC) where as another 35-37ºC up to 4 weeks ( depending on the type of organisms suspected)
  7. Examine colony characteristics.

Result Interpretation  of PDA

  • Control strains i.e. Candida albicans ATCC ® 10231 ( rapid grower)  and Trichophyton rubrum ( slow grower) ATCC ® 28188): Presence of growth
  • Presence of fungi in specimen: Presence of  growth on PDA

Colony Characteristics of various organisms in PDA

Candida albicans: Growth; smooth white colonies after 24-48 hours

Trichophyton rubrum: Growth seen in 7 days and it may take 3-4 weeks for red color on reverse side of colony to be visible.

Modification of PDA

  1. Potato Dextrose Agar with TA (Tartaric Acid): It uses for the microbial examination of food and dairy products.
  2. Potato Dextrose Agar with Chlortetracycline: It uses is for the microbial enumeration of yeast and mold from cosmetics.
  3. Potato Dextrose Agar with Chloramphenicol: It use for the selective cultivation of fungi from mixed samples.

Uses of Potato Dextrose Agar ( PDA) 

  1. PDA uses  for the detection of fungi from dairy products, prepared foods as well as water samples.
  2. It also uses for the cultivation of  fungi (yeasts and molds) from clinical specimens.
  3. This medium is also applicable for pigment expression as well sporulation.
  4. Various modified PDAs  and their applications are as follows-PDA with TA uses for the microbial examination of food and dairy products while PDA with chlortetracycline uses is for the microbial enumeration of yeast and mold from cosmetics. PDA with chloramphenicol uses for the selective cultivation of fungi from mixed samples.

Key Notes on PDA

  1. Heating the medium after acidification (incorporation of tartaric acid) should be avoided as it may hydrolyse the agar which can render the agar unable to solidify.
  2. 4.0gm of potato extract is equivalent to 200 gm of potato infusion.
  3. The original potato dextrose agar not only supports the growth  of fungi but also some acidic bacteria and thus its modifications are preferred.
  4. The amount of chlortetracycline 40.0 mg, chloramphenicol 25.0 mg and tartaric acid 1.4 gm can use in 1000 ml of PDA in modified forms additionally.

Reference

  1. Medical Mycology. Editors:  Emmons and Binford, 2nd ed 1970, Publisher Lea and Febiger, Philadelphia.
  2. Rippon’s JW: Medical Microbiology. The pathogenic fungi and the pathogenic Actinomycetes. 3rd ed 1988 Publisher WB saunder co, Philadelphia.
  3. Clinical Microbiology Procedure Hand book , Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  4. A Text Book of Medical Mycology. Editor: Jagdish Chandar.  Publication Mehata, India.
  5.  Practical Laboratory Mycology. Editors: Koneman E.W. and G.D. Roberts, 3rd ed 1985, Publisher Williams and Wilkins, Baltimore.
  6. https://catalog.hardydiagnostics.com/cp_prod/Content/hugo/PotatoDextroseAgar.htm
  7. http://himedialabs.com/TD/M096.pdf
  8. https://en.wikipedia.org/wiki/Potato_dextrose_agar
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