Sunday, April 15, 2018

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Sabouraud Agar or Sabouraud Dextrose Agar or SDA is a type of agar growth medium containing peptones.SDA  is used to cultivate dermatophytes and other types of fungi, and can also grow filamentous bacteria such as Nocardia.
SDA  was created by Raymond Sabouraud in 1892 reason why it was name after him but it's  formulation was later adjusted by Chester W. Emmons when the pH level was brought closer to the neutral range and the dextrose concentration lowered to support the growth of other microorganisms. The acidic pH (5.6) of traditional Sabouraud agar inhibits bacterial growth.
So normal Sabouraud Dextrose Agar is often abbreviated (SDA)n because it is composed of the following ingredients below.

What is the composition of Sabouraud Dextrose Agar 

          Ingredients             Gms / Litre

  • Dextrose                        40.000
  • Mycological, peptone    10.000
  • Agar                              15.000
The Final pH is  5.6±0.2 at 25°C

Just as earlier said,SDA agar has been modified inorder to favour growth of some particular microorganism.
This is done by the addition of some substance capable of either inhibiting or favouring particular microorganism as seen below : 

Sabouraud Dextrose Broth is composed the same same way as (SDA)n but doesn't contain  agar.It is a broth with Final pH 5.6 +/- 0.2 at 25ºC.

Sabouraud Dextrose Agar + Chloramphenicol (SDA)c contains an addition of 50.0 mg of chloramphenicol mostly used during antifungigram since Chloramphenicol inhibit bacteria growths at a  Final pH 5.6 +/- 0.3 at 25ºC. 

Sabouraud Dextrose Agar with Chloramphenicol and Gentamicin (SDA)cg contains an addition of antibiotics that is  50.0 mg of chloramphenicol and 5.0 mg gentamicin at Final pH of 5.6 +/- 0.3 at 25ºC. 

Sabouraud Dextrose Agar with Chloramphenicol and Tetracycline contains 50.0 mg of chloramphenicol and 10.0 mg of tetracycline at Final pH of 5.6 +/- 0.3 at 25ºC. 

Sabouraud Dextrose Agar, Emmons has only 20.0 gm of dextrose at Final pH of 6.9 +/- 0.2 at 25ºC.

What are the Principle of SDA

The Peptone (Enzymatic Digest of Casein and Enzymatic Digest of Animal Tissue) provide  nitrogen and vitamin source required for organism growth in SDA. 

Meanwhile ,Dextrose (A sugar ) is added to provide energy and carbon source.

Agar is responsible for the medium solidification.

Further antibiotics such as Chloramphenicol and/or tetracycline may be added as broad spectrum antimicrobials to inhibit the growth of a wide range of gram-positive and gram-negative bacteria. Gentamicin is added to further inhibit the growth of gram-negative bacteria.

The neutral pH of the Emmons modification seems to enhance the growth of some pathogenic fungi, such as dermatophytes.

What are the Uses of Sabouraud Dextrose Agar

Clinical laboratories can use this growth medium to diagnose and further speciate fungal infections, allowing medical providers to provide appropriate treatment with antifungal medications. Histoplasma and other fungal causes of atypical pneumonia can be grown on this medium.
  • It is primarily used for the selective cultivation of yeasts, molds and aciduric bacteria.

  • The medium is often used with antibiotics for the isolation of pathogenic fungi from material containing large numbers of other fungi or bacteria.

  • This medium is also employed to determine microbial contamination in food, cosmetics, and clinical specimens.

How is Normal Sabouraud Dextrose Agar Prepared

  • To 1 Litre of deionized or distilled water,
  • Add 65g  of the Powder
  • Bring to the boil to dissolve completely. 
  • Sterilise by autoclaving at 121°C for 15 minutes. 
  • Mix well and pour in to sterile Petri dishes.

What is the Appearance of Prepared Sabouraud Dextrose Agar

Dehydrated medium: Straw coloured, free-flowing powder. 
Prepared medium: Light straw to straw coloured gel.

What is the Storage Condition of Sabouraud Dextrose Agar

  • Store the dehydrated medium at temperature 10-30°C and use before the expiry date on the label.
  • Store the prepared SDA medium at 2-8°C.

How to Culture in Sabouraud Dextrose Agar

  1. Inoculate each specimen in duplicate that is :
  2. Incubate one set of media aerobically at 22-25°C  for molds and the other set  for yeast  28 to 30°C or 37°C  for 5-30 days. Remember if suspected of being dimorphic fungi. Incubation times will vary, from approximately 2 days for the growth of yeast colonies such as Malasezzia, to 2 to 4 weeks for growth of dermatophytes or dimorphic fungi such as Histoplasma capsulatum. Indeed, the incubation time required to acquire fungal growth is one diagnostic indicator used to identify or confirm fungal species.
  3. Loosen the caps of tubes and ensure adequate moisture for the plates to compensate for loss of water vapour.
  5. Examine every 2-4 days.
  6. Describe each specific type of colony morphology and sub-culture to appropriate media for further identification tests.

Interpreting culture colonies growth on Sabouraud Dextrose agar

Yeasts  on SDA grow as creamy to white colonies while Molds will grow as filamentous colonies of various colors.

Quality Control result on SDA (Morphology Growth Characteristics)

Trichophyton mentagrophytes 
Candida spp
Aspergillus glaucus
Aspergillus versicolor

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