A Sabouraud dextrose agar was created in 1892 for dermatophyte culture. The growth of yeast and fungi being tested is enhanced by adjusting the pH level to 5.6.
Sabouraud dextrose agar is a selective medium for it is used to isolate yeast and fungi and at the same time permit the growth of filamentous bacteria like Nocardia. In fact, you can add an antibacterial agent to augment the antibacterial effect. (1, 2)
Image 1: A sabouraud dextrose agar plate.
Picture Source: mkldiagnostics.com
What is sabouraud dextrose agar used for?
- It is used to isolate, cultivate, and maintain pathogenic and non-pathogenic yeast, fungi, and aciduric bacteria.
- It is used to determine the mycological evaluation of food.
- It is used to determine the presence of harmful microorganisms in cosmetics and clinical specimen.
- It is used to clinically aid in the diagnosis of fungal and yeast infections. (1, 2, and 3)
Image 2: Penicillium notatum on SDA.
Picture Source: microbeonline.com
What is the principle of sabouraud dextrose agar?
Peptone, an enzymatic digest of casein and animal tissue is the source of nitrogen and vitamin necessary to grow microorganism in sabouraud dextrose agar. For carbon and energy source, dextrose is used. Agar is used as a solidifying agent.
To prevent the growth of different array of microorganisms including gram positive and gram negative, antibacterial agents can be added such as tetracycline and chloramphenicol. To further prevent the growth of bacteria (gram negative), gentamicin is added. (2, 3, and 4)
How do you make sabouraud Agar (Composition)?
The ingredients used to make sabouraud agar are:
- Dextrose (glucose) – 40 gm/L
- Distilled water – 1 Liter
- Agar – 15 gm
- Peptone – 10 gm
Final pH 5.6 +/- 0.2 at 25ºC (4, 5)
- Gather all the necessary ingredients and combine them in about 900 ml of deionized water.
- Add hydrochloric acid to adjust the pH to the desired level. The final volume should be 1 liter.
- Bring to heat to dissolve the medium.
- Autoclave for 15 minutes at 121 degree Celsius.
- Allow to cool down before placing into Petri dish to test tube for slants.
- The sabouraud dextrose agar plate is inoculated by streaking or exposure the medium to ambient air. Ideally, a mold is incubated at a room temperature, which is typically 22 to 25 degree Celsius. Yeast is incubated at 28 to 30 degree Celsius. The incubation time needed for the organism to grow determine its fungal species. (5, 6, 7, and 8)
To identify a particular strain of fungi, the organism is thoroughly observed using these parameters:
- Colony morphology
- Microscopic structures
- Media used to support the growth of the organism
- Growth rate
- Specimen source (6, 8)
Image 3: SDA is a medium commonly used for fungal isolation.
Picture Source: slidesharecdn.com
A creamy to white colony is observed such as Candida albicans and Aspergillus brasiliensis. (9)
A filamentous colony of varying colors. (10)
Are there any limitations?
- Some strains fail to grow or grow poorly on sabouraud dextrose agar medium.
- In case of overheating the medium with acidic pH, the integrity of the medium can be greatly affected. It could result in a soft medium.
- An antimicrobial agent once added to the medium can inhibit the growth of bacteria as well as other pathogenic fungi.
- For the purpose of identification, the organism being tested must be in pure culture.
- For final identification, an additional test has to be performed such as morphological, serological, and biochemical test.
- Sabouraud dextrose agar does not allow conidiation of filamentous fungi. (1, 4, 8, 9, and 10)
Image 4: Different culture colonies growth on SDA.
Picture Source: 2.bp.blogspot.com
Colony morphology of fungi in sabouraud dextrose agar
There is a noticeable yellow to greenish powdery discoloration. The color is pale yellowish on the reverse.
The initial growth is white but later on turned to black similar to salt and pepper appearance. In reverse, turning pale yellow.
The growth colonies are pinkish to orange in color.
Blue to greenish powdery which appears pale yellow on the reverse.
The colony is greenish to bluish and the edge is whitish in color. On the reverse, the color becomes yellowish to brownish.
The colonies appeared wrinkled and white cream to yellowish in color.
The colonies are whitish cream in color with a distinct flat appearance with aerial mycelium.
Sabouraud dextrose agar is named after Raymond Sabouraud who created the said medium in 1892. Chester W. Emmons later on adjusted the formulation and brought the pH level closer to the neutral range.
The concentration of dextrose is also reduced to support the growth of other microorganisms. (2, 4, 7, 9, and 10)