Sabouraud Agar or Sabouraud Dextrose Agar (SDA) is a selective medium primarily used for the isolation of dermatophytes. Other fungi, yeasts, and filamentous bacteria such as Nocardia can also grow in SDA. The acidic pH of this medium (pH about 5.0) inhibits the growth of bacteria but permits the growth of yeasts and most filamentous fungi. Antibacterial agents can also be added to augment the antibacterial effect.
This medium is also employed to determine the mycological evaluation of food, contamination in cosmetics, and clinically to aid in the diagnosis of yeast and fungal infections.
Antibiotics like chloramphenicol, gentamicin, and tetracycline can be added as selective agents to inhibit the overgrowth of competing bacteria while permitting the successful isolation of fungi and yeasts. Various other modifications are also reported by using cycloheximide, penicillin, streptomycin, neomycin depending upon the intended use.
Sabouraud Dextrose Agar is comprised of enzymatic digest of casein and animal tissues which provide a nutritious source of amino acids and nitrogenous compounds for the growth of fungi and yeasts.
Dextrose is a fermentable carbohydrate incorporated in high concentrations as a carbon and energy source. Agar is the solidifying agent. The addition of antibiotics like chloramphenicol and/or tetracycline acts 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.
Composition of SDA
|Mycological peptone (enzymatic digest of casein and animal tissues)||10 gm|
Preparation of SDA
- Suspend 65 gm of the medium in one liter of purified water.
- Heat with frequent agitation and boil for one minute to completely dissolve the medium.
- Autoclave at 121° C for 15 minutes.
- Cool to 45 to 50°C and pour into Petri dishes or tubes for slants.
- To process specimens, streak the specimen onto the medium with a sterile inoculating loop to obtain isolated colonies.
- Incubate the plates at 25 – 30°C in an inverted position (agar side up) with increased humidity.
- Cultures should be examined weekly for fungal growth and held for 4 – 6 weeks before being reported as negative.
Result and interpretation:
After sufficient incubation, SDA plates should show isolated colonies in streaked areas and confluent growth in areas of heavy inoculation. Examine plates for fungal colonies exhibiting typical color and morphology. Additional procedures should be performed to confirm the findings.
Yeasts will grow as creamy to white colonies. Molds will grow as filamentous colonies of various colors.
Typical Colony morphology of some fungi on SDA
|Aspergillus flavus||Yellow-green, powdery, and pale yellowish on reverse|
|Aspergillus niger||The initial growth is white, becoming black later on giving “salt and pepper appearance” which results from darkly pigmented conidia borne in large numbers on conidiophores and reverse turning pale yellow|
|Rhodotorula species||pinkish-orangish creamy colonies|
|Aspergillus fumigatus||Blue-green, powdery and pale yellow on the reverse.|
|Aspergillus nidulans||Greenish-blue with a whitish edge, yellow to brownish on reverse|
|Trichosporon mucoides||White to cream, yellowish, wrinkled|
|Geotrichum candidum||White to cream-colored, flat with aerial mycelium|
Modifications of Sabouraud Agar
SabHI Agar is formulated by combining Sabouraud Dextrose Agar and Brain Heart Infusion Agar. SabHI agar yields a greater recovery of pathogenic fungi than either medium individually.
Limitations of Sabouraud Agar
- It does not promote the conidiation of filamentous fungi.
- Antimicrobial agents added into a medium to inhibit bacteria may also inhibit certain pathogenic fungi.
- Avoid overheating a medium with an acidic pH; this may result in a soft medium.
Note: I am grateful to Yuri for permitting us to use his photos. For more images please visit http://thunderhouse4-yuri.blogspot.com/
References and further readings
- Acharya T., Hare J. (2022) Sabouraud Agar and Other Fungal Growth Media. In: Gupta V.K., Tuohy M. (eds) Laboratory Protocols in Fungal Biology. Fungal Biology. Springer, Cham. https://doi.org/10.1007/978-3-030-83749-5_2