Sabouraud Dextrose Agar (SDA) was formulated by Raymond Sabouraud in 1892.
SDA is used for the cultivation of fungi, including yeasts and molds.
It is particularly useful for cultivating fungi associated with skin infections, such as dermatophytes.
This medium is also employed to determine microbial contamination in food, cosmetics, and clinical specimens.
The pH of SDA is adjusted to approximately 5.6, creating an acidic environment that enhances fungal growth and inhibits bacterial growth in clinical specimens.
To further inhibit bacterial growth and prevent overgrowth by competing microorganisms, antibacterial agents can be added to SDA.
Commonly added antibacterial agents include chloramphenicol, gentamicin, and tetracycline.
These selective agents allow for the successful isolation and identification of fungi and yeasts while minimizing bacterial interference.
SDA is a staple in microbiological laboratories for studying fungal pathogens and ensuring the microbial safety of various products.
Principle of Sabouraud Dextrose Agar (SDA)
Sabouraud Dextrose Agar (SDA) contains digests of animal tissues (peptones), providing a nutritious source of amino acids and nitrogenous compounds essential for the growth of fungi and yeasts.
Dextrose is added as the primary energy and carbon source.
Agar acts as the solidifying agent.
Chloramphenicol and/or tetracycline may be incorporated as broad-spectrum antimicrobials to inhibit a wide range of gram-positive and gram-negative bacteria.
Gentamicin is added to further inhibit the growth of gram-negative bacteria.
The pH is adjusted to approximately 5.6 to enhance the growth of fungi, particularly dermatophytes, and to slightly inhibit bacterial growth in clinical specimens.
The high dextrose concentration and low pH create an environment that favors fungal growth while inhibiting contaminating bacteria from test samples.
Composition of Sabouraud Dextrose Agar (SDA)
Ingredients (per Liter):
Dextrose: 40.0 grams
Peptone: 10.0 grams
Agar: 15.0 grams
Final pH (at 25°C): 5.6 ± 0.2
Preparation of Sabouraud Dextrose Agar (SDA)
Suspend 65 g of the medium in one liter of distilled 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.
Inoculation:
Sabouraud agar plates can be inoculated by streaking, as with standard bacteriological media, or by exposing the medium to ambient air.
Incubation:
Typically, molds are incubated at room temperature (22 to 25°C).
Yeasts are incubated at 28 to 30°C or at 37°C if suspected of being dimorphic fungi.
Incubation times vary:
Approximately 2 days for the growth of yeast colonies such as Malassezia.
2 to 4 weeks for the growth of dermatophytes or dimorphic fungi such as Histoplasma capsulatum.
The incubation time required to acquire fungal growth is one diagnostic indicator used to identify or confirm fungal species.
Quality Control of SDA
Result Interpretation on Sabouraud Dextrose Agar (SDA)
Uses of SDA
Sabouraud Dextrose Agar (SDA) is primarily used for the selective cultivation of yeasts, molds, and aciduric bacteria.
The medium is often supplemented with antibiotics to facilitate the isolation of pathogenic fungi from samples containing large numbers of other fungi or bacteria.
SDA is employed to determine microbial contamination in food, cosmetics, and clinical specimens.
This versatility makes it a valuable tool in both clinical and industrial microbiology for ensuring product safety and diagnosing fungal infections.
Limitations of SDA
Some strains may exhibit poor growth or fail to grow on Sabouraud Dextrose Agar (SDA).
Antimicrobial agents added to inhibit bacteria can also inhibit certain pathogenic fungi.
Overheating a medium with an acidic pH can lead to softening of the agar.
Organisms must be in pure culture for accurate identification.
Final identification typically involves morphological, biochemical, and/or serological tests.
SDA does not promote the conidiation (formation of spores) of filamentous fungi.
