Raymond Sabouraud created Sabouraud Dextrose Aga or SDA in 1892. Sabouraud Dextrose Aggar is useful in the cultivation of fungi (yeasts and moulds), especially for skin infections. This medium can also be used to detect microbial contamination in food, cosmetics and clinical specimens. To enhance the growth of fungi and dermatophytes as well as to inhibit bacterial growth in clinical specimens, the pH is set to 5.6. To enhance the antibacterial effect, you can also add antibacterial agents. To inhibit the overgrowth of bacteria by competing microorganisms, gentamicin and chloramphenicol are selected agents that allow for successful isolation of yeasts and fungi.
Principle of Sabouraud Dextrose Agar (SDA)
Sabouraud Dextrose Agar is made from digests of animal tissues (peptones), which are a nutritious source for amino acids and other compounds that can be used to grow fungi or yeasts. Dextrose is used as an energy and carbon source. Agar is used as the solidifying agent. To inhibit the growth of many gram-positive and Gram-negative bacteria, tetracycline and chloramphenicol may be added. To further inhibit the growth gram-negative bacteria, gentamicin may be added. To enhance the growth of fungi and dermatophytes as well as to inhibit bacterial growth in clinical specimens, pH is adjusted to 5.6. Low pH and high dextrose content favor fungal growth.
Composition of Sabouraud Dextrose Agar (SDA)
| Ingredients | Gms/Liter |
| Dextrose | 40.0 |
| Peptone | 10.0 |
| Agar | 15.0 |
Final pH ( at 25°C) 5.6±0.2
In addition,
- Sabouraud Dextrose Broth is the same formula as above but without agar. Final pH 5.6 +/– 0.2 at 25oC
- Sabouraud Dextrose agar with Chloramphenicol has 50.0 mg chloramphenicol. Final pH 5.6 +/– 0.3 at 25oC
- Sabouraud Dextrose agar with Chloramphenicol & Gentamicin contains 50.0mg of chloramphenicol, and 5.0mg gentamicin. Final pH at 25°C: 5.6 +/– 0.3
- Sabouraud Dextrose agar with Chloramphenicol & Tetracycline has 50.0 mg chloramphenicol, and 10.0 mg tetracycline. Final pH at 25oC: 5.6 +/– 0.3
- Sabouraud Dextrose Agar Emmons contains only 20.0 grams of dextrose. Final pH 6.9 +/– 0.2 at 25oC
Preparation of Sabouraud Dextrose Agar (SDA)
- In one liter of distilled tap water, suspend 65 grams of the medium.
- To dissolve the medium, heat with constant stirring and boil for 1 minute.
- For 15 minutes, autoclave at 121°C
- Allow to cool to 45 to 50 degrees Celsius before pouring into tubes or petri dishes for slants.
Sabouraud agar plates may be inoculated using standard bacteriological media or streaking. Molds should be incubated at room temperatures (22-25degC), while yeasts should be incubated at 28-30degC or 37degC, if they are suspected to be dimorphic fungal species. The incubation time for yeast colonies like Malasezzia will take approximately 2 days, while it takes 2 to 4 weeks to grow dermatophytes and dimorphic fungal species such as Histoplasma capsuleatum. Incubation time is one indicator that can be used to confirm or identify fungal species.
Result Interpretation of Sabouraud Dextrose Agar (SDA)
| Fungi | Colony morphology |
| Candida albicans | Pasty opaque slightly domed, smooth, and cream or white colonies |
| Aspergillus flavus | Yellow-green powdery on front 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 |
| Aspergillus fumigatus | Bluish green powdery colonies on front and pale yellow on reverse . |
| Trichosporon mucoides | White to cream, yellowish, wrinkled |
| Geotrichum candidum | White to cream colored, flat with aerial mycelium |
Quality Control of SDA
| Positive controls: | Expected results |
| Candida albicans ATCC® 10231 | Good growth; cream colonies |
| Aspergillus brasiliensis ATCC® 16404 | White mycelium; black spores |
| Negative control: | |
| Uninoculated medium | No change |
Appearance
- Dehydrated medium: Straw coloured, free-flowing powder.
- Prepared medium: Light straw to straw coloured gel.
Modifications of Sabouraud Agar
Sabouraud Dextrose Agar is combined with Brain Heart Infusion Agar to create SabHI Agar. SabHI agar is more effective in removing pathogenic fungi than either of the mediums.
Uses of Sabouraud Dextrose Agar (SDA)
- SDA is used primarily for the selective cultivation yeasts, molds, and aciduric bacteria.
- This medium is used often with antibiotics to isolate pathogenic fungi in material containing large amounts of bacteria or fungi.
- This medium can also be used to detect microbial contamination in foods, cosmetics, or clinical specimens.
Limitations of Sabouraud Dextrose Agar (SDA)
- You might encounter some strains that do not grow well or are incapable of growing on this medium.
- Antimicrobial agents may be added to a medium to inhibit bacteria. This could also prevent certain pathogenic fungi.
- Avoid heating a medium that has an acidic pH. This could cause a soft medium.
- Organisms must be grown in pure culture for identification.
- Final identification should include morphological, biochemical, or serological tests.
- It doesn’t promote conidiation by filamentous fungi.
