Sabouraud Dextrose Agar (SDA): Composition, Principle, Preparation, and Uses in Fungal Culture
Sabouraud Dextrose Agar (SDA) is a widely used culture medium designed to isolate and cultivate fungi, yeasts, and molds. Its acidic pH and high dextrose concentration create an environment that inhibits bacterial growth while promoting fungal development. SDA is an essential tool in pharmaceutical, food, and clinical microbiology laboratories.
📜 History and Background
The medium was first developed by Raymond Sabouraud in 1892, a French dermatologist who formulated it specifically for the growth of dermatophytes causing skin infections. Over time, its formula was refined to support a wider range of fungi, including Candida species and filamentous molds such as Aspergillus and Penicillium.
⚗️ Principle of Sabouraud Dextrose Agar
The SDA medium supports fungal growth by providing necessary nutrients and maintaining a slightly acidic environment. The medium contains peptones (source of amino acids and nitrogen), dextrose (source of carbon and energy), and a pH of around 5.6, which inhibits bacterial contamination.
- Peptones: Supply nitrogen, vitamins, and amino acids for fungal metabolism.
- Dextrose: Provides an energy source for fungal proliferation.
- Low pH: Restricts bacterial growth while enhancing fungal recovery.
🧪 Composition of Sabouraud Dextrose Agar (SDA)
| Ingredient | Quantity (g/L) |
|---|---|
| Peptone | 10.0 |
| Dextrose (Glucose) | 40.0 |
| Agar | 15.0 |
| Final pH at 25°C | 5.6 ± 0.2 |
Optional antibiotics such as chloramphenicol or cycloheximide can be added to make the medium selective for fungi by suppressing bacterial and saprophytic fungal growth.
🧫 Types of SDA Media
| Medium Type | Additive | Purpose |
|---|---|---|
| SDA (Plain) | None | Supports general fungal growth |
| SDA with Chloramphenicol | 50 mg/L | Inhibits bacterial contamination |
| SDA with Cycloheximide | 500 mg/L | Suppresses saprophytic fungi |
| SDA with Both Additives | Chloramphenicol + Cycloheximide | Selective for pathogenic fungi |
🧬 Principle Behind Selectivity
The acidic pH (5.6) and the inclusion of selective antibiotics make SDA a selective medium. It allows the recovery of clinically important fungi without bacterial interference. In pharmaceutical and cosmetic microbiology, this medium is used to detect and enumerate yeasts and molds in product testing.
⚙️ Preparation Procedure of Sabouraud Dextrose Agar
Materials Required
- Dehydrated SDA powder
- Distilled water
- Measuring cylinder and conical flask
- pH meter
- Autoclave
- Sterile Petri plates or culture tubes
Preparation Steps
- Weigh and suspend 65 g of SDA powder in 1 liter of distilled water.
- Heat gently while stirring until the medium completely dissolves.
- Adjust the pH to 5.6 ± 0.2 at 25°C.
- Autoclave at 121°C for 15 minutes.
- Cool to 45–50°C and pour into sterile Petri plates or tubes aseptically.
- Store prepared plates at 2–8°C in sealed conditions to avoid drying.
🔍 Quality Control Testing
| Organism | Expected Result |
|---|---|
| Candida albicans ATCC 10231 | Luxuriant growth |
| Aspergillus niger ATCC 16404 | Luxuriant growth |
| Escherichia coli ATCC 25922 | Partial or no growth (inhibited) |
🧠 Interpretation of Results
- Fungal colonies generally appear after 2–7 days of incubation at 25–30°C.
- Candida albicans forms creamy, smooth colonies.
- Aspergillus niger produces black, powdery colonies.
- Penicillium species show blue-green, velvety colonies.
🏭 Applications of Sabouraud Dextrose Agar
| Field | Purpose / Use |
|---|---|
| Clinical Microbiology | Isolation of pathogenic fungi and yeasts from clinical samples |
| Pharmaceutical Industry | Detection of fungal contamination in sterile/non-sterile products |
| Food Industry | Monitoring of yeast and mold contamination in food items |
| Cosmetic Testing | Detection of fungi in creams, lotions, and personal care products |
| Academic and Research Labs | Subculturing and maintenance of fungal cultures |
⚠️ Precautions
- Do not overheat; dextrose may caramelize, affecting medium performance.
- Work aseptically under a biosafety cabinet to avoid contamination and spore inhalation.
- Dispose of contaminated plates via autoclaving or chemical disinfection.
✅ Advantages of SDA
- Supports a broad range of fungi (yeasts and molds)
- Simple to prepare and cost-effective
- Can be modified for selective isolation
- Widely accepted for pharmacopeial testing
❌ Limitations
- Some bacteria may grow if pH drifts above 6.0
- Saprophytic fungi can overgrow pathogens in non-selective media
- Fastidious fungi may require enriched media for optimal recovery
🧬 Incubation Conditions
| Parameter | Recommended Condition |
|---|---|
| Temperature | 25°C to 30°C |
| Atmosphere | Aerobic |
| Incubation Time | 2–7 days (may extend for slow growers) |
🔑 Conclusion
Sabouraud Dextrose Agar (SDA) remains a cornerstone medium in fungal microbiology. Its selective formulation, simple preparation, and reliable performance make it indispensable in the detection, enumeration, and maintenance of fungi and yeasts across clinical, pharmaceutical, and research settings.
📚 References
- Sabouraud, R. (1892). Annales de Dermatologie et de Syphiligraphie.
- Indian Pharmacopoeia (IP 2022), Volume 1, Appendix 10.
- USP <61> & <62> – Microbiological Examination of Nonsterile Products.
- ISO 21149:2017 – Cosmetics—Microbiology—Enumeration and Detection of Yeasts and Moulds.
💬 About the Author
Siva Sankar is a Pharmaceutical Microbiology Consultant and Auditor with extensive experience in sterility testing, validation, and GMP compliance. He provides consultancy, training, and documentation services for pharmaceutical microbiology and cleanroom practices.
📧 Contact: siva17092@gmail.com
Mobile: 09505626106
