Sabouraud dextrose agar (SDA) is a selective culture medium containing peptones, high dextrose concentration and a slightly acidic pH that promotes the growth of fungi while suppressing most bacteria.
Explanation
Sabouraud dextrose agar was developed in 1892 by French dermatologist Raymond Sabouraud for the isolation of dermatophytes. It consists of a base of enzymatic digest of casein and animal tissues, 40 g/L dextrose and 15 g/L agar with a final pH of about 5.6. The high carbohydrate content and acidic environment favour yeasts and molds and inhibit many bacteria. To further suppress bacterial contaminants, antibiotics such as chloramphenicol or gentamicin are often added. Modified versions include potato flake agar or Emmons formulation with lower glucose concentration and neutral pH to support more fastidious fungi.
SDA is widely used in clinical and environmental laboratories to culture dermatophytes, Candida species and other fungi from skin scrapings, sputum, urine and soil samples. Plates are incubated at 25 °C or 30 °C for up to two weeks to allow slow‑growing molds to develop characteristic colony morphology. The medium’s acidic pH discourages most bacteria but does not support some dimorphic fungi like Histoplasma capsulatum, so additional media are required for comprehensive isolation. Despite its limitations, SDA remains a cornerstone for mycological diagnostics and research.
Key Components and Uses
- Composition: peptone base (casein and animal tissue hydrolysates), 40 g/L dextrose and 1.5 % agar with final pH around 5.6.
- Selective action: high sugar and acidic pH promote fungal growth and suppress bacterial contaminants; antibiotics can enhance selectivity.
- Applications: isolation of dermatophytes, yeasts and molds from clinical specimens such as skin scrapings, nails, hair, sputum and urine.
- Variants: SDA with chloramphenicol, SDA with gentamicin, and Emmons modification with lower sugar and neutral pH.
- Limitations: some fastidious fungi and bacteria require enriched or neutral media; long incubation times are needed for slow‑growing molds.
Sabouraud dextrose agar is a simple yet highly effective medium that supports the growth of many pathogenic and environmental fungi while limiting bacterial overgrowth. Its continued use in diagnostic microbiology underscores its value in fungal isolation and identification.
Related Terms: Fungi, Dermatophyte, Culture medium, Selective media, Yeast.