Sabouraud Dextrose Agar (SDA)- Composition, Principle, Preparation, Results, Uses
Sabouraud Dextrose Agar (SDA) is a type of culture medium that is widely used in microbiology for the isolation and identification of various fungi and yeasts. It was first developed by Raymond Sabouraud in 1892 as a modification of the original potato dextrose agar. SDA consists of peptones, dextrose, agar and water, and sometimes antibiotics to suppress the growth of bacteria. SDA has a low pH of around 5.6, which favors the growth of most fungi and inhibits many bacteria. SDA is commonly used to culture dermatophytes, which are fungi that cause skin infections, as well as other pathogenic or opportunistic fungi that may be present in clinical specimens, food, cosmetics or environmental samples. SDA can also support the growth of some aciduric bacteria, such as lactobacilli and propionibacteria, which may be of interest in certain applications. SDA is usually prepared as plates or slants and incubated at 25°C to 30°C for several days to allow the growth of fungal colonies. The colonies can then be observed for their morphology, color, texture and other characteristics that can help in their identification. Some fungi may also produce characteristic pigments or metabolites that can be detected by chemical tests or microscopy. SDA is a simple, inexpensive and versatile medium that has been widely used for over a century in fungal microbiology. It is still one of the most popular media for the isolation and identification of fungi and yeasts in various fields and settings.
Sabouraud Dextrose Agar (SDA) is a selective and differential medium for the isolation and identification of fungi and yeasts. It is based on the following principles:
- SDA contains peptones, which are digests of animal tissues that provide a nutritious source of amino acids and nitrogenous compounds for the growth of fungi and yeasts.
- SDA also contains dextrose, which is a simple sugar that serves as the energy and carbon source for the microorganisms. Fungi and yeasts can metabolize dextrose more efficiently than bacteria, giving them a competitive advantage on this medium.
- SDA has a low pH of about 5.6, which is slightly acidic and favors the growth of fungi, especially dermatophytes. Dermatophytes are a group of fungi that cause skin infections such as ringworm and athlete`s foot. The low pH also inhibits the growth of most bacteria, especially gram-positive and gram-negative bacteria that prefer a neutral or alkaline pH.
- SDA may contain antibiotics, such as chloramphenicol, tetracycline, or gentamicin, to further suppress the growth of bacteria. These antibiotics are broad-spectrum and can inhibit a wide range of bacterial species. However, some fungi and yeasts may be resistant to these antibiotics and still grow on SDA.
- SDA is a differential medium because it allows the differentiation of fungi and yeasts based on their colony morphology, color, texture, and pigmentation. For example, Candida albicans, a common yeast that causes oral thrush and vaginal infections, produces white to cream-colored colonies with a smooth surface and a yeasty odor. Aspergillus niger, a common mold that causes black mold infections in the lungs and ears, produces black colonies with a velvety surface and a musty odor.
Sabouraud Dextrose Agar (SDA) consists of the following ingredients per liter of distilled water:
- Peptone: 10 g
- Dextrose: 40 g
- Agar: 15 g
The peptone is a digest of animal tissues that provides a nutritious source of amino acids and nitrogenous compounds for the growth of fungi and yeasts. The dextrose is added as the energy and carbon source. The agar is the solidifying agent.
The medium may also contain one or more of the following antimicrobial agents to inhibit the growth of bacteria:
- Chloramphenicol: 0.05 g
- Tetracycline: 0.01 g
- Gentamicin: 0.005 g
The pH of the medium is adjusted to approximately 5.6 in order to enhance the growth of fungi, especially dermatophytes, and to slightly inhibit bacterial growth in clinical specimens.
The composition of SDA may vary slightly depending on the manufacturer or the purpose of use. Some variations include:
- Sabouraud Dextrose Broth (SDB): This is a liquid version of SDA without agar. It is used for the cultivation and maintenance of fungi and yeasts.
- Sabouraud Dextrose Agar with Lecithin and Polysorbate 80 (SDA-LP): This is a modification of SDA that contains lecithin and polysorbate 80 as neutralizing agents. It is used for the detection of fungi in cosmetics and other products that may contain antimicrobial substances.
- Sabouraud Dextrose Agar with Chloramphenicol (SDA-C): This is a variation of SDA that contains only chloramphenicol as the antimicrobial agent. It is used for the isolation of fungi from clinical specimens that may contain gram-positive bacteria.
- Sabouraud Dextrose Agar with Gentamicin (SDA-G): This is a variation of SDA that contains only gentamicin as the antimicrobial agent. It is used for the isolation of fungi from clinical specimens that may contain gram-negative bacteria.
Sabouraud Dextrose Agar (SDA) is a commercially available dehydrated medium that can be prepared by following these steps:
- Weigh 65 g of the medium and dissolve it in one liter of distilled water in a suitable container. Mix well to ensure complete dissolution of the medium.
- Transfer the solution to a flask or bottle and plug it with a cotton wool stopper. Label the container with the name and date of preparation of the medium.
- Sterilize the medium by autoclaving at 121° C for 15 minutes. Make sure that the container is not filled more than half and that there is enough space for steam circulation.
- After autoclaving, remove the container from the autoclave and place it on a wire rack to cool. Do not shake or disturb the medium while it is cooling.
- When the medium has cooled to 45 to 50°C, it is ready to be poured into sterile petri dishes or tubes for slants. Use aseptic technique to avoid contamination of the medium.
- If antibiotics are to be added to the medium, they should be dissolved in sterile distilled water and added to the cooled medium before pouring. The final concentration of antibiotics should be as follows: chloramphenicol 50 mg/L, tetracycline 10 mg/L, gentamicin 4 mg/L.
- Store the prepared medium in a refrigerator at 2 to 8°C until use. Do not use the medium if there is any sign of deterioration or contamination.
The preparation of Sabouraud Dextrose Agar (SDA) is simple and straightforward, but it requires careful attention to avoid errors and ensure quality control. The medium should be prepared according to the manufacturer`s instructions and checked for pH, clarity, and sterility before use. The medium should also be used within its shelf life and discarded if expired or contaminated. By following these guidelines, you can prepare Sabouraud Dextrose Agar (SDA) successfully and use it for your microbiological applications. 😊
Sabouraud Dextrose Agar (SDA) is a differential medium that allows the identification of fungi and yeasts based on their colony morphology and microscopic characteristics. The growth of bacteria is usually inhibited by the low pH and the presence of antimicrobials in the medium.
The colonies of fungi and yeasts on SDA vary in size, shape, color, texture, and elevation depending on the species. Some common examples are:
- Candida albicans: This yeast produces smooth, white to cream-colored colonies that may have a greenish fluorescence under ultraviolet light. Microscopically, it shows oval to elongated budding cells and pseudohyphae.
- Aspergillus niger: This mold produces large, black colonies with a velvety to woolly texture and a white to yellowish periphery. Microscopically, it shows septate hyphae with conidiophores bearing black conidia in radiating chains.
- Trichophyton rubrum: This dermatophyte produces flat, red to wine-colored colonies with a powdery to granular texture and a yellowish reverse. Microscopically, it shows septate hyphae with numerous microconidia along the sides and rare macroconidia at the ends.
The identification of fungi and yeasts on SDA should be confirmed by performing additional tests such as biochemical tests, molecular tests, or susceptibility tests. Some fungi and yeasts may require special media or conditions for optimal growth and differentiation. For example, dimorphic fungi such as Histoplasma capsulatum and Blastomyces dermatitidis may grow as yeasts at 37°C and as molds at 25°C. Therefore, SDA alone is not sufficient for the definitive identification of fungi and yeasts.
Sabouraud Dextrose Agar (SDA) is a widely used medium for the isolation and identification of fungi and yeasts. It has several applications in different fields, such as:
- Clinical microbiology: SDA is used to culture clinical specimens such as skin scrapings, hair, nails, sputum, blood, urine, etc. that may contain fungal pathogens or opportunists. SDA can help to diagnose fungal infections such as dermatophytosis, candidiasis, aspergillosis, cryptococcosis, etc. SDA can also be used to perform antifungal susceptibility testing by disk diffusion or broth dilution methods.
- Food microbiology: SDA is used to detect and enumerate fungi and yeasts in food products such as dairy, meat, cereals, fruits, vegetables, etc. SDA can help to assess the quality and safety of food products and prevent spoilage and foodborne illnesses caused by fungi and yeasts.
- Cosmetic microbiology: SDA is used to test the microbial contamination and stability of cosmetic products such as creams, lotions, shampoos, etc. SDA can help to ensure the efficacy and shelf-life of cosmetic products and prevent adverse reactions and infections caused by fungi and yeasts.
- Environmental microbiology: SDA is used to monitor the fungal and yeast flora in different environments such as air, water, soil, etc. SDA can help to evaluate the microbial diversity and ecology of different habitats and assess the impact of human activities and pollution on the fungal and yeast communities.
SDA is a simple, inexpensive and versatile medium that can support the growth of a wide range of fungi and yeasts. It can be modified by adding different antibiotics or supplements to enhance its selectivity or specificity for certain groups of fungi or yeasts. It can also be combined with other media or methods to improve its sensitivity or differentiation of fungal and yeast isolates. SDA is an essential tool for the study and diagnosis of fungal and yeast infections and contamination in various fields.
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