Sabouraud Dextrose Agar (SDA) 50 FAQs and 30 MCQs
Master Sabouraud Dextrose Agar (SDA), the gold-standard medium for fungal isolation, with this comprehensive resource. Perfect for mycologists, microbiologists, and lab technicians, this guide combines 50 expert FAQs and 30 practice MCQs to sharpen your skills in fungal diagnostics.
📌 What’s Covered?
✅ 50 Essential FAQs
✔ Composition & Principle:
- Low pH (5.6) inhibits bacteria
- High dextrose concentration promotes fungal growth
✔ Clinical Applications: - Dermatophytes (Trichophyton, Microsporum)
- Yeasts (Candida, Cryptococcus)
- Molds (Aspergillus, Mucor)
✔ Protocol Optimization: - Incubation (25-30°C, up to 4 weeks)
- Antibiotic supplementation (chloramphenicol/gentamicin)
✔ Troubleshooting: - Bacterial contamination
- Slow-growing fungi
- Media dehydration
✅ 30 Challenging MCQs
◼️ Identifying dermatophyte colony morphologies
◼️ Differentiating yeasts vs. molds
◼️ Selecting SDA variants (with/without antibiotics)
◼️ Comparing SDA vs. PDA (Potato Dextrose Agar)
🎯 Perfect For:
- Medical lab scientists (MLS/MLT)
- Dermatology & microbiology students
- Clinical researchers
Sabouraud Dextrose Agar (SDA) 50 FAQs
What is Sabouraud Dextrose Agar (SDA)?
A selective culture medium formulated by Raymond Sabouraud in 1892 for cultivating fungi (yeasts and molds).
Who developed SDA?
French dermatologist Raymond Sabouraud in the late 1800s.
What is the primary purpose of SDA?
To isolate and cultivate fungi, especially dermatophytes and yeasts, while inhibiting bacterial growth.
What is the pH of SDA?
Approximately 5.6 (acidic to favor fungal growth).
Why is SDA’s pH acidic?
To enhance fungal growth and slightly inhibit bacteria.
What is the Emmons modification of SDA?
A version with neutral pH (6.8–7.0) and reduced dextrose (20g/L), better for some pathogenic fungi.
What are the main ingredients of SDA?
Dextrose (40g/L), peptone (10g/L), agar (15g/L).
Why is dextrose (glucose) added to SDA?
As an energy and carbon source for fungal growth.
What is the role of peptone in SDA?
Provides nitrogen and amino acids for fungal nutrition.
How is SDA prepared?
Suspend 65g in 1L water, boil, autoclave (121°C, 15 min), cool, and pour into plates/slants.
Can SDA be prepared without agar?
Yes, as Sabouraud Dextrose Broth.
What antibiotics are added to SDA?
Chloramphenicol, gentamicin, or tetracycline to inhibit bacteria.
Why are antibiotics added to SDA?
To suppress bacterial contamination while allowing fungal growth.
What is the concentration of chloramphenicol in SDA?
Typically 50 mg/L.
How does the Emmons modification differ from classic SDA?
Lower dextrose (20g/L), neutral pH (6.9), better for some pathogens.
What types of fungi grow on SDA?
Yeasts (e.g., Candida albicans), molds (e.g., Aspergillus spp.), dermatophytes.
How is SDA used in clinical labs?
To diagnose fungal infections (skin, nails, hair).
Can SDA detect food contamination?
Yes, it’s used to test for yeast/mold contamination in food and cosmetics.
What is the incubation temperature for molds on SDA?
Room temperature (22–25°C).
What is the incubation temperature for yeasts on SDA?
28–30°C (or 37°C for dimorphic fungi).
How long does it take for fungi to grow on SDA?
Yeasts: 2–3 days; molds/dermatophytes: 2–4 weeks.
Why is SDA used for dermatophytes?
Its acidic pH and nutrients favor their growth.
Can SDA be used for bacterial cultures?
No, it inhibits bacteria due to low pH and antibiotics.
What is the colony morphology of Candida albicans on SDA?
Creamy, white, smooth colonies.
What does Aspergillus niger look like on SDA?
White initially, then black spores (“salt and pepper” appearance).
What are the limitations of SDA?
Some fungi grow poorly; antibiotics may inhibit certain pathogens.
Why might some fungi fail to grow on SDA?
They may require additional nutrients (e.g., Trichophyton needs thiamine).
Does SDA promote fungal sporulation?
No, it does not encourage conidiation in molds.
What happens if SDA is overheated?
The agar may become too soft due to acidic pH.
How is SDA quality controlled?
Using control strains like Candida albicans (ATCC 10231) and Aspergillus brasiliensis (ATCC 16404).
What is the expected growth of Aspergillus brasiliensis on SDA?
White mycelium with black spores.
Can SDA be re-melted after solidifying?
No, repeated melting degrades the medium.
What is SDA with chloramphenicol and gentamicin used for?
Enhanced inhibition of Gram-negative and Gram-positive bacteria.
What is cycloheximide’s role in SDA?
Inhibits saprophytic fungi but allows pathogenic fungi to grow.
How should SDA plates be stored?
At 4°C, wrapped to prevent contamination.
Why should SDA plates be incubated lid-side up?
To prevent fungal spores from spreading.
What safety precautions are needed when handling SDA?
Work in a biosafety cabinet, avoid opening plates outside it.
Which fungi are safe for teaching labs?
Saccharomyces cerevisiae, Penicillium camemberti, Rhizopus stolonifer.
Which fungi should be avoided in teaching labs?
Aspergillus fumigatus, A. flavus (potential pathogens).
What pharmacopeias recognize SDA?
USP, EP, JP for microbial testing.
What are the commercial forms of SDA?
Tubes (18 mL), bottles (200 mL, 400 mL).
How is SDA used in pour plate methods?
Melt agar, mix with sample, solidify, and incubate.
What is the shelf life of SDA?
Until expiry if stored at 2–25°C, protected from light.
How should used SDA plates be disposed of?
Autoclave (121°C, 20 min) before disposal.
What is the microbial limit test incubation time for SDA?
5–7 days at 20–25°C (EP/USP standards).
Why is SDA called “modified” in some references?
Original peptone (Granulée de Chassaing) is no longer available.
Can SDA detect dimorphic fungi?
Yes (e.g., Histoplasma capsulatum), but incubation may take weeks.
What is the reverse side observation in molds?
Pigment differences (e.g., Aspergillus flavus has a pale yellow reverse).
What is the role of agar in SDA?
Solidifying agent for plate/slant preparation.
Can SDA be used for environmental sampling?
Yes, but with precautions (biosafety cabinet, antifungal agents if needed).
Sabouraud Dextrose Agar (SDA) 30MCQs
1. Who developed Sabouraud Dextrose Agar (SDA)?
A) Louis Pasteur
B) Robert Koch
C) Raymond Sabouraud
D) Alexander Fleming
2. What is the primary purpose of SDA?
A) Cultivation of bacteria
B) Isolation of fungi and yeasts
C) Viral culture
D) Protozoan growth
3. What is the pH of standard SDA?
A) 7.0
B) 5.6
C) 8.2
D) 6.5
4. Which component in SDA acts as a carbon source?
A) Peptone
B) Dextrose (Glucose)
C) Agar
D) Chloramphenicol
5. Which antibiotic is commonly added to SDA to inhibit bacteria?
A) Penicillin
B) Chloramphenicol
C) Streptomycin
D) Vancomycin
6. What is the colony morphology of Candida albicans on SDA?
A) Green powdery colonies
B) Creamy, white, smooth colonies
C) Black spores
D) Yellow pigmentation
7. Which fungus appears as “salt and pepper” on SDA?
A) Aspergillus flavus
B) Candida albicans
C) Aspergillus niger
D) Penicillium spp.
8. What is the incubation temperature for molds on SDA?
A) 37°C
B) 22–25°C (Room temperature)
C) 4°C
D) 55°C
9. Which modification of SDA has a neutral pH?
A) Emmons modification
B) Original Sabouraud formula
C) Blood agar variant
D) MacConkey-SDA blend
10. Why is SDA used in food testing?
A) To detect bacterial toxins
B) To enumerate yeasts and molds
C) To identify viruses
D) To culture probiotics
11. What is the solidifying agent in SDA?
A) Gelatin
B) Agar
C) Silica gel
D) Pectin
12. Which organism is a QC strain for SDA?
A) Escherichia coli
B) Aspergillus brasiliensis (ATCC 16404)
C) Staphylococcus aureus
D) Pseudomonas aeruginosa
13. How long should SDA plates be incubated for yeast detection?
A) 24 hours
B) 2–3 days
C) 1 week
D) 1 month
14. What inhibits bacterial growth in SDA?
A) High sugar and neutral pH
B) Low pH and antibiotics
C) UV light exposure
D) Lack of oxygen
15. Which fungus produces yellow-green colonies on SDA?
A) Candida albicans
B) Aspergillus flavus
C) Geotrichum candidum
D) Trichosporon spp.
16. What is the main nitrogen source in SDA?
A) Dextrose
B) Peptone
C) Agar
D) Chloramphenicol
17. Why should SDA not be re-melted?
A) It becomes toxic
B) Agar degrades and softens
C) Loses antibiotics
D) Changes color
18. Which incubation temperature is used for dimorphic fungi?
A) 4°C
B) 22°C
C) 30°C and 37°C
D) 50°C
19. What is the shelf life of prepared SDA plates?
A) 1 day
B) 1 week
C) A few days (if stored at 4°C)
D) 6 months
20. Which fungus grows as white, wrinkled colonies on SDA?
A) Candida albicans
B) Trichosporon mucoides
C) Aspergillus niger
D) Penicillium spp.
21. What is the purpose of cycloheximide in SDA?
A) Inhibit saprophytic fungi
B) Promote bacterial growth
C) Enhance pigment production
D) Solidify the medium
22. Which standard recommends SDA for microbial testing?
A) US Pharmacopeia (USP)
B) CLSI
C) FDA
D) WHO
23. How should SDA plates be disposed of?
A) Throw in regular trash
B) Autoclave at 121°C for 20 minutes
C) Freeze at -20°C
D) Burn openly
24. What does Geotrichum candidum look like on SDA?
A) Black spores
B) White, flat colonies with aerial mycelium
C) Green powdery growth
D) Red pigmentation
25. Which bacteria are inhibited by gentamicin in SDA?
A) Gram-negative bacteria
B) Gram-positive bacteria
C) Acid-fast bacilli
D) All bacteria
26. What is the heating time for 200 mL SDA in a water bath?
A) 5 minutes
B) 45 minutes
C) 2 hours
D) 10 seconds
27. Which fungus is unsafe for teaching labs?
A) Saccharomyces cerevisiae
B) Aspergillus fumigatus
C) Penicillium camemberti
D) Rhizopus stolonifer
28. What is the reverse color of Aspergillus fumigatus on SDA?
A) Black
B) Pale yellow
C) Red
D) Blue
29. What happens if SDA is overheated during preparation?
A) Turns black
B) Becomes too soft
C) Loses dextrose
D) Kills all fungi
30. Which organism requires thiamine for growth on SDA?
A) Candida albicans
B) Trichophyton verrucosum
C) Aspergillus niger
D) Penicillium spp.
