AP-Micro Microbiology: Specimen Processing and Stains 3 — Questions and Answers
Question 1: Which stain is the gold standard for identifying Pneumocystis jirovecii in bronchoalveolar lavage specimens?
- Gram stain
- Giemsa stain
- Gomori methenamine silver (GMS) stain (Correct answer)
- Ziehl-Neelsen stain
Correct answer: Gomori methenamine silver (GMS) stain
GMS stain blackens the cell wall of fungal organisms including Pneumocystis jirovecii cysts against a green background.
Question 2: A wound swab is collected and transported in Amies transport medium. What is the primary advantage of this medium over Stuart's medium?
- It contains charcoal to absorb toxic fatty acids (Correct answer)
- It provides nutrients to allow organism growth
- It selectively inhibits gram-positive flora
- It maintains anaerobic conditions
Correct answer: It contains charcoal to absorb toxic fatty acids
Amies medium contains charcoal that neutralizes toxic fatty acids on swabs, improving survival of fastidious organisms.
Question 3: Which staining technique uses fluorescent dyes and is useful for rapid screening of mycobacteria in sputum?
- Auramine-rhodamine fluorochrome stain (Correct answer)
- Calcofluor white stain
- Acridine orange stain
- Periodic acid-Schiff stain
Correct answer: Auramine-rhodamine fluorochrome stain
Auramine-rhodamine binds mycolic acids and fluoresces under UV light, allowing faster low-power scanning for acid-fast bacilli.
Question 4: A stool specimen for ova and parasite (O&P) examination should ideally be preserved in which fixative?
- Formalin 10% only
- Polyvinyl alcohol (PVA) only
- Formalin plus PVA (two-vial system) (Correct answer)
- Thioglycolate broth
Correct answer: Formalin plus PVA (two-vial system)
The two-vial system uses formalin for concentration techniques and PVA for permanent stained smears to maximize detection.
Question 5: Calcofluor white stain is particularly useful for detecting which organisms?
- Mycobacteria
- Protozoa
- Fungi and Acanthamoeba cysts (Correct answer)
- Gram-negative bacteria
Correct answer: Fungi and Acanthamoeba cysts
Calcofluor white binds to chitin in fungal cell walls and cellulose in Acanthamoeba cysts, fluorescing bright white-blue under UV.
Question 6: What is the recommended minimum volume of blood for each blood culture bottle in adults?
- 1–2 mL
- 5–10 mL
- 10–20 mL (Correct answer)
- 25–30 mL
Correct answer: 10–20 mL
Inoculating 10–20 mL per bottle significantly increases the sensitivity of blood cultures for detecting bacteremia.
Question 7: Spores of Clostridium species appear as which color after a Schaeffer-Fulton endospore stain?
- Blue-black
- Red (Correct answer)
- Purple
- Colorless
Correct answer: Red
In the Schaeffer-Fulton method, spores retain malachite green and then appear red after safranin counterstaining, while vegetative cells stain pink-red.
AP-Micro Microbiology: Specimen Processing and Stains 3
Which stain is the gold standard for identifying Pneumocystis jirovecii in bronchoalveolar lavage specimens?