AP-Micro Microbiology: Specimen Processing and Stains 2 — Questions and Answers
Question 1: Which fixative is commonly used to preserve bacterial morphology before Gram staining?
- Formalin
- Heat fixation (Correct answer)
- Glutaraldehyde
- Ethanol immersion
Correct answer: Heat fixation
Heat fixation kills bacteria and adheres them to the slide without distorting morphology significantly.
Question 2: A sputum sample is collected from a patient suspected of having tuberculosis. What is the preferred transport medium?
- Thioglycolate broth
- Stuart's transport medium
- No medium — direct smear within 1 hour (Correct answer)
- Löwenstein-Jensen slant at 4°C
Correct answer: No medium — direct smear within 1 hour
Sputum for AFB smear and culture should be processed promptly or refrigerated without transport medium to avoid overgrowth.
Question 3: The Ziehl-Neelsen stain uses carbol fuchsin as the primary stain. What is its function?
- It stains the peptidoglycan layer red
- It penetrates the waxy mycolic acid cell wall when heated (Correct answer)
- It acts as a mordant to fix crystal violet
- It decolorizes gram-negative organisms
Correct answer: It penetrates the waxy mycolic acid cell wall when heated
Carbol fuchsin, aided by heat, penetrates the lipid-rich mycolic acid coat of mycobacteria to stain them red.
Question 4: Which decolorizer is used in the acid-fast stain procedure?
- Acetone-alcohol (95%)
- Acid-alcohol (3% HCl in 95% ethanol) (Correct answer)
- Lugol's iodine
- Safranin
Correct answer: Acid-alcohol (3% HCl in 95% ethanol)
Acid-alcohol (3% HCl in 95% ethanol) removes carbol fuchsin from non-acid-fast organisms but not from mycobacteria.
Question 5: A CSF specimen arrives in the laboratory with a 4-hour delay at room temperature. Which organism is most at risk of being missed due to this delay?
- Cryptococcus neoformans
- Neisseria meningitidis (Correct answer)
- Streptococcus pneumoniae
- Escherichia coli
Correct answer: Neisseria meningitidis
Neisseria meningitidis is extremely fragile and dies rapidly outside the body, making delays in CSF processing critical.
Question 6: What does the counterstain safranin reveal in the Gram stain procedure?
- Gram-positive organisms staining pink
- Gram-negative organisms staining pink/red (Correct answer)
- Fungal elements staining red
- Capsules staining pink
Correct answer: Gram-negative organisms staining pink/red
Safranin counterstains gram-negative bacteria pink/red after they lose crystal violet during decolorization.
Question 7: India ink preparation of CSF is used to visualize which structure?
- Bacterial flagella
- Fungal capsules (Correct answer)
- Spore coats
- Viral inclusion bodies
Correct answer: Fungal capsules
India ink particles are excluded by the polysaccharide capsule of Cryptococcus neoformans, creating a clear halo around the organism.
AP-Micro Microbiology: Specimen Processing and Stains 2
Which fixative is commonly used to preserve bacterial morphology before Gram staining?