Microbiology: Specimen Processing and Stains Flashcards

Which fixative is commonly used to preserve bacterial morphology before Gram staining?

Answer: Heat fixation

Heat fixation kills bacteria and adheres them to the slide without distorting morphology significantly.

A sputum sample is collected from a patient suspected of having tuberculosis. What is the preferred transport medium?

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.

The Ziehl-Neelsen stain uses carbol fuchsin as the primary stain. What is its function?

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.

Which decolorizer is used in the acid-fast stain procedure?

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.

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?

Answer: Neisseria meningitidis

Neisseria meningitidis is extremely fragile and dies rapidly outside the body, making delays in CSF processing critical.

What does the counterstain safranin reveal in the Gram stain procedure?

Answer: Gram-negative organisms staining pink/red

Safranin counterstains gram-negative bacteria pink/red after they lose crystal violet during decolorization.

India ink preparation of CSF is used to visualize which structure?

Answer: Fungal capsules

India ink particles are excluded by the polysaccharide capsule of Cryptococcus neoformans, creating a clear halo around the organism.