Microbiology: Specimen Processing and Stains Flashcards

A throat swab for group A Streptococcus should be refrigerated if processing is delayed more than how long?

Answer: 2 hours

If processing is delayed beyond 2 hours, throat swabs should be refrigerated or placed in transport medium to preserve organism viability.

During the Gram stain, which step distinguishes gram-positive from gram-negative bacteria?

Answer: Decolorization with acetone-alcohol

Decolorization removes the crystal violet-iodine complex from gram-negative bacteria (thin peptidoglycan), while gram-positive bacteria retain it.

Which specimen type is NOT appropriate for anaerobic culture?

Answer: Bronchoalveolar lavage

BAL is contaminated with upper respiratory flora including normal anaerobes, making anaerobic culture results uninterpretable.

The PAS (periodic acid-Schiff) stain colors fungal cell walls because it reacts with which component?

Answer: Chitin and glycogen polysaccharides

Periodic acid oxidizes vicinal diols in polysaccharides like chitin to aldehydes, which then react with Schiff reagent to produce magenta color.

A urethral discharge swab for Neisseria gonorrhoeae must be inoculated directly at bedside because this organism is sensitive to:

Answer: Drying and temperature changes

N. gonorrhoeae dies rapidly when exposed to drying, cold, and delay, requiring immediate bedside inoculation onto warm Thayer-Martin agar.

Acridine orange stain is used in microbiology to detect organisms in blood because it:

Answer: Stains bacterial DNA orange-red and differentiates from RBCs

Acridine orange intercalates into nucleic acids and fluoresces orange-red for bacteria vs. green for mammalian cells under UV, aiding detection in blood.

Which stain is used to identify Legionella pneumophila in tissue sections?

Answer: Dieterle silver stain

Legionella stains poorly with Gram stain and is best visualized using silver impregnation stains like Dieterle or Warthin-Starry in tissue.