Canadian Society for Medical Laboratory Science (CSMLS) Practice Exam

Which organisms are demonstrated with a ZN stain?

1. Ova and parasites

2. Yeast

3. Acid fast bacilli

4. Fungus

The correct answer is: Acid fast bacilli

The Ziehl-Neelsen (ZN) stain is a specific staining technique used primarily to identify acid-fast bacilli, notably Mycobacterium species, which cause diseases such as tuberculosis. This stain exploits the unique waxy cell wall of these organisms, allowing them to retain the primary dye despite decolorization steps that would wash out other types of organisms. The specific properties of acid-fast bacteria make them resistant to standard staining procedures, which is why the ZN stain is so critical in microbiological diagnostics. It provides a vivid contrast, typically showing acid-fast bacilli as bright red or pink against a blue or green background in the microscopy field, making them easily identifiable. This focus on the unique characteristics of acid-fast bacilli allows healthcare professionals and laboratory technicians to quickly diagnose infections associated with these microorganisms, which is key in patient treatment and management.