The Ziehl–Neelsen (ZN) stain is a differential staining technique for detecting acid‑fast bacteria. It exploits the high lipid content and mycolic acids in the cell envelopes of organisms such as *Mycobacterium tuberculosis*, which retain the primary dye despite decolorization with acidic solvents. Cells that do not have these waxy envelopes lose the dye and take up a counterstain, allowing rapid visual differentiation under a light microscope.
Principle and procedure
In the ZN method, heat is used to drive the lipid‑soluble primary stain, typically carbol fuchsin, into the cell wall. A fixed smear is flooded with carbol fuchsin and gently heated until steaming, then allowed to stand so the dye penetrates. After rinsing, the slide is treated with a decolorizing agent, commonly acid–alcohol (3 % hydrochloric acid in ethanol). Acid‑fast organisms resist decolorization and remain red. The smear is then counterstained with a contrasting dye such as methylene blue or malachite green. Non‑acid‑fast cells lose the primary stain and become blue or green. Variations of the method include the Kinyoun cold technique, which uses higher concentrations of phenol to avoid heating, and fluorescent stains like auramine‐rhodamine for increased sensitivity.
Diagnostic relevance and examples
ZN staining is widely used in clinical microbiology to detect *M. tuberculosis* in sputum smears for pulmonary tuberculosis diagnosis. It also assists in identifying other acid‑fast bacteria including *Mycobacterium leprae*, *Nocardia* species (partial acid‑fastness), and oocysts of some protozoa. In positive smears, acid‑fast bacilli appear as slender, slightly curved red rods against a blue or green background. Accurate interpretation requires examination of multiple fields at 1000× magnification and correlation with clinical findings; false negatives can occur with low bacterial load, and culture or molecular assays remain necessary for confirmation and speciation. The ZN stain remains a cornerstone method for acid‑fast microscopy because of its simplicity and low cost. It provides rapid, preliminary evidence of mycobacterial infection and guides early patient management, especially in resource‑limited settings. Related Terms: Acid-fast, Mycobacterium tuberculosis, Carbol fuchsin, Kinyoun stain, Auramine O