ZiehlNeelsen

Ziehl-Neelsen stain is a classic bacteriological staining method used to detect acid-fast bacilli in clinical specimens. It is named after the German scientists Franz Ziehl and Friedrich Neelsen who developed the technique in the late 19th and early 20th centuries. The stain is primarily used to identify mycobacteria, including Mycobacterium tuberculosis and Mycobacterium leprae, as well as some Nocardia species.

The method relies on the waxy cell wall of mycobacteria, which contains mycolic acids that make cells resistant to decolorization. In the Ziehl-Neelsen procedure, the sample is stained with carbol fuchsin (a phenol-based red dye) and heat is applied to facilitate penetration of the dye. After washing, the smear is treated with an acid-alcohol decolorizer. Non–acid-fast organisms are decolorized, while acid-fast bacteria retain the red color. A counterstain, typically methylene blue or brilliant green, is then applied to provide contrast, so acid-fast organisms appear red against a blue/green background.

Applications include rapid screening of sputum and other body fluids for tuberculosis and other mycobacterial infections, as well as staining tissue sections in pathology. The technique is inexpensive and rapid, making it widely used in resource-limited settings. Limitations include variable sensitivity, especially in paucibacillary samples, and the need for careful interpretation. Some Nocardia species may be variably acid-fast, and newer culture and molecular methods complement the Ziehl-Neelsen stain in diagnosis.