acidfastStreiche

Acid-fast staining is a differential colorimetric technique used in microbiology to detect acid-fast organisms, most notably mycobacteria such as Mycobacterium tuberculosis. The method exploits the waxy, lipid-rich cell wall that contains mycolic acids, which makes these bacteria resistant to decolorization by acid-alcohol and enables them to retain a primary dye.

Two main approaches are commonly used: the Ziehl-Neelsen method and the Kinyoun method. In the Ziehl-Neelsen technique, a smear is flooded with carbol fuchsin and heated to facilitate dye penetration, then decolorized with acid-alcohol and counterstained with methylene blue. Acid-fast organisms remain red, while non-acid-fast cells take up the blue counterstain. The Kinyoun method is a cold stain that uses a higher concentration of phenol and similar decolorization steps but without heating. Both methods yield acid-fast organisms as bright red or pink against a contrasting background.

Applications of acid-fast staining include initial screening for tuberculosis and other mycobacterial infections in clinical specimens such as sputum, tissue imprints, and body fluids. Nocardia species can be variably acid-fast, while some non-mycobacterial bacteria are not acid-fast. The technique is valued for its speed and relative simplicity, but it requires careful technique and proper sample quality. False negatives can occur with scanty organisms or over-decolorization, and interpretation is enhanced when used alongside culture, molecular tests, and clinical context.

Safety considerations include handling infectious specimens under appropriate biosafety conditions, as with other diagnostic stains.