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Importance of Acid Fast Staining

Acid-fast staining is the only technique capable of characterizing Mycobacterium, the causative agent of tuberculosis and leprosy. During the 17th and 18th centuries, Mycobacterium tuberculosis was called the "White Plague" in Europe. During this time, one in four deaths was caused by M. tuberculosis. Tuberculosis remains the leading cause of death in the world from any single infectious agent; the disease infects 1.8 billion people every year--a third of the world's population.

  1. Features

    • Acid-fast staining is a differential stain, one that stains based on specific cellular composition. An acid-fast stain distinguishes organisms with waxy cell walls. These organisms repel stain and, once stained, resist decolorization with acid-alcohol, thus the name "acid fast." Their cell walls are lipid-filled capsules composed of mycolic acid, which makes them difficult to characterize with standard microbiological techniques.

    Zielh-Neelsen Stain

    • The most common acid-fast stain is the Zielh-Neelsen stain. Acid-fast bacteria are stained red with a dye called carbol fuchsin. This is called the primary stain and it contains phenol, which allows the dye to enter the cells. In addition, heat is often used during this step to improve cell penetration. Next, the cells are washed with acid-alcohol. Any cells that are acid-fast will retain the stain, and others will be decolorized. Finally, a counter-stain of methylene blue is applied to stain any other cells. At the end, bright red acid-fast cells stand out against a blue background.

    Kinyoun Stain

    • Kinyoun stain is a modified procedure for staining acid-fast organisms. Here, a higher concentration of carbol fuchsin is used, so heat is not required in the primary step. The counter-stain in this procedure is either methylene blue or brilliant green. If brilliant green is used, bright red acid-fast bacilli will stand out clearly against a green background.

    Acid-Fast Microorganisms

    • Acid-fast organisms include Mycobacteria, Nocardia, Cryptosporidium parvum, isospora, cyclospora cysts, Legionella micdadei and the head of a sperm.

    History

    • The acid-fast stain was developed by bacteriologist Franz Ziehl (1859-1926) and pathologist Friedrich Neelsen (1854-1894), both German. Neelsen died when he was only 44 years old; his work with dangerous bacteria was eventually fatal.

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