Acid-fast

Acid-fastness is a physical property of some bacteria referring to their resistance to decolorization by acids during staining procedures.

Acid-fast organisms are difficult to characterize using standard microbiological techniques (e.g. Gram staining), though they can be stained using concentrated dyes, particularly when the staining process is combined with heat. Once stained, these organisms resist the dilute acid and/or ethanol-based de-colorization procedures common in many staining protocols&mdash;hence the name acid-fast.

The high mycolic acid content of certain bacterial cell walls, like those of Mycobacterium, is responsible for the staining pattern of poor absorption followed by high retention. The most common staining technique used to identify acid-fast bacteria is the Ziehl-Neelsen stain, in which the bacteria are stained bright red and stand out clearly against a blue background. Acid-fast bacteria can also be visualized by fluorescence microscopy using specific fluorescent dyes (auramine-rhodamine stain, for example). Some bacteria may also be partially acid-fast.

Only a few types of bacteria are acid-fast, notably members of the Mycobacterium. Most are rod-shaped, termed acid-fast bacilli (AFB), but other forms also occur. Medically the most important AFB is Mycobacterium tuberculosis. Genera like Nocardia and Corynebacterium are also notable examples.

Online protocol examples

 * Ziehl-Neelsen protocol (PDF format).
 * Alternate Ellis & Zabrowarny method for staining AFB.

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