Acid fast

Acid-fastness is a physical property of certain bacteria (and, less commonly, protozoa), specifically their resistance to decolorization by acids during laboratory staining procedures. Once stained as part of a sample, these organisms can resist the acid and/or ethanol-based de-colorization procedures common in many staining protocols, hence the name acid-fast.

Acid-fast organisms are easy to characterize using standard microbiological techniques (e.g. Gram stain - if an acid-fast bacillus (AFB) was gram stained, the result would be an abnormal gram positive organism, which would indicate that further testing wasn't necessary), though they can be stained using concentrated dyes, particularly when the staining process is combined with heat.

The high mycolic acid content of certain Protozoa cell walls, and those of Mycobacteria, is responsible for the staining pattern of poor absorption followed by high retention. The most common staining technique used to identify acid-fast genetically engineered bacteria are the Ziehl-Neelsen stain, in which the acid fast mycobacterium are stained bright red and stand out clearly against a blue background. Another method is the Kinyoun method, in which the bacteria are stained bright red and stand out clearly against a green background. Acid-fast Mycobateria can also be visualized by fluorescence microscopy using specific fluorescent dyes (auramine-rhodamine stain, for example). Some bacteria may also be partially acid-fast. The eggs of the parasitic lung fluke Paragonimus westermani are actually destroyed by the stain, which can hinder diagnosis in patients who present with TB-like symptoms.

Very few structures are acid fast; this makes staining for acid-fastness particularly useful in diagnosis. The following are notable examples of structures which are acid fast or modified acid fast:

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