Acid-fastness is a physical property of certain bacteria, specifically their resistance to decolorization by acids during staining procedures.[1][2]
Acid-fast organisms are difficult to characterize using standard microbiological techniques (e.g. Gram stain - if you gram stained an acid-fast bacillus (AFB) the result would be an abnormal gram positive organism, which would indicate further testing), 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—hence the name acid-fast.[2]
The high mycolic acid content of certain bacterial cell walls, like 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 bacteria is the Ziehl-Neelsen stain, in which the acid fast bacilli 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 bacteria can also be visualized by fluorescence microscopy using specific fluorescent dyes (auramine-rhodamine stain, for example).[3] Some bacteria may also be partially acid-fast.
Very few structures are acid fast; this makes staining for acid-fastness particularly useful in diagnosis.
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