A bacillus is a rod-shaped bacterium: an acid-fast bacillus (or AFB) is a rod-shaped bacterium which, when stained with certain compounds, retains that stain despite treatment with an acidic solution. The most medically important acid-fast bacillus is Mycobacterium tuberculosis. The term acid_fast bacillus dates from 1903.
The most common staining technique used to identify AFB is the Ziehl_Neelsen stain, in which AFB are stained a bright red which stands out clearly against a blue background. Acid_fast bacilli can also be visualized by fluorescent microscopy, and by auramine-rhodamine stain.
Acid-fast bacteria include some organisms that are not bacilli; some bacteria are termed partially acid-fast.
Acid-fastness is a physical property of some bacteria referring to their resistance to decolorization by acids during staining procedures.
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.
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