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-fast bacilli include: -
