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: -

• Mycobacteria
  • Mycobacterium tuberculosis - causes tuberculosis
  • Non-tuberculous mycobacteria
    • Mycobacterium bovis
    • Mycobacterium africanum
    • Mycobacterium microti
    • Mycobacterium leprae - causes leprosy
    • Mycobacterium avium (also known as Mycobacterium avium intracellulare and Mycobacterium avium complex)
    • Mycobacterium kansasii
• Nocardia - causes nocardiosis
• Nocardia asteroides
• Nocardia brasiliensis