Which organism is an acid-fast bacillus?

• A. Leptospira grippotyphosa
• B. Escherichia coli
• C. Streptococcus equi
• D. Mycobacterium tuberculosis

Answer: (D)

Acid-fast bacilli have a waxy, lipid-rich cell wall loaded with mycolic acids that makes them resistant to decolorization by acid alcohol during staining. Because of this, when you perform an acid-fast stain like Ziehl-Neelsen, these organisms retain the primary red dye and appear red against a blue counterstain. Mycobacterium tuberculosis is the classic example of an acid-fast bacillus, reflecting that distinctive cell wall.

The other organisms listed do not share this feature: Leptospira grippotyphosa is a spirochete with a different cell-wall structure; Escherichia coli is a Gram-negative rod that loses the dye with acid alcohol; Streptococcus equi is a Gram-positive coccus, likewise not acid-fast.