Which organism is acid-fast and commonly causes tuberculosis?

• A. Streptococcus pneumoniae
• B. Escherichia coli
• C. Staphylococcus aureus
• D. Mycobacterium tuberculosis

Answer: (D)

Acid-fast staining relies on a cell wall rich in mycolic acids, which makes certain bacteria resistant to decolorization by acid-alcohol and able to hold onto the red primary stain. This property marks the group of acid-fast bacilli, with Mycobacterium tuberculosis being the classic example. Because TB is caused by this organism, its cells appear red after an acid-fast stain, helping it stand out from other bacteria that don’t retain the stain.

The other organisms listed—Streptococcus pneumoniae, Escherichia coli, and Staphylococcus aureus—lack that waxy, lipid-heavy cell wall and do not stay red after acid-alcohol treatment; they are identified by different staining and diagnostic methods.