Which organism is the classic example that stains acid-fast?

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

Answer: (D)

Acid-fast staining relies on a waxy, lipid-rich cell wall that contains mycolic acids. This makes the bacteria resistant to decolorization by acid-alcohol, so they retain the red primary stain after washing. The classic example is Mycobacterium tuberculosis, which has a high mycolic acid content and remains pink/red when viewed after the stain. The other organisms listed lack this waxy cell wall, so they lose the initial stain during decolorization and take up the counterstain, appearing non-acid-fast. Therefore, the organism that stains acid-fast is Mycobacterium tuberculosis.