Which organism is characterized by acid-fast staining?

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

Answer: (C)

Acid-fast staining targets organisms with a waxy, lipid-rich cell wall due to mycolic acids. This thick, lipid layer holds onto the stain (carbol fuchsin) even after treatment with acid-alcohol, so acid-fast organisms appear red under the stain. Mycobacterium tuberculosis has this waxy wall, making it classicly acid-fast and easily identified by this method. Other common bacteria like Staphylococcus aureus, Escherichia coli, and Streptococcus pneumoniae lack that thick mycolic-acid–rich barrier, so they lose the stain during decolorization and do not appear acid-fast. This distinction helps clinicians quickly spot TB-causing organisms in specimens.