Which organism is acid-fast and commonly causes tuberculosis?

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

Answer: (A)

Acid-fast staining highlights bacteria with a waxy, lipid-rich cell wall that resists decolorization; the classic example is Mycobacterium tuberculosis. Its cell wall contains mycolic acids that trap the red dye carbol fuchsin, so after acid-alcohol wash it remains stained, indicating acid-fast. This property is closely linked to tuberculosis, caused by Mycobacterium tuberculosis. The other organisms listed are not acid-fast and cause different diseases: Streptococcus pneumoniae is a Gram-positive diplococcus causing pneumonia and meningitis but lacks the mycolic acid–rich wall; Staphylococcus aureus is a Gram-positive cocci linked to skin and systemic infections; Escherichia coli is a Gram-negative rod associated with GI and urinary infections. Thus the organism associated with TB and acid-fast staining is Mycobacterium tuberculosis.