Acid-fast bacilli seen on sputum smear; which disease?

• A. Tuberculosis (Mycobacterium tuberculosis)
• B. Nocardiosis
• C. Legionnaires' disease
• D. Pneumocystis jirovecii pneumonia

Answer: (A)

Acid-fast staining is used to identify organisms with a waxy, mycolic acid–rich cell wall. Mycobacterium tuberculosis, the cause of tuberculosis, has this characteristic cell wall, so acid-fast bacilli seen on a sputum smear most strongly point to TB. The stain ( Ziehl-Neelsen or similar) makes these bacteria appear red because they resist decolorization by acid after staining.

Other organisms listed do not fit the finding as neatly. Nocardia can be weakly acid-fast on special, modified stains and is a branching filamentous bacterium, but the classic, most expected organism depicted by acid-fast bacilli in sputum is Mycobacterium tuberculosis. Legionella and Pneumocystis jirovecii are not acid-fast; Legionella is a Gram-negative rod, and Pneumocystis shows cysts or organisms on silver stains or immunofluorescence, not acid-fast staining.