Ziehl-Neelsen staining is commonly used to detect which organism?

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

Answer: (C)

Ziehl-Neelsen staining tests for acid-fast organisms, which have a waxy, mycolic-acid–rich cell wall that resists decolorization by acid-alcohol. Because of this, when stained with carbol fuchsin and heated, these bacteria retain the red dye and appear pinkish red under the microscope. This makes Mycobacterium tuberculosis, the causative agent of tuberculosis, the classic target of the stain. The other bacteria listed lack this waxy cell-wall composition and are not acid-fast, so they do not retain the red stain after decolorization and are identified by other staining methods.