An acid-fast bacillus that has mycolic acid in its cell wall and can be seen in a palisade arrangement is which organism?

• A. Mycobacterium tuberculosis
• B. Mycobacterium leprae
• C. Borrelia burgdorferi
• D. Treponema pallidum

Answer: (B)

Acid-fastness comes from the thick, waxy cell wall enriched with mycolic acids found in certain bacteria. This makes them resistant to decolorization and they stain as red bacilli with acid-fast stains. Among the organisms listed, the one that is an acid-fast bacillus and is classically described in granulomatous lesions with macrophages arranged in a palisading pattern is Mycobacterium leprae, the agent of Hansen’s disease. This palisading arrangement reflects how the immune response organizes around infected cells in leprosy lesions. The other organisms shown—spirochetes—do not have mycolic acid in their walls and are not acid-fast, so they don’t fit this description.