## Mycobacterium tuberculosis: Structural and Biochemical Features ### Correct Answer: Gram-positive bacillus with thick peptidoglycan cell wall **Key Point:** M. tuberculosis is NOT a typical Gram-positive organism. It is an acid-fast bacillus (AFB) with a unique mycolic acid-rich cell wall that does not stain by Gram's method. ### Why the Other Options Are Correct | Feature | Details | |---------|----------| | **Acid-fast staining** | Retains carbol fuchsin after acid-alcohol decolorization due to mycolic acids in the cell wall; visualized by Ziehl-Neelsen or auramine-rhodamine stains | | **Obligate aerobe** | Requires oxygen; replicates in well-aerated apical-posterior lung segments; explains predilection for upper lobes | | **Biochemical markers** | Produces niacin (accumulates in culture media), reduces nitrates to nitrites; these are key identification tests | ### Cell Wall Architecture **High-Yield:** M. tuberculosis cell wall is composed of: 1. **Mycolic acids** — long-chain fatty acids (C~60~–C~90~) unique to mycobacteria 2. **Arabinogalactan** — polysaccharide linked to peptidoglycan 3. **Peptidoglycan** — thin layer (unlike Gram-positive bacteria) This mycolic acid-rich outer layer: - Prevents Gram stain uptake - Confers acid-fastness - Provides resistance to antibiotics and immune killing - Accounts for slow growth (doubling time ~15–20 hours) **Clinical Pearl:** The mycolic acid layer is why M. tuberculosis survives intracellularly in macrophages and resists many antibiotics — it is hydrophobic and impermeable to most drugs. **Mnemonic: AFB-MAC** — Acid-Fast Bacillus with Mycolic Acid-rich Cell wall (not Gram-positive).
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