15.3 Acid-Fast Bacteria

The Acid-Fast Cell Wall

The defining feature of acid-fast bacteria is a cell wall rich in mycolic acids — long-chain (C60–C90) branched lipids covalently bound to arabinogalactan, which itself links to the peptidoglycan layer. This waxy coat has five practical consequences the PA-CAT expects you to know:

1. Poor Gram staining — the waxy wall blocks crystal violet; organisms appear as vague Gram-positive or unstained 'ghosts.' Use Ziehl-Neelsen (hot carbol fuchsin, acid-alcohol decolorization) or auramine-rhodamine fluorescent stain instead.
2. Resistance to drying, disinfectants, and antibiotics — mycobacteria survive in dried sputum and resist many beta-lactams.
3. Slow growth — M. tuberculosis divides every 18–24 hours (vs. ~20 min for E. coli), so cultures take 2–6 weeks on Lowenstein-Jensen agar or 1–2 weeks in liquid media (MGIT).
4. Intracellular persistence — the wall shields the organism from lysosomal enzymes and cationic peptides inside macrophages.
5. Cord factor (trehalose dimycolate) — causes serpentine cording in virulent M. tuberculosis and contributes to virulence by inhibiting macrophage migration.

Stain Hierarchy

Mycobacterium tuberculosis

M. tuberculosis is transmitted by airborne droplet nuclei (1–5 μm) generated by coughing, sneezing, or singing. After inhalation, bacilli reach the alveoli where they are phagocytosed by alveolar macrophages but resist killing thanks to their mycolic-acid wall and sulfatides. They replicate within macrophages and spread via lymphatics to regional (hilar) nodes, forming the Ghon complex — the peripheral subpleural granuloma plus enlarged hilar node. In most immunocompetent hosts, cell-mediated immunity (Th1, IFN-γ, TNF-α) walls off the organism in a caseating granuloma with central necrosis; viable bacilli persist for years in a latent state.

Latent vs. Active TB

Mantoux tuberculin skin test (TST) uses 5 TU PPD intradermally; induration ≥5 mm is positive in immunocompromised/contacts, ≥10 mm in moderate-risk, ≥15 mm in low-risk. IGRA (QuantiFERON-TB Gold) measures IFN-γ release from sensitized T cells exposed to ESAT-6 and CFP-10 antigens; it does not cross-react with BCG vaccine and requires only one visit.

Tuberculosis Pathogenesis — Step by Step

1. Inhalation of droplet nuclei → alveolar deposition.
2. Uptake by alveolar macrophages (occasionally via complement or surfactant receptors).
3. Intracellular replication; macrophage apoptosis or necrosis releases bacilli.
4. Dendritic cells prime CD4+ Th1 cells in draining lymph nodes.
5. IFN-γ activates macrophages; TNF-α recruits monocytes; granuloma forms.
6. Caseation (cheesy necrosis) provides a hypoxic, lipid-rich niche for persistence.
7. Reactivation (years later, with waning immunity) → apical cavitation → contagious disease.

Mycobacterium leprae and Hansen Disease

M. leprae cannot be grown on artificial media and has the longest known doubling time (~14 days). It infects Schwann cells via binding to α-dystroglycan and prefers cooler tissues (skin, peripheral nerves, anterior eye, testes). Disease manifests along a spectrum governed by host cell-mediated immunity:

Lepromatous leprosy carries a high burden of bacilli and is more transmissible via prolonged close contact; tuberculoid is paucibacillary and less contagious. Skin biopsy with Fite-Faraco stain and slit-skin smears confirm the diagnosis. Multi-drug therapy (dapsone, rifampin, clofazimine) prevents resistance.

Nontuberculous Mycobacteria (NTM)

Important NTM include M. avium-intracellulare (disseminated disease in advanced AIDS; prophylaxis with azithromycin when CD4 <50), M. kansasii (TB-like pulmonary disease), and the rapid growers M. abscessus and M. fortuitum (skin/soft-tissue infections post-procedure, catheter infections). Rapid growers produce colonies within 7 days, unlike M. tuberculosis.

Treatment: The RIPE Regimen

Standard therapy is 2 months of RIPE followed by 4 months of rifampin + INH (continuation phase) for drug-susceptible pulmonary TB. Directly observed therapy (DOT) improves adherence.