MedCompanion

Mycobacterium tuberculosis

MTB

Morphology: Slow-growing, acid-fast bacillus (AFB, Ziehl-Neelsen + auramine-rhodamine). Obligate aerobe; doubling time ~24 h. Waxy mycolic-acid cell wall confers acid-fast staining and resistance to many drugs.

MTBMycobacterium

Typical drugs

  1. #1Rifampin— Backbone of every regimen. Concentration-dependent; orange secretions; potent CYP inducer.
  2. #2Isoniazid— Bactericidal early phase. Always with pyridoxine in adults. Hepatotoxicity dose-limiting.

Empiric therapy when resistant

Standard active TB: RIPE × 2 months → RH × 4 months (HRZE/HR, 6 months total) for drug-sensitive disease. For MDR-TB: WHO BPaL (bedaquiline + pretomanid + linezolid, 6 months) for select cases. Pre-XDR / XDR-TB: highly individualized; consult TB clinic + ID.

Resistance mechanisms

  • altered-target

    katG / inhA mutations → INH resistance

    Example: Most common single-drug resistance. Treat with longer / broader regimen guided by susceptibility.

  • altered-target

    rpoB mutations → rifampin resistance (often surrogate for MDR-TB)

    Example: MDR-TB = INH + RIF resistant. Requires WHO BPaL or longer regimens; treatment 6–18 months.

  • altered-target

    embB, pncA, fluoroquinolone resistance → XDR-TB

    Example: XDR = MDR + resistance to FQ + at least one second-line injectable. Limited options; consult TB specialist.

Resistance notes

INH resistance ~7% of new US isolates; MDR-TB <2% in US but much higher in some immigrant-origin populations. DST (drug susceptibility testing) mandatory for every culture-positive isolate. Genotype-based assays (Xpert MTB/RIF) detect rifampin resistance within hours.

Pearls

Active vs latent: active TB has symptoms + radiographic findings + (usually) positive smear/culture; latent TB has positive IGRA/TST + no symptoms + normal CXR. DOT (directly observed therapy) recommended for active TB to ensure adherence. Pott's disease = vertebral TB (chronic back pain, kyphosis). Drug-induced hepatotoxicity — INH + RIF + PZA all hepatotoxic; baseline + monthly LFTs; stop at AST/ALT >3× ULN with symptoms. TB-HIV co-infection: start ART within 2 weeks of TB tx (8 weeks if CD4 >50). MDR-TB classic vignette: prior TB treatment + immigrant from MDR-endemic region + culture growth on standard RIPE — must repeat DST.

References