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Microbiology

Antimicrobial Therapy

This section provides a brief overview of the antimicrobial therapy for M. tuberculosis, M. avium complex (MAC), M. kansasii, M. marinum, and rapid growers (RGM)

The Foundation: Drug Susceptibility Testing (DST)

Regardless of the organism, DST is a cornerstone. It helps determine which drugs are most likely to be effective against the specific isolate causing the infection. This is especially critical because resistance patterns vary significantly among these organisms

Mycobacterium tuberculosis

  • Goal: Eradicate the infection, prevent relapse, and prevent transmission
  • Standard Regimen: A 6-month course of combination therapy
    • Intensive Phase (2 months): Isoniazid (INH), Rifampin (RIF), Pyrazinamide (PZA), Ethambutol (EMB)
    • Continuation Phase (4 months): Isoniazid (INH), Rifampin (RIF)
  • Drug Resistance: A major concern. MDR-TB (resistant to INH and RIF) and XDR-TB (resistant to INH, RIF, fluoroquinolones, and at least one injectable second-line drug) require complex, prolonged treatment with second-line drugs
  • Key Strategies: Directly Observed Therapy (DOT) to ensure adherence, rapid diagnosis, and DST

Mycobacterium avium complex (MAC)

  • Goal: Control symptoms, prevent disease progression, and improve quality of life
  • Treatment: Combination therapy with a macrolide (clarithromycin or azithromycin) as the foundation
    • Typically combined with ethambutol and rifamycin (rifampin or rifabutin)
    • Aminoglycosides may be added in severe cases
  • Duration: Prolonged, often lasting at least 12 months after sputum cultures turn negative
  • Special Considerations: Drug interactions (especially with HIV medications), macrolide resistance

Mycobacterium kansasii

  • Goal: Eradicate the infection
  • Treatment: Rifampin-based regimen
    • Isoniazid (INH), rifampin, and ethambutol
  • Duration: At least 12 months after sputum cultures turn negative
  • Key Feature: Generally susceptible to rifampin, making treatment more straightforward

Mycobacterium marinum

  • Goal: Eradicate the infection, especially in skin and soft tissues
  • Treatment: Options include tetracyclines (doxycycline, minocycline), macrolides (clarithromycin, azithromycin), fluoroquinolones (ciprofloxacin, levofloxacin), rifampin, ethambutol, or TMP/SMX
  • Duration: Typically 3-6 months, depending on severity
  • Special Considerations: Often associated with aquatic exposures (fish tanks, swimming pools)

Eapidly Growing Mycobacteria (RGM)

  • Goal: Eradicate the infection, which can be challenging due to resistance
  • Species: M. abscessus group, M. chelonae, M. fortuitum
  • Treatment: Highly variable depending on the species and DST results
    • M. abscessus: Often requires a combination of amikacin, cefoxitin or imipenem, and clarithromycin/azithromycin
    • Other RGMs: May be susceptible to macrolides, aminoglycosides, carbapenems, linezolid, fluoroquinolones, or tetracyclines
  • Key Challenges: M. abscessus is notoriously difficult to treat, inducible macrolide resistance
  • General Principles: Surgical intervention is often necessary, prolonged treatment durations, inhaled amikacin may be useful for pulmonary infections