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Microbiology

Rapid Growers

This section focuses on the antimicrobial therapy for infections caused by rapidly growing mycobacteria (RGM). These mycobacteria species, which include the Mycobacterium abscessus group, Mycobacterium chelonae, and Mycobacterium fortuitum, pose unique treatment challenges due to their intrinsic resistance to many antibiotics

General Principles

  • Drug Susceptibility Testing (DST): DST is essential for guiding treatment decisions for RGM infections, as susceptibility patterns can vary significantly among species and even within species
  • Species-Level Identification: Accurate species-level identification is crucial, as different RGM species have different susceptibility profiles
  • Combination Therapy: Treatment of RGM infections typically involves a combination of multiple drugs to improve efficacy and prevent the emergence of drug resistance
  • Prolonged Treatment Duration: Treatment durations for RGM infections are often longer than those for M. tuberculosis infections
  • Surgical Intervention: Surgical excision or debridement of infected tissue may be necessary in some cases, particularly for skin and soft tissue infections
  • Biofilm Formation: RGM, particularly M. abscessus, can form biofilms, which can make them more resistant to antibiotics
  • Monitoring for Adverse Effects: Anti-RGM drugs can cause a variety of adverse effects, and patients should be closely monitored during treatment
  • Route of Administration: Intravenous administration of antimicrobials is usually preferred

Mycobacterium abscessus Group

  • Treatment Challenges: M. abscessus is highly resistant to many antibiotics, making treatment particularly challenging. The M. abscessus group consists of three subspecies: M. abscessus subsp. abscessus, M. abscessus subsp. massiliense, and M. abscessus subsp. bolletii. M. abscessus subsp. massiliense is usually more susceptible to macrolides, therefore, speciation is critical
  • Recommended Treatment Approach
    • Inducible Macrolide Resistance: M. abscessus often exhibits inducible macrolide resistance, meaning that resistance develops during prolonged exposure to macrolides. To overcome this, a macrolide (clarithromycin or azithromycin) should be administered with other drugs
    • Initial Intensive Phase
      • Amikacin or tobramycin
      • Cefoxitin or imipenem
      • Clarithromycin or azithromycin (if susceptible)
    • Continuation Phase
      • Clarithromycin or azithromycin (if susceptible)
      • Linezolid
      • Moxifloxacin or tigecycline (based on susceptibility)
    • Treatment Duration: Treatment should continue for at least 12 months after clinical improvement
    • Cystic Fibrosis (CF) Patients: M. abscessus pulmonary infections are common in CF patients. Treatment is similar to that for non-CF patients, but may require prolonged intravenous therapy and inhaled amikacin
  • Drug Susceptibility Testing: DST is essential for guiding treatment decisions. Susceptibility testing should include macrolides, aminoglycosides, carbapenems, linezolid, tigecycline, and other potential agents
  • Surgical Management: Surgical excision of infected tissue or drainage of abscesses may be necessary
  • Inhaled Amikacin: Inhaled amikacin may be used as adjunctive therapy for pulmonary infections
  • Monitoring: Patients should be monitored for adverse effects of all drugs, including ototoxicity and nephrotoxicity with aminoglycosides

Mycobacterium chelonae

  • Treatment Regimen
    • M. chelonae is generally more susceptible to antibiotics than M. abscessus
    • Recommended Drugs
      • Clarithromycin or azithromycin
      • Amikacin or tobramycin
      • Linezolid
      • Ciprofloxacin or moxifloxacin
    • Treatment Duration: Treatment duration varies depending on the site and severity of the infection, but typically ranges from 4-6 months
  • Drug Susceptibility Testing: DST should be performed to guide treatment decisions
  • Surgical Management: Surgical excision of infected tissue or removal of foreign bodies may be necessary for localized infections

Mycobacterium fortuitum

  • Treatment Regimen
    • M. fortuitum is generally susceptible to several antibiotics
    • Recommended Drugs
      • Amikacin or tobramycin
      • Cefoxitin or imipenem
      • Doxycycline or minocycline
      • Ciprofloxacin or levofloxacin
      • Trimethoprim/sulfamethoxazole (TMP/SMX)
    • Treatment Duration: Treatment duration varies depending on the site and severity of the infection, but typically ranges from 3-6 months
  • Drug Susceptibility Testing: DST should be performed to guide treatment decisions
  • Surgical Management: Surgical excision of infected tissue or drainage of abscesses may be necessary for localized infections

Key Terms

  • RGM (Rapidly Growing Mycobacteria): Mycobacteria species that grow relatively quickly in culture (colonies appear within 7 days)
  • DST (Drug Susceptibility Testing): A laboratory test to determine the susceptibility of an organism to antimicrobial drugs
  • Macrolides: A class of antibiotics that includes clarithromycin and azithromycin
  • Aminoglycosides: A class of antibiotics that includes amikacin and tobramycin
  • Carbapenems: A class of beta-lactam antibiotics that includes imipenem and meropenem
  • Linezolid: An oxazolidinone antibiotic used to treat various bacterial infections
  • Tigecycline: A glycylcycline antibiotic used to treat various bacterial infections
  • Cefoxitin: A cephalosporin antibiotic used to treat various bacterial infections
  • Ciprofloxacin/Levofloxacin: Fluoroquinolone antibiotics used to treat various bacterial infections
  • Minocycline/Doxycycline: Tetracycline antibiotics used to treat various bacterial infections
  • TMP/SMX (Trimethoprim/Sulfamethoxazole): A combination antibiotic used to treat various bacterial infections
  • Biofilm: A community of microorganisms attached to a surface, enclosed in a self-produced matrix
  • Surgical Debridement: The removal of dead or damaged tissue from a wound
  • Ototoxicity: Damage to the inner ear caused by certain drugs, leading to hearing loss or balance problems
  • Nephrotoxicity: Damage to the kidneys caused by certain drugs
  • Inducible Resistance: Resistance to an antibiotic that develops during prolonged exposure to the drug
  • Cystic Fibrosis (CF): A genetic disorder that affects the lungs and other organs, leading to chronic lung infections
  • Inhaled Amikacin: Amikacin administered directly to the lungs via inhalation
  • Speciation: The process of determining the specific species of an organism
  • Intravenous Administration: Administration of a drug directly into a vein
  • Localized Infections: Infections that are confined to a specific area of the body
  • Foreign Body Removal: The removal of a foreign object from the body
  • Adjunctive Therapy: Additional treatment used in conjunction with the primary therapy
  • Monitoring: Regular assessment of a patient’s condition and response to treatment
  • MIC (Minimum Inhibitory Concentration): The lowest concentration of an antimicrobial agent that inhibits the growth of an organism
  • MDR (Multidrug-Resistant): Resistance to multiple antibiotics