Amphotericin Susceptibility Testing 

Amphotericin B is a polyene antifungal agent used to treat severe and systemic fungal infections. It works by binding to ergosterol in fungal cell membranes, disrupting membrane integrity and causing cell death. Despite its broad-spectrum antifungal activity, susceptibility testing is necessary for some fungal pathogens to assess the drug's effectiveness and guide treatment decisions, especially in cases of emerging resistance.

Content:

• Testing Methods:
  • Broth Microdilution (Gold Standard):
    • This method is recommended by CLSI (Clinical and Laboratory Standards Institute) and EUCAST.
    • Amphotericin B is tested in a series of two-fold dilutions in RPMI 1640 medium.
• • E-test (Gradient Diffusion):
    • A strip containing a gradient of amphotericin B concentrations is placed on an agar plate inoculated with the fungal isolate.
    • The MIC is determined where the elliptical zone of inhibition intersects the strip.
  • Disk Diffusion:
    • Rarely used for amphotericin B, as it is less reliable compared to MIC-based methods.

Application:

• Clinical Use:
  • Amphotericin B is a drug of choice or alternative for severe fungal infections, including:
    • Candidiasis: For invasive or resistant Candida infections.
    • Aspergillosis: When triazoles are ineffective or contraindicated.
    • Cryptococcal meningitis: Often combined with flucytosine.
    • Mucormycosis: One of the primary treatment options.
    • Endemic fungal infections: Such as histoplasmosis, blastomycosis, and coccidioidomycosis.
• Antifungal Stewardship:
  • Guides targeted therapy, reducing unnecessary amphotericin B use and its associated toxicity.
  • Promotes the use of alternative agents when resistance is detected.
• Epidemiological Surveillance:
  • Tracks resistance trends in fungal populations.
  • Helps identify emerging resistance, such as in Aspergillus terreus or Candida auris.

Amphotericin B susceptibility testing is an important tool in clinical mycology for guiding the treatment of invasive fungal infections. Standardized methods like broth microdilution and E-test ensure reliable results.