Azithromycin  Susceptibility Testing 

Azithromycin is a macrolide antibiotic commonly used to treat bacterial infections, including respiratory, urogenital, and skin infections. It inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit, preventing translation. Susceptibility testing for azithromycin is crucial to determine its efficacy against specific bacterial pathogens, especially with the increasing prevalence of macrolide resistance in some bacteria.

Content:

• Testing Methods:
  • Disk Diffusion (Kirby-Bauer Method):
    • Disks containing azithromycin (15 µg) are placed on agar plates inoculated with the bacterial isolate.
• • Minimum Inhibitory Concentration (MIC) Testing:
    • MIC is determined using broth microdilution or agar dilution methods.
    • The MIC value is the lowest concentration of azithromycin that inhibits visible bacterial growth.
  • E-test (Gradient Diffusion):
    • A strip with a gradient of azithromycin concentrations is placed on an agar plate inoculated with the bacterial isolate.
  • Automated Testing Systems:
    • Systems such as Vitek or Phoenix can provide azithromycin MICs for certain pathogens.

Application:

• Clinical Use:
  • Azithromycin is used to treat:
    • Respiratory infections: e.g., community-acquired pneumonia (CAP), pharyngitis, bronchitis.
    • Sexually transmitted infections (STIs): e.g., Chlamydia trachomatis and Neisseria gonorrhoeae.
    • Enteric infections: e.g., traveler's diarrhea caused by Campylobacter spp. and Shigella spp..
• Antimicrobial Stewardship:
  • Prevents the overuse of azithromycin in infections where resistance is likely (e.g., Neisseria gonorrhoeae).
  • Promotes switching to alternative antibiotics when resistance is detected, reducing selective pressure.
• Epidemiological Surveillance:
  • Tracks macrolide resistance trends, particularly in pathogens such as Streptococcus pneumoniae, Neisseria gonorrhoeae, and Campylobacter spp..
  • Identifies emerging resistance mechanisms in clinical isolates.

Azithromycin susceptibility testing is essential to determine its efficacy against bacterial pathogens, particularly in regions with high macrolide resistance. Standardized methods, including disk diffusion, MIC testing, and E-tests, provide reliable results.