Susceptibility Testing 

Susceptibility testing is a laboratory method used to evaluate the effectiveness of antimicrobial agents against specific microorganisms. The process determines whether a microorganism, such as bacteria or fungi, is susceptible, intermediate, or resistant to a particular antimicrobial. This testing is essential in clinical microbiology, guiding the selection of appropriate therapeutic agents for the treatment of infections and in monitoring resistance patterns in pathogens.

Content

• Purpose of Susceptibility Testing:
  • Determines the efficacy of antibiotics, antifungals, or antivirals against a given pathogen.
  • Supports clinicians in selecting effective antimicrobial therapy.
  • Tracks the emergence and spread of antimicrobial resistance.
• Methodologies:
  • Disk Diffusion (Kirby-Bauer Method):
    • Involves placing antibiotic-impregnated disks on an agar plate inoculated with the test organism.
    • Zones of inhibition (clear areas around the disks) indicate the organism’s susceptibility.
• • Broth Dilution (MIC Determination):
    • Involves serially diluting the antimicrobial agent in liquid media and inoculating with the test organism.
    • The minimum inhibitory concentration (MIC) is the lowest concentration that inhibits visible growth.
  • E-Test (Gradient Diffusion):
    • Combines features of disk diffusion and MIC determination.
    • Uses a strip impregnated with a gradient of antimicrobial concentration.
• Applications:
  • Clinical Decision-Making:
    • Assists in the selection of targeted antimicrobial therapy, improving patient outcomes.
    • Reduces the use of broad-spectrum antibiotics, limiting collateral damage to the microbiota.
  • Antimicrobial Stewardship Programs:
    • Enables institutions to monitor and optimize antimicrobial use.
    • Helps in reducing the development of resistance.
  • Epidemiological Surveillance:
    • Monitors trends in antimicrobial resistance globally and locally.
    • Supports the development of policies to control the spread of resistant strains.

 Susceptibility testing is a cornerstone of infectious disease management, enabling precise and effective treatment of infections. Advances in both phenotypic and genotypic methods continue to improve the speed and accuracy of susceptibility testing.