Antibiotic Susceptibility Testing)

• Antimicrobial


  A chemotherapeutic agent designed in nature to inhibit or prevent the growth of microorganisms
• Types of antimicrobials


  • Bactericidal agents - result in bacterial death
  • Bacteriostatic agents - cause temporary inhibition of growth
• Mechanisms of antimicrobial action

  • Interference with the cell wall
  • Interference with the cell membrane
  • Interference with folic acid synthesis
  • Interference with nucleic acid synthesis
  • Interference with protein synthesis
• Kirby-Bauer test


  Also known as the disc-diffusion method, the most widely used antibiotic susceptibility test in determining what treatment of antibiotics should be used when treating an infection
• Kirby-Bauer test


  • Relies on the inhibition of bacterial growth
• Antimicrobial sensitivity test


  1. Use a culture medium (Mueller-Hinton agar)

  2. Uniformly and aseptically inoculate culture medium with the test organism

  3. Place filter paper discs impregnated with a specific concentration of a particular antibiotic on the medium
• Antimicrobial sensitivity test


  • The size of the inoculum must be standardized (using barium sulfate standards, McFarland standards)
  • If the inoculum is too small, the zone of inhibition will be larger than what it is supposed to be
  • If the inoculum is too large, the zone of inhibition will be smaller
• Importance of antimicrobial sensitivity test

  To direct & predict antimicrobial chemotherapy<|>To review & monitor epidemiological trends<|>To set national & local antibiotic policies<|>To test the activity and effectiveness of a new antimicrobial agent<|>To presumptively identify isolates
• Interpreting antimicrobial sensitivity test results

  1. Measure the diameter of the clearing zone around each antibiotic disc in millimeters (mm)

  2. Compare the measurements to a zone-size interpretive chart

  3. Characterize the organism as being resistant, intermediate or susceptible to the specific antibiotic
• Susceptibility interpretations


  Sensitive (S): Zone radius is wider than or equal to, or not more than 3mm smaller than the control<|>Intermediate (I): Zone Radius is > 2 mm but smaller than the control by > 3mm<|>Resistant (R): No zone of inhibition or zone radius measures 2mm or less
• Materials for Kirby-Bauer test

  • Petri plate containing Mueller Hinton Agar
  • 0.5 McFarland Standard
  • Sterile cotton swabs
  • Antibiotic discs
  • Forceps
  • Alcohol
  • Vernier caliper
  • Alcohol lamp
  • Tryptic Soy Broth or NSS