Antimicrobial Susceptibility Testing

• Antimicrobials are compounds that kill or inhibit microorganisms [natural, semi-synthetic, or synthetic]
• Antibiotics are antimicrobials, usually of low molecular weight, produced by microorganisms that inhibit or kill other microorganisms
• Antibiotics and antimicrobials are our first line of defense in battling with the microorganisms or diseases that can be fatal [now, manageable or curable with the discovery of antibiotics]
• G+ and G- Cells Affected

  • Penicillin
  • Fluoroquinolones
  • Tetracydines
  • Streptomycin
  • Chloramphenicol
  • Sulfa Drugs
• G+ Cells Affected

  • Vancomycin
  • Bacitracin
• Mycobacterium tuberculosis

  • Isoniazid
• ANTIMICROBIAL SUSCEPTIBILITY TESTING

  • Performed on bacteria and fungi isolated from clinical specimens to determine which antimicrobial agents might be effective in treating infections caused by these organisms

  1. Specimen collection
  2. Microscopic examination
  3. Macroscopic examination
  4. Biochemical identification
• Step by Step Antimicrobial susceptibility

  1. Performed on bacteria and fungi isolated from clinical specimens

  2. Determine the organism that is causing the disease

  3. Determine whether the bacterial isolate is capable of expressing resistance to the antimicrobial agents selected for treatment

  4. Often performed by a disk diffusion or dilution (minimal inhibitory concentration [MIC]) method

  5. Standards that describe these methods are published and frequently updated by the Clinical and Laboratory Standards Institute (CLSI)
• Primary goal


  To determine whether the bacterial isolate is capable of expressing resistance to the antimicrobial agents selected for treatment
• STANDARDIZED COMPONENTS OF ANTIMICROBIAL SUSCEPTIBILITY TESTING

  • Bacterial inoculum size

  - Disk Diffusion [petri dish: Mueller Hinton Agar]: 1.5x10^8 CFU per mL

  - Basis: McFarland Standard

  - Broth microdilution: 5x10^5 CFU per mL
• STANDARDIZED COMPONENTS OF ANTIMICROBIAL SUSCEPTIBILITY TESTING

  • Growth medium (typically a Mueller-Hinton base or agar; Mueller Hinton Broth)

  - pH 7.2 - 7.4

  - Cation concentration - Calcium 25mg/L and Magnesium 12.5 mg/L

  - Blood and serum supplements

  - Thymidine content
• • Incubation atmosphere - Ambient air
  • Incubation temperature - 35°C
  • Incubation duration - 16-18 hours
  • Antimicrobial concentration
  • Lowest concentration of those antibiotics [CLSI standard]