Ampicillin Susceptibility Testing is used to determine the effectiveness of ampicillin, a broad-spectrum β-lactam antibiotic, against bacterial pathogens. Ampicillin is effective against a range of Gram-positive and some Gram-negative bacteria. Here’s a detailed overview of the testing process:

Testing Methods

• Disk Diffusion Method:
  • Preparation: An agar plate is inoculated with a bacterial suspension to create a uniform lawn of growth.
  • Application: An ampicillin-impregnated disc is placed on the agar surface.
  • Incubation: The plate is incubated at 35-37°C for 18-24 hours.
  • Measurement: After incubation, the diameter of the zone of inhibition around the disc is measured. A larger zone indicates susceptibility to ampicillin, while a smaller or no zone suggests resistance.
• Broth Microdilution Method:
  • Preparation: Bacterial suspensions are diluted in a microtiter plate containing broth with varying concentrations of ampicillin.
  • Incubation: The plate is incubated at 35-37°C for 18-24 hours.
  • Evaluation: The minimum inhibitory concentration (MIC) of ampicillin is determined by identifying the lowest concentration that inhibits visible bacterial growth. This MIC value is compared to standard susceptibility criteria.
• Etest Method:
  • Preparation: An agar plate is inoculated with the bacterial strain.
  • Application: A strip containing a gradient of ampicillin concentrations is placed on the agar surface.
  • Incubation: The plate is incubated at 35-37°C for 18-24 hours.
  • Measurement: The MIC is determined by reading the point where bacterial growth intersects with the strip’s gradient. This provides an exact MIC value for ampicillin.

Technical Specifications

• Antimicrobial Concentration: The concentration of ampicillin in the disc or dilution ranges is standardized according to established guidelines to ensure accurate susceptibility testing.
• Quality Control: Testing follows guidelines from organizations such as CLSI (Clinical and Laboratory Standards Institute) to ensure accuracy and reproducibility.

Applications

• Clinical Diagnostics: Ampicillin susceptibility testing is crucial for determining the efficacy of ampicillin in treating infections caused by susceptible bacterial strains. It helps in selecting the appropriate antibiotic and optimizing treatment strategies.
• Antibiotic Stewardship: This testing aids in managing antibiotic use, ensuring that ampicillin is used effectively and helping to prevent the development of resistance.