Ampicillin  Susceptibility Testing 

Ampicillin is a broad-spectrum beta-lactam antibiotic that belongs to the penicillin class. It is widely used to treat infections caused by susceptible Gram-positive and some Gram-negative bacteria. However, its efficacy depends on bacterial susceptibility, which is determined using standardized laboratory methods. Ampicillin susceptibility testing ensures its appropriate use, minimizes resistance development, and guides antimicrobial therapy.

Content:

• Testing Methods:
  • Disk Diffusion (Kirby-Bauer Method):
    • A disk impregnated with ampicillin is placed on an agar plate inoculated with the bacterial isolate.
    • After incubation, the zone of inhibition is measured and interpreted using CLSI or EUCAST breakpoints.
  • Minimum Inhibitory Concentration (MIC) Testing:
    • MIC is determined using broth microdilution, agar dilution, or automated systems (e.g., Vitek, Phoenix).
    • MIC values are compared to interpretive breakpoints (e.g., CLSI: MIC ≤ 8 µg/mL for susceptible strains).
  • E-test (Gradient Diffusion):
    • A strip with a gradient of ampicillin concentrations is placed on an inoculated agar plate.
    • The MIC is determined where the growth inhibition ellipse intersects the strip.

Application:

• Clinical Use:
  • Treats infections such as:
    • Urinary tract infections (UTIs) caused by susceptible E. coli or Proteus mirabilis.
    • Listeriosis (caused by Listeria monocytogenes).
• Antimicrobial Stewardship:
  • Promotes appropriate use of ampicillin over broader-spectrum antibiotics to minimize resistance development.
  • Supports local antibiograms for guiding empirical therapy.
• Epidemiological Monitoring:
  • Tracks beta-lactamase-mediated resistance in bacterial populations.
  • Identifies emerging resistance trends, such as ESBLs (Extended-Spectrum Beta-Lactamases).

Ampicillin susceptibility testing plays a critical role in guiding the appropriate use of this antibiotic for treating bacterial infections. Standardized methods, such as disk diffusion and MIC testing, ensure reliable and reproducible results.