[Antibiotic susceptibility testing by the disk method. Comparative evaluation of 5 commercial systems]

Area of Science:

• Microbiology
• Clinical Diagnostics
• Pharmaceutical Science

Background:

• Accurate antibiotic susceptibility testing (AST) is crucial for effective antimicrobial therapy.
• Standardization of AST methods and reagents is essential for reliable results.
• Variations in media composition and interpretation standards can impact AST outcomes.

Purpose of the Study:

• To evaluate and compare the performance of four different antibiotic susceptibility testing systems: bioMerieux, Difco, Institut Pasteur Production, and Oxoid.
• To assess the reproducibility and inter-system variability of AST results.
• To determine the reliability of each system based on established standards.

Main Methods:

• Testing of media batches for pH, calcium, and magnesium concentrations.
• Determination of antibiotic activity using three reference strains: E. coli ATCC 25922, S. aureus ATCC 25923, and P. aeruginosa ATCC 27853.
• Optimization of inoculum preparation and susceptibility testing performed 30 times per strain/antibiotic/system.
• Calculation of reproducibility, inter-system variance, and interpretation according to manufacturer, SFM, and WHO standards.

Main Results:

• Difco medium yielded the most accurate results.
• Other media tended to produce excessively large zones of inhibition.
• An overall discordance of 0.29% was observed, considered of little practical importance.
• Homogeneity groups between systems were defined based on reproducibility and variance.

Conclusions:

• The Difco system demonstrated superior accuracy in antibiotic susceptibility testing.
• While minor variations exist, the tested systems show acceptable concordance when interpreted using established standards.
• Further evaluation of media performance is recommended to optimize AST accuracy and clinical decision-making.