The disk diffusion test—also known as the Kirby-Bauer method—is a standardized technique for determining the susceptibility of bacteria to antimicrobial agents. Antibiotic-impregnated paper disks are placed on Mueller-Hinton agar that has been uniformly inoculated with a standardized bacterial suspension. During incubation, antibiotics diffuse into the agar, inhibiting bacterial growth and forming a clear zone around each disk. Its diameter is measured and interpreted using clinical breakpoints to classify the organism as susceptible, resistant, or susceptible with increased exposure (EUCAST, 2024; Jorgensen & Ferraro, 2009). A larger zone indicates higher susceptibility, while a smaller or no zone suggests resistance.
This method is widely used due to its simplicity, low cost, and reliability in routine microbiology laboratories and qualitative or semi-quantitative data to inform the effectiveness of antibiotic candidates and guide antimicrobial therapy selection. However, it may not be suitable for all bacterial species, resistance detection needs or distinction between bacteriostatic and bactericidal effects.