Diskdiffusion

Disk diffusion, also known as the Kirby-Bauer disk diffusion test, is a microbiology method used to assess the susceptibility of bacteria to antimicrobial agents. It involves placing antibiotic-impregnated disks on an agar plate inoculated with a standardized bacterial suspension. After incubation, zones of inhibition around the disks indicate susceptibility to the corresponding antibiotics.

Procedure basics include preparing a bacterial suspension adjusted to 0.5 McFarland and swabbing it uniformly on Mueller-Hinton agar, which has a defined depth. Disks containing known amounts of antibiotics are applied to the surface. Plates are incubated at 35°C for 16–18 hours in ambient air. The diameter of each inhibition zone is measured and interpreted using standardized breakpoints from guidelines such as CLSI or EUCAST to categorize the organism as susceptible, intermediate, or resistant.

Interpretation and utility: The test provides a phenotypic profile of antibiotic susceptibility and is widely used for routine clinical testing of many pathogens, including Staphylococcus aureus, streptococci, and Enterobacterales. It offers qualitative results and is often used to screen antibiotics while MIC testing provides quantitative data when necessary. Quality control strains, such as Staphylococcus aureus ATCC 25923 or Escherichia coli ATCC 25922, are used to verify test accuracy.

Limitations and considerations: Accuracy depends on proper inoculum density, agar depth and composition, pH, disk storage, and diffusion properties. Some antibiotics diffuse poorly in agar, slow-growing organisms may require longer incubation, and certain pathogens may require alternative media or methods. The test may not detect resistance mechanisms not expressed under test conditions, so results should be interpreted with clinical context and guidelines.