A disk diffusion method used to assess bacterial susceptibility to antibiotics
Explanation
The Kirby–Bauer test, also known as the disk diffusion assay, is a standardized technique used in clinical microbiology to evaluate the susceptibility of a bacterial isolate to antimicrobial agents. In this method, a suspension of the test organism is uniformly spread over the surface of a Mueller–Hinton agar plate to create a confluent lawn of growth. Sterile filter paper disks impregnated with defined concentrations of different antibiotics are then placed on the agar surface. During incubation, the antibiotics diffuse radially into the agar. If the organism is susceptible to a particular agent, growth is inhibited around the disk, producing a clear zone.
After incubation, the diameters of the zones of inhibition are measured and compared with interpretive criteria provided by organizations such as the Clinical and Laboratory Standards Institute or the European Committee on Antimicrobial Susceptibility Testing. The organism can then be categorized as susceptible, intermediate or resistant to each agent. The test’s simplicity and cost ‑effectiveness make it a cornerstone of routine antimicrobial susceptibility testing. However, factors such as inoculum density, agar depth, incubation time and temperature can influence results. This method is not appropriate for organisms that grow very slowly, require special media or conditions, or for agents that do not diffuse well in agar.
Testing Examples and Limitations
The Kirby–Bauer test is commonly used to evaluate susceptibility of pathogens such as Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa to antibiotics including cefoxitin, ciprofloxacin and meropenem. A cefoxitin disk can help detect methicillin resistance in S. aureus, while carbapenem disks identify carbapenemase‑producing Enterobacteriaceae. The assay is also used to screen community isolates for resistance profiles to guide empiric therapy.
Despite its utility, the test has limitations. It cannot determine exact minimum inhibitory concentrations, and results may not translate to clinical efficacy for drugs with poor diffusion or concentration ‑dependent killing. It is unsuitable for anaerobes, mycobacteria and intracellular pathogens, which require other methods. Laboratories must strictly adhere to standardized protocols to ensure reproducibility.
The Kirby–Bauer disk diffusion assay provides a straightforward means to screen bacterial isolates for antimicrobial susceptibility. When performed correctly and interpreted against established breakpoints, it informs clinical decision‑making and supports antibiotic stewardship programs.
Related Terms: Antibiotic susceptibility, Disk diffusion, Minimum inhibitory concentration, Mueller–Hinton agar, CLSI