Definition
The disk diffusion test is a standardized microbiological assay used to assess the susceptibility of bacterial isolates to antimicrobial agents. It involves placing paper disks impregnated with defined concentrations of antibiotics onto the surface of an agar plate inoculated with the test organism; zones of inhibited bacterial growth around the disks are measured and interpreted to determine susceptibility, intermediate resistance, or resistance.
Overview
The procedure, commonly referred to as the Kirby‑Bauer test, is widely employed in clinical laboratories for routine antimicrobial susceptibility testing (AST). Standardization bodies such as the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) provide detailed guidelines on media composition, inoculum density, disk potency, incubation conditions, and interpretive criteria. The test enables rapid, qualitative categorization of bacterial isolates, informing therapeutic decisions and surveillance of antimicrobial resistance.
Etymology / Origin
The term combines “disk,” referring to the circular paper filter disks used as carriers for antibiotics, and “diffusion,” describing the passive spread of the antimicrobial compound into the surrounding agar matrix. The method originated in the 1960s, with the seminal work of Kirby and Bauer (1966) establishing its clinical utility and forming the basis for subsequent standardization.
Characteristics
| Aspect | Details |
|---|---|
| Media | Mueller‑Hinton agar is the preferred medium because of its reproducible diffusion properties and lack of antagonistic substances. |
| Inoculum | A standardized bacterial suspension (typically 0.5 McFarland turbidity) is swabbed uniformly across the agar surface to create a confluent lawn. |
| Disks | Commercially prepared disks contain fixed antibiotic amounts (e.g., 10 µg penicillin). Disk potency is selected according to guidelines for each organism‑drug combination. |
| Incubation | Plates are incubated aerobically at 35 ± 2 °C for 16–18 hours; specific conditions may vary for fastidious organisms. |
| Interpretation | The diameter of the clear inhibition zone is measured with a ruler or caliper. Measured values are compared with breakpoint tables (CLSI/EUCAST) to assign “susceptible (S),” “intermediate (I),” or “resistant (R).” |
| Advantages | Simple, inexpensive, requires minimal equipment, provides results within 24 hours, and allows simultaneous testing of multiple agents. |
| Limitations | Provides only qualitative results, may be affected by agar depth, inoculum size, and diffusion characteristics; not suitable for certain organisms (e.g., anaerobes, mycobacteria) without protocol modifications. |
Related Topics
- Antimicrobial susceptibility testing (AST) – broader category of methods used to determine bacterial resistance.
- Kirby‑Bauer method – the specific standardized protocol for disk diffusion.
- Minimum inhibitory concentration (MIC) – quantitative measure of the lowest antimicrobial concentration inhibiting visible growth, often determined by broth microdilution or agar dilution.
- Clinical and Laboratory Standards Institute (CLSI) – organization that publishes widely adopted AST standards.
- European Committee on Antimicrobial Susceptibility Testing (EUCAST) – European counterpart to CLSI, providing breakpoint criteria.
- Resistance mechanisms – genetic and biochemical bases (e.g., β‑lactamases, efflux pumps) that can influence disk diffusion outcomes.
- Alternative AST methods – such as broth microdilution, Etest (gradient diffusion), and automated systems (e.g., VITEK).