Disk Diffusion Test
Disk diffusion test (or Kirby-Bauer method) is the most widespread variant of antibiotic susceptibility determination. Its technique includes several consecutive steps.
- First, the Disk diffusion test microbial culture is pour plated onto the solid medium, providing optimal microbial growth.
- At the next step, the filter paper disks with a standard quantity of certain antibiotic are placed on the medium surface.
- After overnight incubation, the diameter of the growth inhibition zone around the disk is measured. Diameter is proportional to the inhibitory power of the drug for investigated bacterial culture.
- This method is influenced strongly by the nature of the solid medium, its diffusion permeability for antibiotics, drug molecular size and stability, etc. Nonetheless, in standard experimental conditions, the method was shown to give reliable and reproducible results.
- Growth inhibition zone diameter is compared with standard data obtained previously for a particular antibiotic and given bacterial species. Standard data has been determined by comparing the results of diffusion and dilution testing methods.
- The culture is considered to be susceptible to particular antibiotic if the testing strain growth is inhibited by antibiotic concentration, which corresponds to the average therapeutic dose for the drug.
- Resistant microbial culture is not inhibited even by the maximally tolerated dose of certain antibiotics.
- Another version of diffusion susceptibility testing allows determining the MIC of antibiotic for tested microbial culture.
- The method called E-test can be more suitable in some cases and may produce more precise results.
- In this method, a narrow strip of polymer carrier containing descending concentration gradient of the antibiotic is placed upon solid medium after microbial inoculation. Inhibition of microbial growth appears only where antibiotic concentration in the strip exceeds MIC.
- Being rather simple, this diffusion technique makes possible the direct determination of MIC, because different antibiotic concentrations are designated on the strip surface.