Disk Diffusion Methods

Disk (or disc) diffusion susceptibility refers to whether or not an antiseptic, disinfectant or antibiotic chemical is effective at inhibiting the growth of certain kinds of microbes. The disc diffusion susceptibility test associated with antibiotics is called the Kirby-Bauer Antimicrobial Susceptibility Test. The disc diffusion susceptibility test associated with antiseptic and disinfectants is called the Antiseptic/Disinfectant Susceptibility Test.

Purpose

• This method allows scientists to determine which chemicals may be used for pharmaceutical purposes. Sometimes these chemicals are antiseptics, which are chemicals used on living tissue to remove pathogens. Other times they are disinfectants, which are like antiseptics, but are used on inanimate objects. Still other times they are antibiotics which are chemicals produced by living organisms (e.g., penicillin). Collectively, these are all known as antimicrobials.

General Approach

• When scientists seek to test whether or not an antiseptic, disinfectant or antibiotic is effective at stopping the spread of microbes, they impregnate a disc of filter paper with the chemical under test and place it on agar. The chemical diffuses from the disc into the agar, but only around the perimeter of the disc. If an organism (group of microbes) is placed on the agar it will only grow on the area around the disc if it is not susceptible to the chemical. If it is susceptible to it, it will not grow on or around the agar and the scientists will have evidence that the chemical inhibits the organism. The area on which the organism does not grow is called the "zone of inhibition."

Kirby-Bauer Method

• In this method, scientists obtain a plate culture of the organism to be tested and use a sterile loop to repeatedly emulsify a colony from the plate into a sterile saline solution. The culture is then applied to the agar disc. The antibiotic disc is then placed on top of the agar using a dispenser and the plate is incubated for 24 hours. After incubation, the inhibition zone (if it exists) is measured and recorded.

Antiseptic/Disinfectant Method

• This method is almost identical to the Kirby-Bauer method, but there is one core difference. In the Kirby-Bauer Method, the zone sides are standardized. This means that the scientific community has agreed upon the areas that are to count as the zone of inhibition. In the Antiseptic/Disinfectant Method, they are not standardized. However, the general effectiveness of the chemical can be determined by comparing the results of the experiment with other experiments that test the same chemical on different organisms.