In a typical disk diffusion experiment, when are the chemical-saturated disks placed on the agar plate?

• A. Before sterilizing the agar plate in the autoclave
• B. Before pouring the molten agar into the petri dish
• C. After the agar plate has been inoculated with the test microorganism
• D. After incubation of the agar plate for 24 hours

Answer: (C)

Verified step by step guidance

1. Understand the purpose of the disk diffusion experiment: it is used to test the effectiveness of antimicrobial agents against specific microorganisms grown on an agar plate.
2. Recognize that the agar plate must first be prepared and sterilized to prevent contamination before any testing can occur.
3. Note that the agar plate is poured and solidified, then inoculated with the test microorganism to create a uniform lawn of bacteria.
4. The chemical-saturated disks are placed on the agar surface only after the plate has been inoculated with the microorganism, so the chemicals can diffuse into the agar and interact with the bacteria.
5. Finally, the plate is incubated to allow bacterial growth and diffusion of the chemical, which will result in zones of inhibition if the chemical is effective.