An example of a antimicrobial assay using the disk diffusion assy. Samples are adsorbed onto a paper disk and put onto agar which is seeded with the bacteria. The plate is grown up for 24 hours. If an extract is active, there will be a clear circle around the paper disk where no bacteria is growing. The three extracts on the top row are all active. Image courtesy of Islands in the Sea 2002, NOAA/OER.