The agar diffusion method was developed as a practical alternative to the agar and broth tube dilution procedures. The most popular agar diffusion method is the Kirby-Bauer disk diffusion method. In this method, the test agar plate is swabbed with a standardized concentration of the test organism, and then paper disks containing a defined antibiotic concentration are placed on the lawn of bacteria. After overnight incubation, the diameter of the zone of inhibited growth around the disk is measured. This zone is influenced by a number of variables, including the susceptibility test medium (Mueller-Hinton agar is the standard for bacterial tests), the concentration of the test organism, the rate of growth of the test organism, the concentration of antibiotic in the disk, the diffusion of the antibiotic in the agar, and the susceptibility of the organism to the antibiotic. The method was originally devised by Ouchterlony. 
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