The disk diffusion test. A standardized inoculum of Streptococcus pneumoniae is swabbed onto the surface of Mueller-Hinton agar with 5% horse blood. Filter paper disks impregnated with antimicrobial agents are placed on the agar and after overnight incubation, the diameter of the zone of inhibition is measured around each disk.
In "strain 11" good susceptibility to all tested antibiotics (wide zones of growth inhibition around disks) can be seen. "Strain 23" is resistant to penicillin (disk on the top) and sulfonamides (growth to the disks).