Disc diffusion assays

Disc diffusion assays test microbes for chemical susceptibility (often used to test common antibiotics)
Filter paper discs containing chemicals are placed on a Petri dish plated with a lawn of bacterial strain of interest
A plain filter paper disc serves as a negative control
Bacteria are allowed to try to grow, then the zone of inhibition where they were not able to grow (due to chemical sensitivity) is measured
Chemicals diffuse away from the discs. There are higher concentrations of chemicals closest to discs, lower concentrations of chemicals further from discs
Bacteria will be prevented from growing anywhere the concentration of chemicals is higher than their resistance level
The more sensitive they are to the chemical, the larger this zone of inhibition
The more resistant they are to the chemical, the smaller this zone of inhibition
For antibiotics, the zone of inhibition is compared to a table of cut-off values to determine resistance classification.