Using the Diffusion (Cylinder-Plate) Method
Both the European Pharmacopoeia and the US Pharmacopeial Convention (USP) require testing of antibiotic potency using microbiological assays for some products. When the potency of an antibiotic is measured chemically, its concentration in solution is all that is determined. Conversely, a microbiological assay verifies the ability of the antibiotic to kill the target organisms, as well as the concentration at which the antibiotic will kill effectively.
A microbiological potency assay measures the effectiveness of an antibiotic by the degree of growth inhibition on susceptible strains of microorganisms at differing concentrations. One of the two methods defined in the pharmacopoeia to make this determination is the Diffusion (Cylinder-Plate) method.