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.
The Diffusion (Cylinder-Plate) method is based upon the diffusion of the antibiotic throughout solid culture media. A metal cylinder or paper disk containing the antibiotic is placed on a solid agar growth media containing a target microorganism. This test can be performed at multiple concentrations in order to determine the minimum effective concentration. After incubation, the growth of the target organism is measured. An antibiotic should inhibit the growth of the target organism.
One major advantage of microbiological potency testing, as opposed to chemical assays which simply provide a concentration, is that the effect of the antibiotic is directly measured on a living microorganism, providing a real measure of the antibiotic action.