Antibiotic potency assays using microbiological methods are required by both the European and United States Pharmacopoeia for some antibiotic products. Physicochemical methods, such as immunological assays and High-Performance Liquid Chromatography (HPLC) have become commonplace for determining the concentration of API and for the concomitant determination of Assay and impurities. However, microbiological assays provide a true measure of how effectively an antibiotic kills or inhibits a target microorganism at a specified concentration.
A microbiological potency assay measures the effectiveness of an antibiotic by the degree of growth inhibition on susceptible strains of microorganisms at differing concentrations. The second method defined in the pharmacopeia to make this determination is the turbidimetric method.
The turbidimetric method is based upon the effectiveness of the antibiotic to inhibit the growth of a target microorganism. Known concentrations of antibiotics are added to a liquid culture media containing a target microorganism. This mixture is incubated and the turbidity of the solution measured using spectrophotometric methods. The turbidity of the solution positively correlates with microbial growth and is, therefore, a direct measure of the effectiveness of the antibiotic. The choice of antibiotic potency test method has been determined for each antibiotic and is found in the applicable current pharmacopeia.