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Gibraltar Laboratories performs certification of label claim testing regarding the potency of antibiotics (antibiotic testing). Certification is necessary to conform to US FDA current good manufacturing practice regulations. Potency means that the labeled strength of the antibiotic is accurate and therefore an infection caused, for example, by gram positive bacteria can be treated successfully by bacitracin. The potency of antibiotics may be demonstrated under suitable conditions by their inhibitory effect on microorganisms.

A reduction in antimicrobial activity also will reveal subtle changes not demonstrable by chemical methods. Accordingly, microbial or biological assays remain gnerally the standard for resolving doubt with respect to possible loss of activity. The antibiotic testing performed at Gibraltar Laboratories is described in the chapter <81> ANTIBIOTICS—MICROBIAL ASSAYS of the USP. This chapter summarizes procedures for the antibiotics recognized in the USP for which microbiological assay remains the definitive method. The cylinder-plate or “plate” assay depends upon diffusion of the antibiotic from a vertical cylinder through a solidified agar layer in a Petri dish or plate to an extent such that growth of the added microorganism is prevented entirely in a circular area or “zone” around the cylinder containing a solution of the antibiotic.

The antibiotic test is a very demanding test that requires expert care in the preparation of the media and organisms used in the assay. The fundamental principle is that the potency of the antibiotic is directly proportional to area of death it caused. If the antibiotic is not effective all of the bacteria added to the agar will form a confluent lawn. However, if the antibiotic is potent it will diffuse throughout the test system causing a clear zone where bacterial growth was prevented . This zone can be precisely measured with a sophisticated analytical instrument. This clear area of growth is also referred to as a zone of inhibition. Standards are always included in the assay so that one knows that a given zone area corresponds to a known potency. A regression curve is obtained from several concentrations of the standard and the unknown’s potency is obtained from its zone area.