Antibiotic Microbial Potency Assays

Lancaster Laboratories offers antibiotic microbial potency assays at Michigan facility

Del Martin
Analyst Del Martin inoculates cycliners for an antibiotic assay.

A unique service offering, Lancaster Laboratories’ antibiotic microbiological potency assays can be performed under USP and EP guidelines in the Portage, MI, laboratory. Stemming from a long history of supporting antibiotic potency assay testing to support the regulated medicated feed industry, this GMP laboratory has aided pharmaceutical clients in method development and modification of the compendial methods listed in USP and EP chapter 2.7.2 for API, in-process, product release and stability testing.

There are two typical approaches to antibiotic microbiological potency assays – the plate-based zone of inhibition and tube based turbidimetric assays. In both cases, the subject sample is compared to a drug standard against a reference organism.

The plate method results in a no-growth zone surrounding a cylinder that is measured either automatically or manually. In the plate-based method, the reference organism is seeded in an agar layer in a petri dish. Antibiotic is applied to cylinders on the surface of the agar or in wells cut into the agar. Antibiotic diffusion from the cylinder or well prevents growth of the organism, creating a zone of inhibition. The diameter of the zone is proportional to the concentration/potency of the antibiotic. Test samples are diluted to an appropriate concentration and a reference standard solution is diluted in a similar manner to create a standard curve. The diameter of the zones for both the sample and standards are measured, and results for the sample are calculated from the standard curve.

Turbidimetric assays work on a similar principal. The turbidimetric assay uses a spectrophotometer to measure the opacity of a tube containing the reference organism, growth medium and drug. Inhibition of growth of the reference culture in a solution containing the antibiotic and growth media is measured turbidimetrically. As in the zone of inhibition, assay test samples are diluted to an appropriate concentration and a reference standard solution is diluted to create a standard curve. Turbidity is measured spectrophotometrically for the test samples, and standards and results are calculated from the standard curve.

The Lancaster Laboratories facility contains all the necessary equipment to conduct USP, EP and proprietary microbiological assays, including an Autoturb II diluter and reader, Purdue well cutter, automatic cylinder dropper and Fisher Lilly zone reader. Experienced staff can assist in method development, validation or qualification of compendial methods specific to our clients’ unique antibiotic potency assay needs. Also, the Lab can support modification of compendial methods for application to non-compendial matrices.

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