USP (81) Antibiotics – Microbial Assays

The activity (potency) of antibiotics may be demonstrated under suitable conditions by their inhibitory effect on microorganisms. A reduction in antimicrobial activity will also reveal subtle changes not demonstrable by chemical methods. Accordingly, microbial or biological assays remain the standard for resolving doubt concerning possible loss of activity.

Two general methods are employed, the cylinder-plate or “plate” assay and the turbidimetric or “tube” assay. The first 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 turbidimetric method depends upon the inhibition of growth of a microbial culture in a uniform solution of the antibiotic in a fluid medium that is favorable to its rapid growth in the absence of the antibiotic.

Nova’s capabilities include both the cylinder-plate assay and the turbidimetric/tube assay.

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