USP <61> and <62> Microbial Testing

Living contaminants can change the experiment

Bacteria, yeast and mold may enter through contaminated materials, equipment or handling. In cell-based work, a contaminant can grow alongside the intended cells, change the culture environment or create a response wrongly attributed to the test compound.

A documented example is mycoplasma contamination of cell cultures, which can change gene expression, metabolism and growth. Routine enumeration does not establish that a dedicated mycoplasma test was performed; it requires appropriate detection methods. ATCC: contamination and cell research.

What the count represents

These procedures have defined purposes and conditions. They are distinct from a sterility test. USP: microbiological testing standards.

Why a low number is not “sterile”

Some organisms may not grow under the conditions used. A general “microbial safety” heading cannot expand the actual scope of a method.

The sample can interfere with the test

If a sample suppresses microbial growth, a negative observation may reflect inhibition rather than absence. Method suitability checks whether the procedure can recover organisms in the sample’s presence. Controls help distinguish a valid negative result from a test that could not detect the intended target.

Three separate findings

• Microbial enumeration assesses recoverable counts.
• Sterility testing looks for growth in selected samples under a sterility procedure.
• Endotoxin testing measures bacterial residues that can remain without viable bacteria.

Read all three where reported. A result for one does not replace a missing result for another.