Microbial limits testing

Microbial limits testing counts the bacteria, yeasts and molds that can grow from a sample of a nonsterile product, following methods such as USP <61>.

USP <61> gives two counts, both in colony-forming units (CFU) per gram: the total aerobic microbial count (TAMC), for bacteria, and the total combined yeasts and molds count (TYMC). The sample is plated or filtered onto growth media, incubated, and the colonies counted. The laboratory first shows that the product itself does not suppress growth, or a low count would mean nothing. A companion chapter, USP <62>, checks for specific organisms that should be absent, such as Escherichia coli and Staphylococcus aureus, and USP <1111> gives typical acceptance limits.

A lyophilized peptide offers microbes little water to grow in, so counts are usually low. The test shows that the powder was made and handled cleanly, and that it is not carrying a load of microorganisms into the laboratory.

The test has a defined scope. It is meant for nonsterile products and sets a ceiling on how many organisms are present, not a requirement that there be none: passing it does not make a product sterile, which is a separate test. It also counts only living organisms that grow under its conditions, so it says nothing about endotoxin left behind by bacteria that have died.

Related terms

• Sterility testing
• Endotoxin
• Certificate of analysis

UPDATED OCTOBER 30, 2026 · REVIEWED BY DEL POTTER, PHD