Chapter 55 of the USP is titled “Biological Indicators – Resistance Performance Tests”. Within the introduction, a definition of biological indicators is provided: “A biological indicator (BI) is a well-characterized preparation of a specific microorganism with a known resistance to a specific sterilization process.” To expand on this definition, a BI is essentially a microbiological test system. It is made up of a known viable population of a particular microorganism (bacterial spores) which is inoculated onto a carrier. A BI is ready for use, and provides a defined resistance to a specific sterilization process.
There are at least three forms of biological indicators: (1) spores are added to a carrier and then packaged to maintain the integrity of the inoculated carrier; (2) spore suspensions, which in turn can be inoculated directly onto units to be sterilized; (3) self-contained BIs, which are designed so that the primary packaging contains the growth medium for recovery of the process-exposed spores.
A BI can be inoculated with a single species of spores. It could also contain up to two different species, without having detrimental effect on the performance of the biological indicator. Because the spores used in BIs are the most resistant spores to a particular sterilization process, a BI that does not grow out indicates that other potential spores or bacteria in the sterilization load have been killed.
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