USP <51> AET has the following text regarding preparation of test strains:
"Use standardized suspensions of test strains or prepare as stated below."
"The viable microorganisms used in the procedure should be part of a freshly growing culture (e.g., in logarithmic growth phase) with the exception of A. brasiliensis spores."
I've encountered at least one contract lab who uses pre-made commercially available lyophilized cultures specifically marketed for AET. The lyophilized cultures are hydrated and then used for testing and that lab tells me they consider this the mentioned "standardized suspensions" from the USP text. The contract laboratory does perform proper method suitability using the premade AET cultures with passing results and I have seen the validation work for using AET pre-made cultures, but I still question the acceptability of the practice.
My personal opinions/talking points are as follows:
- Why? It's simpler and more cost effective to start a fresh culture and any analyst with the title microbiologist should be able to adjust suspensions accordingly
- Introducing recently desiccated, starved, frozen, and stressed cells as a challenge to a preservative system seems like a very bad idea - a weak preservative system might pass when it would otherwise fail when challenged with fresh cultures
- The validation work cannot possibly represent the varied materials and preservative systems that may be tested for AET
At this time, I've requested laboratories performing AET for my site to always use fresh cultures.
With that said, I'm open to hear what others may say on the topic. Can anybody defend using pre-made commercial cultures for AET?
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