I will take this opportunity to share the following. The question I have
always had about USP <51> is how 'real' world it is. Real world
microbiology is not a microbe growing in a Petri dish. I have seen
formulations containing preservatives be very effective when evaluated
according to USP <51>. However the preservatives were not effective when
the microbial growth was as a biofilm. Has anyone else observed something
similar or evaluated preservative effectiveness against microbial growth as
Richard A. Albert
Kleen Test Products
On Tue, Oct 1, 2019 at 5:04 PM Robert Pritchett <
[log in to unmask]> wrote:
> Hello all,
> 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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