Do you use positive controls with reference strains for routinely?

I want to find if in your lab do you use positive controls with reference strains for routinly? So, if every day put an bacterial inoculum on culture media (with or without matrix) as a measure to verify if the methods works. No dubt, for test such coagulase positiv test for staphylococcus where it is necessary to use a positive control. This observations are based on recommandation of ISO 7218:2007 - 15"Quality assurance of results" I mention that I have al lot of experience in microbiological QC in pharma and this practice seem to be extreme and unuseful with a few exception (such coagulase test).

Thx, Ionut

Dear ionutc,

IMO such routines as you mention are crucial to trusting one's results. Although the frequency may depend on specific situation / criticality.

Implementations undertaken may  depend on lab-handling capability / status for specific species.

Not quite sure if yr query only related to controls applied to final identification tests or you include evaluation of initial growth/selective media.

Some food manufacturing locations forbid in-house testing for pathogens / bacterial microorganisms in general.

Commercial service labs and other professional units "frequently" do control activities such as you mention because (a) it protects their reputation/results and (b) it may relate to certification to standards such as ISO 17025. Together with involvement in interlab group testing on standard test samples, etc.

Factory in-house food micro.labs IMEX vary. The rich ones buy commercial reference cultures, eg Salmonella strains. The less rich cultivate strains which they come across in routine operations after having them fully categorised by typing labs.

These days many labs use test kits for all the final stages if they can afford.

Rgds / Charles.C

Dear Charles C,
Thank you very much for your respose.
It is about a commercial microlab accredited 17025, who performes analysis for customers. Analysis are generally quantitative plate counting methods or qualitative for detection of specified microorg.
I mention that I perform verification of culture media for every batch prepared (productivity, selectivity, specificity) using reference strains. I use reference strain for validation and evaluation of uncertainty. Periodicaly, I check methods with reference strains in the presence of product tested.
In this context is it really necssary to inoculate culture media daily with reference strains as a positive control. Ex. To put an inoculum of Stapylococus aureus on BP Agar in paralel with the test for product. Also, I use a negative control.
It is not about the final stage of identification where sometimes must use a positive control.

Ionut

Dear ionutc,

Thks yr clarification.

I interpret yr query as to the routine necessity (or not) of  carrying out "continuous"  parallel reference tests for assurance that culture media remain “suitable” for their designated purpose within the context of ISO 17025.

Should say that my recent experience is mainly with factory micro. labs which have less demanding requirements as compared to a commercial service unit.  For certification to Food Safety standards, factory micro.labs typically only need to demonstrate a capability “equivalent” to ISO 17025 rather than be officially certified, although some are.

I have done a little net searching concerning micro. lab  accreditation via ISO17025. It is quite a complicated subject . Seems the relevance of 17025 to Quality Assurance of micro.lab media is particularly defined via  ISO 11133 Pts 1 and 2 respectively.

I did not see any requirement in either 17025 or 11133 (1/2) for maintaining a permanent, continuous parallel activity  as (I think) you describe.  IMO such a frequent repetition is logistically impractical as a general SOP although I can envisage there might be some particularly sensitive situations where extra care is temporarily justified.

However  the particular media characteristics eg type / origin / supply scenarios can influence the methodology / frequency of "validation" of culture media. Details are given in ISO 11133. Some examples / interpretations  are shown in acc attachments / PS below which include cases ranging from nil (eg  acc2, sec. 3.1[a] ) requirement up to extended test periods (eg PS-SOP).

Here is a commercial document illustrating some operational aspects of the significance of iso 11133 for evaluation of culture media .

 Guidelines for Culture Media re ISO 11133.pdf   2.44MB   27 downloads

For comparison, I hv attached 3 example sets of accreditation-related requirements for micro.labs/media below (acc 2-4), as "interpreted"  within the context of ISO 17025. Examples of text relating to performance of culture media are at -

acc2 / Sec.3.1

acc3 / sec. 8.3

acc4x / Sec.6.4

 acc2.pdf   39.38KB   31 downloads

 acc3.pdf   233.97KB   21 downloads

 acc4x.pdf   632.82KB   18 downloads

I expect the above ideas are  similar to yr present actions, other than the parallel testing.

I can understand, particularly for say "home-made" media, or based on experience of brand variations  the advantage of doing some parallel testing intra-batch. Maybe on a time-based program, eg weekly, monthly ? It also obviously relates to how frequently you start new batches of media  and maybe on the nature of your samples / tests.

Hope the above is helpful. Pls revert if any queries.

Rgds / Charles.C

PS -This is IMO a rather impressive SOP for the topic under discussion. Note the comments on testing procedures Pgs 4-9 and associated Appendices.

 SOP Quality Control Culture Media, 2012.doc   2.47MB   24 downloads