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Storage of Reconstituted Sterile Media and Poured Plates

The quality of prepared plates can deteriorate from the time of preparation. The rate of deterioration will vary according to the stability of the medium constituents.

The optimal shelf life should be determined by microbiological testing carried out at the time of preparation and during the storage period. This investigation would need to be undertaken by each individual laboratory/customer.

Plates should be wrapped in appropriate storage film or placed upside down in sealed containers. The following shelf lives are suggested for the majority of prepared media when stored at 2-8°C in the absence of light:

Nutrient agars w/o blood
2-4 weeks
Blood agars
7 days
Blood agars (with unstable additives)
2-5 days
Most selective media
5-7 days

Certain media such as Baird Parker Agar show an increase in selectivity towards some staphylococci upon storage, even after a few days. Similarly the selectivity of Bismuth Sulphite Agar increases within 48 hours of preparation.

Simple non-selective bottled broths, diluents and agars, once reconstituted and sterilised, may be stored in a cool or ambient temperature environment away from light for up to 6 months. Storage of Buffered Peptone Water, however, may lead to the development of a precipitate unless the water used is pure.

Prepared selective media should not be stored for more than 3 weeks at 2-8°C. A few selective broths, e.g. Selenite Broth, are slightly more stable and may be satisfactory for up to 2-3 months when stored in the dark. Selective agars cannot be stored in bottled form as they should not be re-melted.

Most reconstituted sterile supplements (SR’s) can be aliquoted and stored frozen below -10°C for up to three months. These aliquots should be thawed only once.

Reference: Draft Campden 'Microbiology Methodology' Working Party 1991

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