The Shelf-life of prepared culture media
Prepared culture media dispensed in plates, tubes, or bottles, which are not used immediately should be protected against light and desiccation, so that the composition and the performance is not changing during storage. The stability of prepared culture media is limited. For most of the prepared agar plates and bottled media the optimal storage temperature is 4-12 °C in the dark, and wrapped in a bag to avoid contamination and dehydration. Prepared culture media containing blood, egg yolk, egg yolk tellurite emulsion and antibiotics should be stored in the refrigerator at 2-8 °C.
If plating agars are to be stored for a longer period of time, they must be prevented from drying out by sealing each Petridish with adhesive tape along the joint between the lid and base or by packing several dishes into airtight plastic bags. Before packaging the plates are not dried. They should be cooled down, because hot to warm agar plates produces condense water. Excessive condense water may ultimately result in contamination of the plates. The storage of agar plates before drying limits the adverse effects of drying of the agar surface during storage. A loss of more than 15 % of the water content (during storage and incubation) can adversely affect the growth of microorganisms, particularly Gram-negative. Liquid media in test tubes or flasks should also be sealed airtight. Loss of water can result in precipitation and crystallization of certain substances in the culture media.
The extent of water loss depends on the composition, the amount of medium in the plates, the type of incubator i.e. fan-assisted or otherwise, the humidity of the atmosphere in the incubator, the position and number of the plates in the incubator and the incubation temperature.
In the case of culture media which contain unstable additives, it is often better to store the prepared medium without required additives and to add these later when the final medium is needed.
Some culture media contain ingredients which are extremely light sensitive, e.g. rose bengal in yeast agars like Rose Bengal Chloramphenicol Agar. Upon exposure to light sensitive an inhibitory substance is formed. Rose bengal containing culture media must be kept in the dark both during storage and incubation.
The expiration date of prepared culture media depends on the quality of the basic ingredients, the formulation, the quality of the preparation procedures, the sterilization, the packaging and the storage conditions. Each laboratory should assess the expiration date of each prepared culture medium for its typical preparation, packaging and storage conditions.
ISO 11133 part 1 (2000-06-01) recommends that media to which final components are added immediately before use shall be kept in a refrigerator for not more than 3 months, at room temperature for not more than 1 month and in sealed bags for a maximum period of one week.
