In a microbiology laboratory, to swiftly respond to experimental needs, we often need to prepare agar based medium in advance. These media must be properly stored after sterilization to maintain their quality and effectiveness.
We can melt it by placing it in a boiling water bath or in the laminar steam of an autoclave. For media that have already been autoclaved, we should minimize the reheating time and avoid excessive heating to prevent affecting the quality of the agar based medium. The melted media should be briefly placed at room temperature to prevent the glass bottle from breaking due to a significant temperature difference.
After that, the media should be placed in a thermostatic water bath set at 47℃ to 50℃ for cooling and maintenance until use. This temperature range can be appropriately adjusted according to the actual gelatinization temperature of the agar based medium. The melted media should be used as soon as possible and generally should not be left for more than 4 hours. Unused media should not be kept for the next use.
We need to pour the melted agar based medium into petri dishes to form a layer of agar at least 3mm thick. After covering the dishes, place them on a level surface to cool and solidify the agar. If the plates need to be stored, or if the incubation time exceeds 48 hours, or if the incubation temperature is above 40℃, then more media needs to be poured. Most media on the market have a dispensing volume of 20mL, but in practice, pouring 25mL to 30mL of media per plate is more ideal to prevent the quality decline of the media due to moisture loss during the incubation process.
The solidified agar based medium should be used immediately or stored in a refrigerator at 5℃±3℃, using sealed bags to avoid direct sunlight which can alter the media components. Mark the bottom or side of the plates with necessary information including the name, preparation date, and expiration date. An appropriate media coding system can also be used for marking. Place the poured plates in sealed bags for refrigerated storage to extend their shelf life. To avoid condensation, the plates should be cooled before putting them into the bags. Do not dry the media surface before storage.
For solid media used for surface inoculation of microorganisms, we need to dry the agar surface first. This can be done by opening the petri dish lid, inverting the plates in an oven or incubator (set at 25℃ to 50℃), or placing them in a sterile laminar flow hood with convection until the moisture on the agar surface evaporates. Be careful not to over-dry, which could affect the quality of the media.
However, self-prepared agar based medium need careful preservation to extend their limited shelf life. There are also some ready-to-use media on the market, such as 3-minute media, which can be stored at room temperature for up to 12 months. These media only need to be heated in a microwave for 3 minutes before pouring plates when inoculation is required. This ready-to-use media simplifies the preparation work, ensuring the efficiency and accuracy of the experiment, and becomes an ideal choice for microbiology laboratories.
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