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Extending shelf life without compromising quality
Microbiology labs can spend tens of thousands of dollars on agar plates in per and significant amounts of time preparing them. Preparing plates in large batches can save time in the lab, but they must be used before they dry out or their components become inactive. Here we discuss how to store agar plates properly, to help to extend their shelf life and ensure that money and time are not being wasted.
Before storage
Steps you can take while preparing your agar plates to help extend their shelf life include:
- Allow your plates to cool completely after pouring them. This will prevent moisture from evaporating and condensing on the lids during storage. Keep a close eye on your plates, as they will begin to dry out from being exposed to air, and put them in storage as soon as they are cool.
- Label the plates with the type of agar, the date they were poured, and the expiration date so that there is no guesswork about their age when it’s time to use them.
- Wrap the plates in parafilm or seal them in a bag. The sleeve that the empty plates were stored in can be used if sealed securely. This will prevent the plates from drying out during storage.
During storage
Storing agar plates in the right location and conditions will also help to extend their shelf life:
- Store the plates in a cold room or refrigerator at 2°C to 8°C. In the cold room, store the plates away from areas of high air flow to prevent the plates from drying out. In the refrigerator, store the plates away from the freezer compartment, as freezing will ruin the plates and as a result they will need to be discarded. Monitor the storage temperature daily.
- Store plates upside down to prevent moisture from condensing out of the agar and onto the plate lids.
- Do not store dry ice in the same location as the plates. The CO2 that dissipates from the dry ice can change the pH of the media.
- Keep plates out of direct sunlight. If plates are in a refrigerator or cold room, this likely won’t be a problem during storage. But be sure to avoid leaving plates in the sun before and after storage, as sunlight can cause photo-oxidation and alter the components of your media.
- Rotate your plates in the storage area so that the oldest plates are closest at hand and can be used first. Always check the dates on plates before using them to make sure that you are using the oldest ones that have not exceeded the expiration date.
- The length of time that plates can be stored varies depending on the type of agar. While each type of agar plate should be shelf life validated (see below) to determine the correct shelf life in each lab, some general guidelines are:
- Non-selective agars - up to four weeks
- Non-selective blood agars - up to seven days
- Most selective agars - up to seven days
- Purchased prepared media – up to expiration date indicated by the manufacturer
Validating shelf life
To ensure that your storage times aren’t compromising the quality of your agar plates, each type of stored media should be validated for every organism that will be cultured on that media. To validate the shelf life of your plates, follow these steps:
- Make a batch of media large enough for the number of organisms times the number of weeks that you want to test.
- Pour, package, and store your plates following the best practices above.
- Every week, remove one plate per organism from storage for testing and make a batch of fresh media for comparison. Inoculate the plates with the target organisms.
- Incubate the plates and record the amount of growth and any indicators relevant to your organism and media type.
- Continue to test every week until you see noticeable differences between the stored and fresh plates such as reduction in colony size and number, or color changes and cracking in the media.
- Note the week at which the last acceptable results were obtained. This is the upper limit of the plates’ shelf life. Your lab may choose to add a safety margin of a week or two and reduce the expiration date by that margin.
- For selective or blood agars with shorter shelf lives, perform these tests over days rather than weeks to validate the shelf life. Be sure to prepare plates for controls such as resistant and non-resistant organisms in the case of selective media.
- Repeat these tests if any changes occur in your lab’s storage protocol, for any new type of media used, and for any new organism your lab is culturing.
After storage
Even though you have validated your agar plates to determine their optimal shelf life, they should always be checked for quality issues before they are used. There are also steps you can take to optimize culture growth on your plates after they come out of storage.
- Check each plate you are using for visible signs of deterioration. These include contamination, uneven filling, bubbles on the surface of the agar, color changes, hemolysis, and signs of freezing or dehydration such as shrinking, cracking, and loss of volume. Discard any plates that appear to be damaged.
- Check the agar surface for wetness. If plates are wet, dry them by placing them upside down with the lids off at 25°C to 50°C or in a laminar flow cabinet for 15 to 30 minutes at 37°C. Don’t over-dry the plates in order to keep water loss to a minimum.
- Allow your plates to come to room temperature before plating them to prevent thermal shock to the organisms you are culturing.
Learn more
By storing your agar plates properly, you can help to ensure that your culture results aren’t compromised by expired media and avoid wasting time and money in your lab. Download our guide, Tips for Successful Culture Media Preparation, to learn more about storing agar plates and other topics such as:
- Preparing the best quality media every time
- Avoiding failed media batches
- Avoiding atypical colonial morphology or reduced shelf-life
- Maintaining efficiency in your test processing
