How Long Do Agar Plates Last?
In the field of science, materials, particularly biological materials, make up a large part of the lab work. Whether you are studying bacteria and viruses or manufacturing pharmaceuticals, whether your intent is to understand a human or animal function or examine a little-understood living organism, when you work with organic matter, time is of the essence. And because so much biological work is done on agar plates, it is helpful to answer the question, “How long do agar plates last?”
The Purpose of Agar Plates Matters
First, it is critical to understand what type of agar you are using in your lab and what the purpose of the agar plates you are using is.
Agar plates are typically made by pouring agar (a gelatin formed from seaweed) into a Petri dish and allowing it to settle into a solid film on the plate.
Also read: Agar Plates vs Petri Dishes: Are They the Same?
However, many agar plates are also made with additional nutrients and organic materials, like blood, in order to work with and understand various cultures.
These substances matter as, for example, we might be considering how long to store plain agar versus how, and for how long, to store blood agar. Obviously, blood agar will expire much more quickly than a substance made just from plant matter like seaweed.
Thus, when considering how long agar plates last, you have to take into consideration what materials are living or dead on your agar.
How to Store Agar Plates
In terms of storage, you should store your agar plates upside down in a cool, dry room. Storing them upright runs the risk of releasing condensation.
If you do store your agar plates upright, be sure to check on them after a few hours. If you notice drops of condensation on the lid, immediately turn them upside down.
The drops of liquid on the lid signify your agar drying out. Once your agar is dried out, it will be useless.
You can also store your agar plates in a cold room or refrigerator to lock in moisture for longer.
Of course, if the plates have any sort of nutrient or inoculation on them, they should be kept in cold storage unless the specifications for your particular experiment call for a warmer environment.
As you likely already know, cold storage will preserve organic matter for longer, but it will also inhibit natural growth.
So, for example, if you are growing yeast or bacteria on a petri dish, you may actually want a warmer environment.
Always follow the guidelines of your specific test.
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How Long Do Agar Plates Last?
In general, plain, poured agar plates will last anywhere from 30 to 90 days. Some newer agar plates are being designed to last up to 24 months, which will allow labs in developing countries that don’t have ready access to cold storage to preserve their plates much longer.
Inoculated
Again, inoculated plates or those with additional nutrients will go bad much more quickly as you are dealing with additional organic matter.
Blood agar typically lasts only 7 days. Nutrient-rich agar can last up to 4 weeks. Other agar plates with additives and inoculations are typically only good for 2 to 7 days.
The answer, of course, to the question of how long agar plates can last, is “it depends.”
Expired
Now, if your plates have an expiration date on them, and that date has passed, you have a few things to consider.
If the agar is just classic growth media with no culture, nutrients, or other additives, you are likely fine to continue to use them. Obviously scan the agar and check that nothing has begun to grow on the plates first.
However, if you have plates with nutrients or antibiotics on them, you must be aware that organic matter may no longer be effective once the expiration date has passed.
These factors are simply those to be mindful of anytime you are dealing with expired plates.
How Can I Tell if the Agar Plates Can Be Used?
If you pull out plates and you’re not sure of the expiration date, you’re not sure how long the plates have been there, but you want to use them if you can, take into consideration a few factors.
If anything is growing on the agar, it will of course interfere with anything else you try to add to the plate.
If you were hoping for nutrients on the agar, it may not be effective.
If you were hoping to test a bacterium on an antibiotic, it may not be as powerful.
All of these factors come into play when deciding if an agar plate can be used.
Like with most issues in biology, the answer to a simple question is not always simple. In fact, it rarely is. There is much to consider when it comes to using agar plates and figuring out how long they can last.
The best thing you can do is be aware of what is on your agar plates, when you got them, and how you intend to use them. Then, be sure to mark everything clearly and neatly in case you forget anything one month, two, or more down the road.
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