Why do we use Agar plates and how should you best handle them?

By Andrew Ramage

 Prepared agar plates are one of the most popular forms of prepared culture media. While agar is chiefly used in the laboratory, this polymer has an interesting history and is very useful outside of the scientific world too. But where does it come from, and why do we favour this substance over any other?

The origins of agar

The word “agar” originates from agar-agar which is the Malay name for the red seaweed (algae) from which the jelly is produced. Legend has it that agar was discovered by an innkeeper in 1658, who threw away leftover red seaweed (algae) soup and later noticed it gelatinised overnight in the freezing winter weather.

Agar was first used in cuisine and has been used in Japanese and Indian cooking for centuries. It is still widely used today as a vegan substitute for gelatin, primarily to thicken soups, sauces and desserts. Since then, we have found many other uses for such a versatile substance from dentistry (impressions material) to creating ant farms or modelling clay for children. Perhaps one of agar’s most surprising uses is as both appetite suppressant and laxative, thanks to its high fibre content.

Agar usage in the lab

Agar was first described for use in the laboratory environment in 1882. Fanny Hesse, wife of Walther Hesse, was working in Robert Koch’s laboratory and suggested agar as an alternative to meat gelatin, as she’d had such success using it in her jams. In 1887, when Julius Richard Petri perfected his small glass plate with its protective lid, the modern agar plate as we know it was created.

Agar is now a staple of all laboratories because of its remarkable physical properties. In gel form, it can withhold its shape and strength up to temperatures as high as 65°C and has a high melting point of around 85°C.  

Providing it is prepared and stored in a reliable, consistently monitored environment, agar is now an essential tool for the pharmaceutical, food preparation, biochemical and many other industries who rely on accurate environmental tests for safety and profitability. And to think it all started with some frozen seaweed.

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If your business uses agar, here are some important things you need to know:

How to know your agar is suitable for use

Before a new batch of agar plates can be used, particularly under pharmaceutical industry regulations, it is imperative to make sure it is properly tested for suitability. This is done by performing growth promotion testing (GPT).  The purpose of GPT is to assure the nutritive properties of a new batch of agar by challenging it with a small number of microorganisms. However, this process itself carries certain challenges.

A frequent complaint is the non-growth or low recovery of specific microorganisms. If you experience this issue, then it is recommended you first retest using a freshly made control strain to eliminate the possibility of a problem with the control and to avoid the need of making a whole new batch.

There are several best practice steps to be followed for GPT, which begin well in advance of starting the actual test. Your media should never be used prior to testing, for instance, and the test batch must be labelled for quarantine and segregated from any that have been successfully tested and are in use.

Avoid excess condensation

Excess condensation renders your agar virtually useless. There are different reasons for excess condensation in your packs. Your supplier should condition the plates a couple of days before packing, to ensure there is no condensation. However, when the plates leave the supplier, condensation can occur due to large temperature changes over a short period of time. This is referred to as temperature shock. This can occur in transit and may happen during both the summer and winter months.

Once your media has been delivered, if you begin to notice excess condensation forming, it’s most likely an issue with your storage facility. When your media continues to present with excess condensation, and you know you have set up your storage facility correctly, then you might be feeling stumped for answers.  

Generally, the issue is something simple and easily remedied.  Common problems are that the plates are stored next to the door or a radiator or they have been stored near an open window, or where direct sunlight shines on the plates or the boxes in which they’re packed. These are all examples of temperature shock which will inevitably result in excess condensation and a very wet agar surface is useless.

Not all agar must be stored in the fridge

On the subject of storage, some customers will unwittingly store plates in the fridge. This might be due to microbiologists with a clinical background automatically refrigerating prepared media. This is because media used in this sector has components that rapidly degrade, so it needs to be refrigerated to extend the shelf life. Microbiologists can make a simple mistake of refrigerating media unnecessarily out of habit, or because the supplier has changed and they are not familiar with the storage requirements of the new product.

If your prepared media has been stored in the fridge but was validated for ambient storage it can, most times, still be used. A smart prepared media supplier will have validated the media for storage outside of the recommended range for up to approximately five days before they can no longer guarantee accuracy and quality.

Even when your agar is exposed to extreme temperatures, which most commonly cause excess moisture as condensation in the packaging, the plates may recover by reabsorbing this moisture if they are allowed to return to room temperature for a few days. It’s also important to note that the recommended storage temperature for shelf life can be different from the transport or delivery condition.

The shelf life of your media is based on optimal storage conditions, so working outside of this parameter could hamper your testing. You can find out more about this topic here.

What to do when you are faced with sterility issues

If you make use of non-selective media, you might often experience contaminated colonies on fresh (non-irradiated) media towards the end of their shelf life. This is because the manufacture of agar plates is performed aseptically rather than being terminally sterilised.

Despite stringent controls, no supplier can 100 percent guarantee sterility of fresh agar plates. In the environment in which the plates are filled and packed, microorganisms are few and extremely stressed as it is a hostile environment to their survival. Any that do make it into the media will either die before growing or will take many weeks or months to grow. However it is more likely that media that shows growth at the end of a long shelf life has become contaminated following damage to the packaging during handling.

A trustworthy supplier will always replace your media if you have found contaminating colonies on your unopened plate packs, or if you found them upon opening. However, once a plate pack is opened your supplier cannot be held responsible for any contamination if it was not immediately discovered.

If you want more in-depth insight into prepared culture media then please download The Pharmaceutical and Cleanroom Industry’s Pocket Guide to Prepared Culture Media.




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