Last Updated on June 24, 2022

A Messy Affair: Sterilizing and Disposing of Used Agar and Petri Dishes

Agar plates, otherwise known as Petri dishes, serve as an integral and irreplaceable part of microbiological research, particularly in their functional and utilitarian role in culturing bacteria, fungi, and other microorganisms.

Though relatively easy to prepare and requiring a minimal amount of equipment to culture, clean-up and disposal can be a messy affair, requiring the user to undertake procedures to sterilize and minimize the risk of potentially harmful microorganisms contaminating both inside and outside the laboratory environment.

Standard protocol requires the use of autoclaves to sterilize petri dishes, as only high heat and pressure can effectively kill the full range of microorganisms, which can persist even under unusually harsh conditions. Alternative agar petri dish sterilization methods not requiring the use of autoclave sterilizers, such as microwave ovens or caustic chemicals, have been recommended in place of using autoclaves. However, these methods have been proven ineffective because of the resistance of certain microorganisms.

The method in which we place petri dishes in bio-waste/biohazardous waste bags and sterilized them in autoclaves has become the standard protocol at many laboratories. However, this method comes with downsides, as liquified agar mixed with biomaterial can easily leak and cause an enormous mess within the laboratory or on its way to waste disposal areas and beyond. When paired with the correct autoclaving accessories, the autoclave sterilization method for plastic agar plates/Petri dishes can be both effective in deactivating harmful microorganisms and preventing an enormous mess.

Please find below the following plastic petri-dish agar plate sterilization and disposal method using an autoclave sterilizer, broken down into a few easy steps.

1. Collect Used Plates

Collect used Petri dishes/culture plates with agar. Cultured agar plates/Petri dishes should be left as is with agar and covered to prevent the spread of microorganisms and reduce the risk of contamination.

2. Load Agar Plates/Petri Dishes

Load used culture plates in the plastic petri dish/agar plate sterilization basket. Removing covers and placing them aside before loading the agar-filled portion into the specialized perforated basket. Place dish/plate covers on top of the agar-filled portions at the end.

3. Set Up Bucket and Stand

Set up the stand inside the specialized solid bucket and fill it with water to prevent melted media from solidifying inside the bucket. The stand will place the specialized perforated basket on top of the water to allow the melted media to fall through. Fill water to about half of the height of the stand.

4. Load Bucket with Stand

Place the water-filled bucket inside the autoclave. Next, place the perforated basket with disassembled plastic Petri dishes/agar plates, resting the basket on the stand.

5. Run Cycle

Close autoclave and run a normal sterilization cycle for at least 20 minutes. Please note that the sterilization time differs with the number of plastic Petri dishes/agar plates.

6. Unload

After the sterilization cycle is complete and the autoclave temperature has cooled down to a safe level, remove the perforated basket. Even though the temperature is registered at a safe level, please be mindful of hot steam, liquids, and media.

7. The Chunk

Note that the plastic Petri dishes/culture plates and lids have melted and re-forged into a sizeable chunk. Some of the agar material has been trapped/fused into the plastic, and though it should be sterile, avoid touching parts where agar is exposed to prevent a mess. Most of the agar has melted and mixed with the water inside the bucket.

8. Throw Away- No Mess!

Dispose of the plastic chunk and pour water into a biohazardous liquid receptacle or a drain where it can be safely disposed of and eventually processed. Safe, easy, and no mess!

A Means to a Mess

Agar plates, otherwise known as Petri dishes, serve as an integral and irreplaceable part of microbiological research, particularly in their functional role in culturing bacteria, fungi, and other microorganisms. Though relatively easy to prepare and requiring a minimal amount of equipment to culture, clean-up and disposal can be a messy affair, requiring the user to undertake procedures to sterilize and minimize the risk of potentially harmful microorganisms contaminating both inside and outside the laboratory environment.

Standard protocol requires the use of autoclaves to sterilize Petri dishes, as only high heat and pressure can effectively kill the full range of microorganisms, which can persist even under unusually harsh conditions. Alternative agar petri dish sterilization methods not requiring autoclave sterilizers, such as microwave ovens, have been recommended in place of using autoclaves. However, these methods have been proven ineffective because of the resistance of certain microorganisms. The method by which we place Petri dishes in bio-waste/biohazardous waste bags and sterilize them in autoclaves has become the standard protocol at many laboratories. However, this method comes with downsides, as liquified agar mixed with biomaterial can easily leak and cause an enormous mess within the laboratory or on its way to waste disposal areas and beyond. When paired with the correct autoclaving accessories, the autoclave method is the most effective agar dish and plate sterilization method.

​As you can see, petri dish sterilization can be done safely and easily with little mess, decimating the risk of contamination.

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