What is agar agar?

Agar agar is a jelly-like substance that is extracted from various species of red algae, especially from the genera Gelidium and Gracilaria. It consists mainly of polysaccharides, especially agarose, which is responsible for its gel-forming properties. Agar is primarily used as a culture medium in laboratories to grow microorganisms such as bacteria, fungi and mould.

When agar is mixed with water and heated, it liquefies, and when it then cools, it solidifies into a solid gel. This makes it ideal for use in petri dishes, as it provides a stable surface on which microorganisms can grow. In addition to its significant applications in laboratory research, agar is also used in the food industry as a thickening agent and in some vegetarian recipes as an alternative to gelatin.

Procedure for making agar petri dishes

NB! We take no responsibility for any damage that may occur by making agar petri dishes yourself and operating a pressure vessel such as a pressure cooker or autoclave.

Making agar petri dishes requires some basic ingredients and equipment, including sterilization tools such as a Sterile Air Box (SAB) or a Laminar Air Flow (LAF) / Clean Room Bench and an autoclave / pressure cooker.

Here's how to make them with three popular types of agar: Malt Extract Agar (MEA), Malt Yeast Agar (MYA), and Potato Dextrose Agar (PDA).

Equipment and Ingredients

• SAB or LAF : To ensure a sterile environment.
• Petri dishes : Sterile, disposable or reusable.
• Sterile pipettes : For handling liquids.
• Bottles for autoclaving : Heat-resistant glass or plastic.
• Autoclave or pressure cooker : For sterilizing the agar medium.

Recipes

1. Malt Extract Agar (MEA)

   • 20g ground extract
   • 20g agar
   • 1 liter of water

2. Malt Yeast Agar (MYA)

   • 20g ground extract
   • 0.5-2g yeast extract
   • 20g agar
   • 1 liter of water

3. Potato Dextrose Agar (PDA)

   • 200g potato (boiled and mashed to make extract)
   • 20g dextrose
   • 20g agar
   • 1 liter of water

Procedure

1. Prepare the medium : Mix the dry ingredients with water in a heat-resistant bottle. Shake well to mix.

2. Sterilize the solution : Place the bottle in an autoclave or pressure cooker and sterilize at 121°C for 15-20 minutes. Allow the pressure cooker to cool to atmospheric pressure before opening. NB! Remember to leave the bottle slightly open to prevent explosion. The bottle can result in cracking if the container is fully closed during the sterilization process AND if opened before proper cooling, i.e. down to atmospheric pressure.

3. Work in a sterile environment : If you use a SAB or LAF, make sure the area is well cleaned and disinfected. Wipe all surfaces with 70% isopropanol or another suitable disinfectant.

4. Pour into petri dishes : Carefully open the petri dishes in SAB/LAF. Pour into agar with a sterile pipette or directly from the bottle, filling about 1/3 of the depth of the dish. Try to minimize exposure to air to prevent contamination.

5. Cooling : Leave the dishes in SAB/LAF until the agar has completely solidified. This usually takes 30 minutes to 1 hour.

6. Storing the bowls : If not using them immediately, seal them in plastic bags or foil to prevent drying or contamination. Store in the refrigerator for longer shelf life. NB. Long shelf life when storing petri dishes requires that they are 100% sealed, so that no other microorganisms get in.

By following these steps, you can prepare agar plates ready for both mycological and microbiological studies. Always ensure that the work environment is as sterile as possible to minimize the risk of contamination. Good luck with your experiments!