Preparing Agar Media

Here we have two common preparations for agar media; the Potato Dextrose Agar (PDA) and Malt Extract Agar (MEA). The follow example details the process of making 1 litre of PDA as this is the most accessible low cost method. This amount is enough to pour 20 x 80mm petri dishes.

PDA

  • 300 – 400 grams potatoe
  • 20 grams agar agar
  • 10 grams dextrose powder
  • 2 grams yeast (optional)
  • The broth should be about 1litre in volume once boiled and strained

Materials:

  • Potatoes
  • Agar agar
  • 2-3l water
  • Glucose Dextrose Powder

Equipment:

  • erlenmeyer flask / narrow, long neck bottle
  • cotton wool or polyester filling
  • pressure cooker
  • alcohol solution in spray bottle
  • infra red thermometer
  • 20 x 80mm petri dishes (just this example, you could us other sizes obviously)
  • latex gloves
  • face mask
  • tyvex sleeves or hazmat suit

Overview:

  • Take a potato, cut into chunks or shred
  • add to pot with 2 – 3l of water (enough to make 1litre when boiled down)
  • boil for one hour, don’t let it boil over
  • strain liquid
  • pour into flask / bottle
  • add agar and dextrose powder
  • 15 psi 30 minutes
  • when below 3psi move pressure in sterilised area, ideally in front of HEPA filter (laminar flow)
  • open just above 0psi
  • remove with sterilise paper towel
  • check temperature with infra red thermometer pouring temperature is 38c
  • while waiting, wipe down outside of petri dishes bag with Alcohol Solution
  • carefully open bottom of the bag with scalpul / scissors
  • Place stack on the work surface and remove bag
  • When ready to pour, swirl mixture
  • carefully lift the lid from the petri dish at the bottom of the stack. Make sure your fingers just touch the top of the sides of the lid.
  • Pour agar
  • Place lid down, lift up the lid of petri dishes second from the bottom and so on until all petri dishes are filled
  • wait until cooled below 20c and use, or wrap with para film and refrigerate for future use
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