Solid media is a wonderful thing! It allows you cultivate bacteria, fungi, slime molds, and other cell cultures. Usually media consists of water and a thickening agent (makes water solid). Optionally, depending on your goals (and what you are culturing), you can add nutrients and antibiotics to protect from invasive species. To use it, you will pour it on a petri dish or a DIY equivalent.

There is a large variety of agar media recipes that suit different needs. This recipe is the most basic one, and if you want to add more ingredients to it, reduce the amount of water that you use by the weight of other ingredients. Example: you decide to add 4 grams of sugar to your agar, therefore you will need to add 4ml (1ml of water weights 1 gram) less water.

In this protocol we will be making 400ml of 2% agar media. This basic media will work great for culturing slime molds like Physarum polycephalum that only require solid water to grow on. This media is also resistant to mold contamination because there is nothing nutrient in it.

Supplies

  • Glass bottle with stopper or a screw-on lid
  • Agar powder
  • Water (ideally you should use distilled water, but tap or drinking water works fine most of the time)
  • Optional: malt extract
  • Optional: sugar
  • Optional: black tea leaves

Procedure

  1. Open and evaluate your glass bottle

    When deciding how much agar to make, a good rule of thumb is to use 80% of your bottle’s capacity - if the bottle is labeled as 500ml, make 400ml. Another important thing - the bottle must not have any cracks!

  2. Weight out 8 grams of agar and add it to the bottle

    8 grams will make up 2% of 400ml in the total mixture. If you are looking for softer media, try using 4 grams (1%) or if you want something really solid, go ahead and use 12 grams (3%). A funnel is a great tool to add stuff to bottles with narrow necks, and if you don’t have one, you can make one out of paper!

  3. Optional: add other ingredients

    If your cultures require carbohydrates, a good carbohydrate to start with is sucrose - table sugar. You can add up to 8% of total weight. If your cultures require protein1, you may want to add some malt extract, however keep in mind that it contains mostly sugars. Malt extract will a sufficient source of both proteins and carbohydrates on its own.

    You can also add food coloring to have agar media contrast your cultures for easier visual inspection and imaging! A few drops will do the job.

  4. Measure out 392ml water and add it to the bottle

    Easy way to measure water volume is by using a scale - put a container (tupperware works great) on a scale that can read down to a gram, press the 0 button to reset the scale to 0, and pour water into the container until you have a desired amount. Keep in mind that if you have added additional ingredients (or used a different amount of agar) in previous steps, you will have to add less water to make up for 400ml total.

    Antibacterial media for fungi culturing can be made frugally by aggressively brewing 12-24 grams of tea leaves in the water. Adjust the amount of tea leaves depending on the amount of water you are using. Using less tea leaves will make the mixture more prone to contamination, and less will slow down to culture growth substantially.

  5. Close the bottle lid but to not tighten it, you should be able to remove it without using force

  6. Sterilize the bottle with the media

  7. Tighten the lid and store

    This mixture will be good to use for 2-3 months at room temperature. If you have added extra ingredients to the mixture (such as malt extract), store in the refrigerator.

Validated Equipment

Glass bottle with stopper or a screw-on lid:

  • IKEA KORKEN 34 oz
  • Pyrex Slimline Media Bottle

Agar powder:

  • Telephone Brand Agar-Agar Powder
  • Aling Conching Agar-Agar (Red)

Notes

  1. This part might be very confusing. Personally, I don’t have much experience making nutrient media outside a lab setting, so you might have to find other ingredients that work for your purposes. It would be interesting to see if protein powders used as supplements for humans will work on cultures. ↩