How to Make Agar Petri Dishes

Agar Petri Dishes

Agar petri dishes are commonly used in microbiology to culture microorganisms. Agar is a gelatinous substance derived from seaweed that provides a solid surface for microorganisms to grow on. Here is a simple recipe for making agar for petri dishes:


  • 500 mL distilled water
  • 12 g potato flakes
  • 8 g agar
  • 4 g corn syrup
  • 1/2 g peptone
  • 1 tsp blue food coloring


  • saucepan
  • rubber spatula
  • digital scale
  • glass jars or bottles
  • autoclave or pressure cooker
  • petri dishes or plastic cups


  1. In a saucepan, mix together water, potao flakes, agar, corn syrup and peptone. Stir until dissolved.
  2. Bring to a boil over medium heat.
  3. Remove from heat, then add blue food coloring and stir.
  4. Pour liquid into jars or bottles, then place in pressure cooker or autoclave. Make sure lids are loose and covered with foil to prevent water from getting inside.
  5. Cook at 15 psi for 30 minutes.
  6. Once pressure is completely released, remove jars or bottles and set aside to cool slightly.
  7. Pour liquid into sterilized petri dishes or plastic cups.
  8. Let the agar cool and solidify before using.

If you have an autoclave or pressure cooker, you can also sterilize the agar after it has been poured into the petri dishes. This will kill any microorganisms that may be present. It also prevents contamination of your cultures. To sterilize the agar, place the petri dishes in the autoclave or pressure cooker and heat at 121°C for 15-20 minutes.

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How to Identify Contamination on Agar

Identifying contamination on agar petri dishes is an important aspect of microbiology. Contamination can occur if microorganisms from the environment or improper technique enter the culture. Here are some steps to help you identify contamination on agar petri dishes:

  1. Observe the agar surface. The surface of a healthy culture should be smooth, even, and without any discoloration or texture. If there are raised or sunken spots, or the agar is discolored or cloudy, this may indicate contamination.
  2. Look for visible growth. Contaminants can grow on the surface of the agar and appear as fuzzy or opaque spots. Healthy cultures should only have growth of the desired microorganisms. No other types of growth should be visible.
  3. Smell the culture. If the culture has a foul or unpleasant odor, this may indicate contamination.
  4. Check for any changes in the agar’s appearance. If the agar becomes liquid or gelatinous, this can be a sign of contamination. Additionally, if there is any bubbling or frothing on the surface of the agar, this can also indicate contamination.
  5. If you suspect contamination, transfer a small sample of the growth to a new petri dish containing fresh agar. Should the growth on the new plate differ from the original culture, this is a sign of contamination.
  6. Problems identifying contamination at this point requires microscopy or biochemical testing to identify the contaminating organism.

It is important to maintain a sterile working environment. Also, use proper techniques to prevent contamination in your cultures. Always wear gloves and a lab coat, and sterilize any equipment and surfaces that will come into contact with your cultures.