How To Make Your Own Agar For Petri Dishes

Scientists and biology students use agar, a substance extracted from red-purple algae, to grow bacteria cultures in petri dishes. Sugar galactose, a substance prevalent in red-purple algae cell walls, is agar's main active ingredient. Agar is ideal for growing bacterial cultures; it becomes firm when cooled to room temperature, and bacteria does not eat away at it. Although you can purchase pre-poured agar petri dishes, preparing your own takes little time and is more cost effective.

Step 1

Dissolve 10 agar tablets for every 500 ml of water. If you prefer to use agar powders, add 6.9 g of agar to 500 ml of water.

Step 2

Heat the agar and water solution in a saucepan or microwave in a microwave-safe container until the solution reaches 185 degrees F, or 85 degrees C. This is the point at which agar completely melts.

Step 3

Open the petri dish lids as little as possible and hold the dish at an angle.

Step 4

Pour enough agar to form a 1/8 inch layer at the bottom of each petri dish.

Step 5

Replace petri dish lids and allow agar to cool to room temperature. At this point the agar will set and your petri dishes are ready to use.