My Science Box

Getting Ready
To prepare agar plates:

1. Thoroughly wash your hands with soap and water.
2. Set the pot (or pressure cooker) on the stove.
3. In the pot, mix together 1 liter of distilled water, 15 g of agar agar, and 2 beef bouillon cubes.
4. Heat to boiling, stirring occasionally to fully dissolve the agar agar and bouillon.
5. If using a regular pot, simmer on medium-low for at least 30 minutes to get as sterilize as possible. (It is impossible to completely sterilize a solution simply by boiling but it’s good enough for a rough classroom experiment such as this one – 25% of the plates I made were contaminated, but that also means more microbes for the students to observe). To ensure a sterile starting solution, use a pressure cooker and cook for 15-20 minutes at 15 psi.
6. While the nutrient agar is boiling, clean your work surface with warm water, soap, and perhaps even an antiseptic cleaning solution.
7. Open the packages of Petri dishes and line them up on your work surface. Keep the lids on!
8. When the nutrient agar has finished cooking, open the lid of each Petri dish as little as possible while carefully pouring 15-20 ml of the solution into the dish – each dish should end up between a third to a half full. If possible, hold the lid directly above the dish while pouring to prevent air-borne particles from settling onto the surface of the agar. Replace the lid immediately. This homemade nutrient agar will contain flecks of protein and fat from the beef bouillon, it will not disturb the experiment but students should carefully note the location of these flecks so that they are not mistaken with microbe colonies.
9. Once the nutrient agar has solidified (10-15 minutes) it will turn cloudy and opaque as opposed to translucent. At that time, turn the plates upside-down to prevent condensation from pooling on the surface of the agar.
10. Store them stacked upside-down in the refrigerator in their original plastic sleeves until ready to use.

To sterilize Q-tips:

1. Wrap 10 Q-tips in a paper towel. Make several packets of Q-tips, enough for 1 Q-tip per agar plate.
2. Set a pot (or pressure cooker) on the stove with water and a steamer basket in place.
3. Place the Q-tip packets into the steamer basket and boil the water.
4. Steam the Q tips for at least 30 minutes. If using a pressure cooker, cook for 15-20 minutes at 15 psi.
5. When the Q-tip packets finish steaming, place them into a new ziplock bag and seal them until ready to be used. While the exterior of the paper towels will be contaminated, the Q-tips inside should be relatively sterile.