Growing Bacteria in Petri Dishes

Let's Try It

1. 

   You’ll need a clean, microwave-safe container (a quart-sized bowl works great) to mix the agar with water and then boil it. These proportions make enough nutrient agar to prepare two Petri dishes. Stir these together well:
   • ½-teaspoon agar (about 1.2 grams)
   • ¼ cup (60 mL) of hot water

2. 

   Bring this mixture to a boil for three minutes to completely dissolve the agar. CAUTION: Adult supervision is required to boil water. If you are using a microwave oven to boil the mixture, be careful not to let it boil over. The mixture should be clear with no particles floating in it after boiling.

3. 

   Remove the mixture from the microwave and allow it to cool for 3 to 5 minutes before moving on to the next step.

4. 

   Take the lid off of the Petri dish (the lid is larger than the dish) and carefully cover the bottom-half of the Petri dish with warm  nutrient agar mixture.

5. 

   Loosely cover the bottom portion (set the lid ajar so excess moisture can escape) and allow the mixture to cool and harden for at least an hour.

   NOTE: Just like gelatin, agar needs to boil for a certain amount of time to properly gel. If necessary, pour any unset mixture in each Petri dish back into the bowl (Cover the empty dishes!) and microwave it again until you see it boil. Watch it boil (but not boil over) for 10-15 seconds before turning off the microwave. There should be no “floaties” in it. Pour the hot agar mixture back into the dishes (cover them!) as you did before and it should solidify within an hour.

6. 

   It’s time to collect some bacteria on the end of a cotton swab! The classic test is to roll a clean cotton swab in your mouth and then to lightly draw a squiggle with it on the gelled agar. However, many people like to test something even more gross like the keys on a computer, a cell phone case, the pump handle of a soap dispenser, or the TV remote control. Unless someone recently cleaned the buttons on the remote, you may be seeing some real goobers in a short time. Dampen a cotton swab and roll it in your fingers as you pull it across the surface of your choice.

   NOTE: You might want to collect a sample from a computer keyboard on half of the Petri dish and leave the other half for the test in Steps 8/9. Remember, you MUST use clean cotton swabs for EACH sample. In order to get a good sample, lightly dampen the cotton swab with water. Be sure to roll the swab in your fingers so all of it touches all of the surface to be tested. You want to cover the entire cotton end of the swab with invisible bacteria. Other things that you might test include: door handles, your hands, under your fingernails, the top of a desk, a pencil or a pen, the area around a bathroom sink, a calculator, or a favorite toy.

7. 

   Lift the lid off the Petri dish and LIGHTLY draw a squiggly line in the agar with the end of the cotton swab. Roll the swab in your fingers as you draw the line. Replace the lid and label the dish with the date and the name of the item you tested.

   NOTE: Moisture coming from the agar can be a problem. Many people turn the Petri dish over during this time to prevent moisture from dripping onto the growing colonies.  A suggestion: Place a drop (no more) of a hand sanitizing gel in the middle of one of the squiggles. The hypothesis is that the antibacterial chemical in hand sanitizer will keep bacteria from growing there.

8. 

   Use a sanitizing wipe to thoroughly clean one of the surfaces you tested in Step 6, e.g. cell phone

9. 

   With a clean swab, redo the squiggle test in the other half of the Petri dish from Step 6 to confirm your cleaning efforts.

10. 

    Before growing anything, some people place each Petri dish into a separate zipper-lock bag. Place the upside down dishes into a warm – about 98°F (37°C) is fine – and totally dark place to grow. In a closed box on a cable box is a great place. In a short time, you’ll be greeted by an amazing variety of bacteria, molds, and fungi. You likely see more and larger colonies over the next few days. You shouldn’t see too much growth where the disinfectants (hand sanitizers) were used. You might even see a “halo” around each location. This halo is called the “kill zone.” Measure and compare the size of the kill zone to determine the effectiveness of different antibacterial agents.

    Remember: Do NOT open the dishes once things begin to grow. You could be culturing some serious goobers and not even know it. The comfort is that they were around you all the time anyway and now you can see them. Just be careful!

11. 

    Goobers like you’re growing will often stink and make their presence known after a short time. These are not toys or curiosities you’re growing. Proper disposal is essential for both safety and sanitation. Seal all the Petri dishes into larger zipper-lock plastic bags. You can add a generous shot of chlorine bleach to the bag before sealing it to add another level of destruction. Remember: do NOT open the zipper-lock bags… ever! When you’re finished analyzing the cultures, dispose of the entire sealed bag in the trash.

    Golly, Mom is right! It IS important to wash your hands with soap and warm water whenever you can!