Science Fair Experiment: Grow Bacteria

A good science fair experiment will have one main factor: it will ask a question. The science fair experiment described here asks, “Are bacteria present in our mouths?” Using adult supervision, this is an excellent addition to any 4th grade class or higher studying biological science or microbiology. Grow bacteria for the science fair with the tutorial below.

Materials Needed

The materials needed for this science fair experiment are available at any science supply outlet or major online retailers like this kit from Amazon. There are bacterial culture kits that will allow you to skip the part of the instructions for preparing your petri dish. The list of materials given here assumes you’ll be preparing your own growing medium:

Agar
Sterile petri dish
Heatproof dish or bowl
Hot water
Measuring spoons and measuring cups
Oven mitt or pot holder
Sterile cotton swab
Tape
Permanent marker
Magnifying glass
Microwave
Science journal or notebook

Once you’ve collected all your materials, you’re ready to begin your experiment.

Prepare Your Petri Dish With Agar

A sterile petri dish is a lidded, shallow cylindrical glass or plastic dish. It is clear for easy viewing of what’s inside. Biologists and microbiologists use them to grow bacteria, fungi and small mosses.

You’ll be coating the bottom of your petri dish with a thin layer of agar, a jelly-like substance made from a type of algae that bacteria love. It comes in gel or powder form. To prepare your petri dish with powdered agar, follow these steps:

For each petri dish you’re preparing, you’ll need approximately half a teaspoon of agar powder.
After placing ½ teaspoon of agar powder into a microwave safe dish or bowl, add ¼ cup of hot water and stir. You’ll need to increase this equally for each petri dish.
Place bowl in the microwave for one minute or until boiling.
Use oven mitt or pot holder to remove dish from microwave.
Set it aside to cool for a minute or two.
Remove petri dish from the packaging and separate the two halves.
Carefully pour warm agar into the bottom half of the dish. Do not overfill. Put just enough to cover the bottom with a thin layer.
Replace the lid and let the petri dish set for about two hours.

Swabbing Your Mouth for Bacteria

To collect your bacteria sample, you’ll need your sterile cotton swab. Carefully remove it from the package, taking care not to touch anything with the cotton tip. Swipe the cotton tip around the inside of your mouth – over your teeth, along your tongue and along the inside of your cheeks.

Inoculating Your Petri Dish

Now you’re ready to introduce your bacteria sample to the agar in your petri dish, or inoculate your dish. Using the cotton tip you swabbed through your mouth, gently streak it across the surface of your agar, being careful not to dig into the gel. Swipe it in a back and forth motion across the entire surface of the agar. Replace the swab in its original package then dispose of it in the trash.

Label, Seal and Wait

Place the lid on your dish, then tape it closed. Put a strip of tape on the bottom, then write your name and the date on it. Place your dish in a warm, dark place. Leave it there and wait.

Check your dish at 24 hours, then again at 48 hours and 72 hours. Use a magnifying glass to get a close look at your growing bacteria.

Record Your Observations

Record the steps of your experiment in your science journal. Upon completion, record the results and your observations. Research and answer these questions in your journal: