Bacteria Science Project Guide

Projects with bacteria are fun and easy! Use this simple procedure as a basis for designing your own experiments or science fair projects. Ideas for other projects are listed at the end.

Watch our video for an overview:

Bacteria are one-celled, or unicellular, microorganisms. They are different from plant and animal cells because they don't have a distinct, membrane-enclosed nucleus containing genetic material. Instead, their DNA floats in a tangle inside the cell. Individual bacteria can only be seen with a microscope, but they reproduce so rapidly that they often form colonies that we can see. Bacteria reproduce when one cell splits into two cells through a process called binary fission. Fission occurs rapidly in as little as 20 minutes. Under perfect conditions a single bacterium could grow into over one billion bacteria in only 10 hours! (It's a good thing natural conditions are rarely perfect, or the earth would be buried in bacteria!)

Growing and testing bacteria is a fun any-time project or a great science fair project. Bacteria are everywhere, and since they reproduce rapidly they are easy to study with just a few simple materials. All you need are some petri dishes, agar, and sterile swabs or an inoculating needle. Agar is a gelatinous medium that provides nutrients and a stable, controlled environment for bacteria growth. Most bacteria will grow well using nutrient agar, but some more fastidious bacteria (those with more complex nutrient requirements like Bacillus stearothermophilus, Branhamella catarrhalis, and Bacillus coagulans) prefer tryptic soy agar.

You also need a source for bacteria, and this is not hard to find! You can swab your mouth or skin, pets, soil, or household surfaces like the kitchen sink or toilet bowl. If you want to study a particular type of bacteria, you can also purchase live cultures. Adult supervision is recommended when working with bacteria. Growing bacteria is a safe activity if you keep the lids closed and always wash your hands after handling the dishes.

Preparing Culture Dishes

Before you can grow bacteria, you'll need to prepare sterile culture dishes. A 125ml bottle of nutrient agar contains enough to fill about 10 petri dishes.

1. Melt the agar in either a hot water bath or a microwave. The bottles will get very hot, so use tongs or a potholder to pick them up.

Water Bath Method - Loosen the agar bottle cap, but do not remove it completely. Place the bottle in hot water at 170-190 ºF until all of the agar is liquid.
Microwave Method - Remove the bottle cap and microwave on high for 30 seconds. Swirl the bottle to distribute the heat. Repeat in 15-20 second increments until the first signs of boiling appear and all of the agar is liquid.


2. Let the agar cool to 110-120 ºF (when the bottle still feels warm but not too hot to touch) before pouring into petri dishes.
3. Slide open the cover of the petri dish just enough to pour agar into the dish. Pour enough agar to cover 1/2 to 2/3 of the bottom of the dish (about 10-13ml). Don't let the bottle mouth touch the dish. Cover the dish immediately to prevent contamination and tilt it back and forth gently until the agar coats the entire bottom of the dish. (Fill as many dishes as you have agar for: you can store extras upside down in the refrigerator until you're ready to use them.)
4. Let the petri dishes stand one hour for the agar to solidify before using them.

Preparing Sensitivity Squares

One method for testing the antibacterial effectiveness of a substance is to use "sensitivity squares." Cut small squares of blotter paper (or other absorbent paper) and then soak them in whatever substance you want to test: iodine, ethyl alcohol, antibacterial soap, antiseptics, garlic, etc. Use clean tweezers to handle the squares so you don't contaminate them. Label them with permanent ink, soak them in the chosen substance, and blot the excess liquid with a paper towel.

Setting Up an Experiment

Each experiment should have a control dish that shows bacteria growth under normal conditions and one or more test dishes in which you change certain variables and examine the results. Examples of variables to test are temperature or the presence of antiseptics. How do these affect bacteria growth?

1. Using a sterile swab or an inoculating needle (heat-sterilized in a flame), collect bacteria from your chosen source (e.g., kitchen sink, inside of your cheek, or live culture). Remove the cover of a petri dish and lightly rub the swab across the surface of the agar in a zigzag pattern. (You may want to turn the dish a quarter turn and zigzag again for maximum coverage.) Cover the dish again immediately. Repeat with a new swab for the other dishes you are using in your experiment.

2. Label one dish "Control." Then in your test dish, use tweezers to add the sensitivity squares that have been soaked in a substance you wish to test for antibacterial properties. It's a good idea to add a plain square of blotter paper to see if the paper by itself has any effect on bacteria growth. For best results, use multiple test dishes and control the variables so the conditions are identical for each dish: bacteria collected from the same place, exposed to the same amount of antibacterial substance, stored at the same temperature, etc. The more tests you perform, the more data you will collect, and the more confident you can be about your conclusions.
3. Place all the dishes in a dark, room-temperature place like a closet. (Unless you are testing them for reaction to different temperatures, of course.)

Wait 3-7 days and examine the bacteria growth in the dishes, without removing the lids. You will see multiple round dots of growth; these are bacteria colonies. Depending on where you collected your bacteria samples, you may have several types of bacteria (and even some mold!) growing in your dishes. Different types of colonies will have different colors and textures. If you have a compound or stereo microscope, try looking at the colonies up close to see more of the differences.

Compare the amount of bacteria in the control dish to the amount in the test dishes. Next, compare the amount of bacteria growth around each paper square. Which one has bacteria growing closest to it? Which one has the least amount of bacteria growing near it? If you did more than one test dish, are the results similar in all the test dishes? If not, what variables do you think might have caused the results to be different? How does this affect your conclusions?

Disposing of Bacteria Cultures

Remember to use care when experimenting with bacteria. The kind of bacteria you're using may be types that are normally present in your house, but you are culturing them in greater numbers than usual, and this can be hazardous. When you are finished with your experiment, pour a little bleach into each petri dish, seal the dishes in a plastic bag, and throw them away.

More Bacteria Experiment Ideas

Here are some other project ideas for you to try on your own or use as a basis for a bacteria science fair project:

• Household cleaners. Which household cleaners work best against bacteria? Try swabbing a surface in your home, like the kitchen sink or a toilet, and then use sensitivity squares to test different cleaners such as Lysol, bleach, Windex, etc.
• Natural substances. Test to see if garlic really has antibacterial properties. What about tea tree oil, or red pepper, or curry?
• Mouthwash. Swab your teeth and gums and see how well toothpaste or mouthwash work against the plaque-causing bacteria on your teeth.
• Antibiotics. Use an antibiotic disc set to see what different antibiotics can do against bacteria. For a more advanced project, learn how gram staining relates to the use of antibiotics.
• Have you heard people say that dogs' mouths are cleaner than humans'? Design an experiment to test whether this is really true!
• Some band-aids are advertised as being antibacterial. Test to see if they really work better than regular band-aids at inhibiting bacteria.
• Is it safe to keep refilling a water bottle without washing it? Test a sample of water from the bottom of a water bottle that has been used for a couple days and compare it to a sample from a freshly-opened, clean water bottle. You can also test to see if a bottle gets more bacteria in it if you drink with your mouth or with a straw.
• Do bacteria grow in your shoes? Is there a difference in bacteria growth between fabric shoes and leather? Do foot powders work to cut down on bacteria?
• Do bacteria grow on your toothbrush? What are some ways you could try to keep it clean? Mouthwash? Hot water?
• Some people recommend getting new mascara every six weeks because bacteria can grow in the tube. Test this by comparing bacteria growth from old mascara and new, unused mascara. You can also test how much bacteria is on other kinds of makeup.
• What about good bacteria? Try making some homemade yogurt with this kind!

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