Growing Bacteria in Petri Dishes
Take samples and see what will grow in an agar Petri dish.
This activity will prove that Mom was right... "Wash your hands with soap and warm water!" A Petri dish prepared with nutrient agar (a seaweed derivative with beef nutrients) is an ideal food source for the bacteria you'll be growing. In this experiment, Steve Spangler collected samples from items around the office - you will not believe what he found.
- 1 Petri dish (4-inch size)
- Agar nutrient (5 grams)
- Container to boil water
- Cotton swab
- Hand sanitizer
- Zipper-lock bag
- You'll need a clean, microwave-safe container (a quart-sized bowl works great) to mix and heat the agar with water. These mixing proportions make enough nutrient agar to prepare two halves of the Petri dish. Mix 1/2 teaspoon agar (about 1.2 grams) with 1/4 cup (60 mL) of hot water and stir. Bring this mixture to a boil for one minute to completely dissolve the agar. CAUTION: Adult supervision is required to boil water. If you are using the microwave oven to boil the mixture, be careful not to let the solution boil over. The mixture should be clear with no particles floating around in the solution. Allow the mixture to cool for 3 to 5 minutes before moving on to the next step.
- Separate the Petri dish (there's a top and a bottom) and carefully fill the bottom half of the Petri dish with warm agar nutrient solution. Use the top half of the Petri dish to loosely cover the bottom portion (set the lid ajar to allow moisture to escape) and allow the solution to cool and harden for at least an hour.
- 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 your computer or the television remote control. Unless someone recently cleaned the buttons on the TV remote, you're in for some real YUCK in a few days.
- Consider all of your options below (or come up with your own) to collect samples. You might want to collect a sample from a computer keyboard for one half of the Petri dish and collect a sample from a door handle for the other half. Remember, you must use clean cotton swabs for each sample. In order to get a good sample collection, dampen the end of the cotton swab with water. Be sure to wipe the end of the cotton swab all over the surface to be tested to cover the end of the swab with invisible bacteria. Things that you might want to test: door handles, your hands, under your fingernails, your mouth, the top of a desk, computer keyboard, remote control, pencil or a pen, area around a bathroom sink, fax machine, calculator, cell phone, or your favorite toy.
- Lift the top off the Petri dish and LIGHTLY draw a squiggly line in the agar with the end of the cotton swab. Cover the Petri dish with the top half and use a piece of paper or tape to label the dish with the name of the item you tested. For your protection, place the sealed Petri dish inside a zipper-lock bag and seal it closed. For safety reasons, do not ever open the zipper-lock bag - you can view the growing bacteria through the clear plastic bag.
- Here's a clever test: Try placing a drop (no more) of hand sanitizing gel in the middle of one of your squiggles. Your hypothesis might be that the antibacterial chemical in hand sanitizer will keep any bacteria from growing. We'll see if you're right.
- Place the plates in a warm dark place to grow - not too warm, but anything up to about 98 degrees F (37 degrees C) should be fine. In a short time, you'll be greeted by an amazing variety of bacteria, molds, and fungi. You should continue to see more and larger colonies for the next few days, but you should not see any growth where the disinfectants (hand sanitizers) are. You might even see a "halo" around each spot where you placed the hand sanitizer. 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 plates once things begin to grow. You could be culturing a pathogen.
- Remember not to open the zipper-lock bag... ever! When you're finished analyzing your growing bacteria, dispose of the entire bag in the trash.
Golly, Mom is right! It is important to wash your hands whenever you can!
How does it work?
You're likely to have a huge variety of colors, shapes, and smells in your tiny worlds. Count the number of colonies on the plate, note the differences in color, shape, and other properties. Getting bacteria to grow can be a little tricky, so don't get discouraged if you have to make more than one attempt. Allow enough time for them to grow, too. You need millions of them in one place just to see them at all. They're really tiny! In a lab, you'd use your trusty inoculating loop to pick up a bit of the bacteria in order to create a slide for further study under a microscope.
Most bacteria collected in the environment will not be harmful. However, once they multiply into millions of colonies in a Petri dish they become more of a hazard. Be sure to protect open cuts with rubber gloves and never ingest or breathe in growing bacteria. Keep your Petri dishes sealed in the zipper-lock bags for the entire experiment. When you're finished with the experiment, some people recommend placing the Petri dish bag in a larger zipper-lock bag along with a few drops of bleach. Seal the larger bag and dispose of it in the trash.
Science Fair Connection:
Just growing bacteria in a Petri dish is not a science fair experiment. Yes, it is gross and cool and fascinating, but it doesn't meet the requirements of a science fair project. If you want to do a science fair project about germs, you have to add a variable, or something that changes in the experiment.
- In the Growing Bacteria activity described above, adding an anti-bacterial hand sanitizer is a variable. Make one dish of germs and one dish of germs with a drop of the anti-bacterial sanitizer or, better yet, make three dishes--one as the control (just germs), one with an anti-bacterial sanitizer, and a third dish with another brand of anti-bacterial sanitizer. Then you can see which anti-bacterial sanitizer is more effective in killing germs. Just make sure that all three Petri dishes have germs from the same place in your home or classroom so that you know they are all exposed to the same bacteria. They also need to be grown in the same warm, dark place for the same amount of time so that the conditions are standardized as much as possible.
Growing Bacteria is such a popular activity that we've written it up as a sample science fair project (see the link below). The sample project describes the swabbing technique to collect the germs and gives you lots of helpful hints about growing bacteria. It makes suggestions about variables and gives you some ideas to make the project your own. What it doesn't give you is the data. What fun would that be? Don't you want to do the experiment for yourself and see what discoveries you make?
If you want to do a science fair project on germs, check out the Growing Bacteria science fair project.
Petri Dish Experiment
Katy - February 24, 2010
I used this experiment to test different brands of soap. It was amazing to see which soaps actually cleaned my hands and which ones did not. Dial soap turned out to be almost as strong as a surgical soap called Betadine. The students really loved this experiment, especially the boys!
Erin Maryland - January 21, 2010
My five year old and I had so much fun testing door knobs, remote controls, dog noses, toilet seats, etc...It was amazing to see after a few days how much bacteria (and different types of bacteria) was growing in the dishes