Grow Bacteria from Items in Your Home

(Biology for ages 8+)

Do you wash your hands after using the bathroom or before eating? Have you ever helped your parents clean the bathroom or the kitchen? I bet you’ve noticed that all of these things tend to get dirty again and you just have to clean them again and again! What’s the point of cleaning anyways?

Just because something looks clean doesn’t mean it is actually clean. There are teeny, tiny creatures that live everywhere in the world, including on your skin, on your television, and even inside your body!

These creatures are called bacteria and they are microorganisms, meaning you need a microscope to see them because they are too small to be seen with the naked eye. Most bacteria in the world are harmless, and some are even helpful, like the ones that live inside your gut to help you digest the food you eat.

But there are also bacteria that cause diseases that can be very harmful and even deadly sometimes. They cause diseases like food poisoning (or the stomach flu), strep throat, and pneumonia.

Cleaning is important because you get rid of the bacteria that are living around your house and on your body. Even though most of these won’t hurt you and some even protect you from the harmful bacteria, it is important to clean things every now and then to keep yourself from running into any bad ones that may have snuck in.

The video above shows a really cool way of comparing the bacteria that are present before and after cleaning to show how important it is to practice good hygiene. Here’s what you’ll need:



EITHER sterilized Petri dishes and nutrient agar powder

OR pre-poured agar Petri dishes

Cotton swabs

Cup of clean, room-temperature water (distilled if possible)

Clorox wipes or other household disinfectant cleaner

Zip-lock bags

Latex gloves (optional but highly recommended)

Fine-point felt pen or marker (fine-point Sharpies work well)

Adult supervision (Adult Supervision at all times please)



  1. The video above shows how to make your own agar and prepare your petri dishes at home. You can easily purchase nutrient agar and sterilized Petri dishes online and closely follow the directions to prepare the agar. You can also purchase Petri dishes that have been prepared with agar.
  2. The agar-prepared Petri dishes will be used to grow colonies of bacteria that you collect from surfaces and objects around your home both before and after the surface or object has been cleaned. Keep track of your Petri dishes by labeling them with the fine-point marker or setting them on a labeled paper or card that designates the surface you swabbed it with.

**Pro-tip: Make the most of your Petri dishes by using one dish for both the before and after swabs of the same surface. Using the fine-point felt marker, draw a straight line down the middle of the bottom half of the Petri dish (the half with the agar in it). Do not write on the agar, but the plastic dish itself.

Then label the side that will be used for the before swab by writing a small “B” along the edge, and writing a small “A” along the edge to mark the after side. It is important to make small marks along the edges because large writing in the middle will make it more difficult to see the bacterial colonies that grow.

  1. Dip a clean cotton swab into your cup of water to moisten it, and then wipe it over your test surface. Immediately wipe this swab on the agar of the appropriately labeled Petri dish. If you followed the pro-tip advice above, be sure to only wipe the swab on the correct side of the Petri dish and do not cross the middle line.
  2. Next, clean the surface or object you just swabbed. Use Clorox wipes or another household cleaner to thoroughly clean it and then let it dry.
  3. Once the test surface is dry, using a new cotton swab, again dip the swab into the cup of water and then wipe the cleaned surface. Immediate wipe this swab on the agar of a new Petri dish or the “after” side if you followed the pro-tip advice.
  4. Repeat steps 2 through 6 for each surface or object you would like to test. We recommend things that are often touched, like door knobs, light switches, your favorite toy, the toilet seat, the toilet handle, phone, television remote or game console controllers.
  5. Once you have swabbed all of your Petri dishes, place them flat in a box or other darkened space to allow the bacteria to grow undisturbed. For best results, wait one week before rechecking the Petri dishes. Don’t forget about them though! Set an alarm or reminder to check them so the bacterial colonies do not grow out of control.
  6. When it comes time to check on your bacteria, we recommend wearing latex gloves when touching them to avoid picking up any bacteria that may be harmful. Talk about what you see. Are there big differences between the before and after bacteria? Do you see some surfaces or objects that grew lots of bacteria compared to others? Do the bacterial colonies all look the same or do you see different shapes and sizes?
  7. After you are finished checking out your bacteria, be sure to safely dispose of the Petri dishes by sealing them in a zip-loc bag before throwing them in the garbage. Remember to properly wash your hands after handling the bacterial colonies.



The agar in the Petri dishes is a specially designed “jelly” that has tons of nutrients and sugars to provide nourish the bacteria you collect. You may never see the fuzzy bacterial colonies on the surfaces of your phone or toilet handle like you see after a week on the agar.

That’s because there usually are not a ton of nutrients present on those surfaces in your house so the bacteria do not grow as much. They still exist on those surfaces and when you place them on the agar, they are able to grow very quickly so we can see them on the agar jelly.

After you clean the surfaces and objects in your house, most of the bacteria living on them are gone so there are likely fewer and smaller colonies from the cleaned surfaces.