How to Grow Bacteria in a Petri Dish Safely at Home

Learn how to conduct safe bacteria-growing experiments with your kids using simple household materials and proper safety measures.

1. What You'll Need. Gather these materials before starting: plastic petri dishes (available online or at science supply stores), unflavored gelatin packets, beef or chicken bouillon cubes, measuring cups, a pot for boiling water, cotton swabs, permanent markers, masking tape, disposable gloves, hand sanitizer, and a warm, dark place for incubation. You'll also need bleach for cleanup afterward. If you can't find petri dishes, clean plastic containers with lids work as substitutes.
2. Preparing Your Workspace. Set up in a kitchen or area that's easy to clean and disinfect. Wash all surfaces with soap and water, then disinfect with a bleach solution. Have everyone wash their hands thoroughly and put on disposable gloves. Keep hand sanitizer nearby and establish a 'clean zone' where you'll handle the sterile materials. Make sure pets and non-participants stay away from your workspace during the experiment.
3. Making the Growth Medium. Boil 2 cups of water and dissolve 2 bouillon cubes to create a nutrient broth. In a separate cup, mix 2 packets of unflavored gelatin with 1/4 cup of cold water and let it bloom for 5 minutes. Add the bloomed gelatin to the hot broth and stir until completely dissolved. Let this mixture cool to room temperature but don't let it set completely. This nutrient-rich gelatin will feed the bacteria and help them grow.
4. Preparing the Petri Dishes. While wearing gloves, pour the cooled gelatin mixture into your petri dishes, filling them about halfway. Don't fill them too full or the bacteria won't have room to spread. Put the lids on immediately and let them sit until the gelatin is completely set, about 30 minutes. Label each dish with masking tape, writing what you plan to test and the date. Keep the dishes sealed until you're ready to collect your samples.
5. Collecting Bacteria Samples. This is where the fun begins. Using clean cotton swabs, gently rub surfaces you want to test - like door handles, phone screens, keyboard keys, or even clean-looking hands before and after washing. Quickly lift the petri dish lid, gently rub the swab across the gelatin surface in a zigzag pattern, then immediately replace the lid. Use a new swab for each dish and work quickly to avoid contamination from the air. Try testing different areas to compare bacteria levels.
6. Incubation and Observation. Place your sealed dishes in a warm, dark place like the top of a refrigerator or a closet shelf. The ideal temperature is around 70-80°F. Check them daily but don't open the lids. Within 24-48 hours, you should start seeing small spots or colonies forming. Over 3-5 days, these will grow larger and may change colors. Keep a daily log with your kids, noting changes in size, color, and number of colonies. Take photos to track the growth progression.
7. Safe Disposal and Cleanup. Never open the dishes once bacteria has started growing. When your experiment is complete, dispose of the entire sealed dishes in your regular trash - the bacteria you've grown are generally harmless environmental bacteria. Thoroughly disinfect all surfaces with a bleach solution. Wash all reusable materials in hot, soapy water, then sanitize. Remove and dispose of gloves carefully, and have everyone wash their hands thoroughly with soap and warm water for at least 20 seconds.