Culturing Bacteria

Students grow bacteria on nutrient agar in Petri dishes by swabbing everyday surfaces and observing colony growth. The activity demonstrates the presence of microbes all around us and can be extended into a science fair project by testing antibacterial products.

1. Prepare nutrient agar by mixing agar powder with hot water and boiling until fully dissolved.
2. Pour the warm agar into Petri dishes, cover loosely, and allow it to cool and solidify.
3. Use a clean cotton swab dampened with water to collect bacteria from a chosen surface (hands, phone, keyboard, door handle, etc.).
4. Lightly streak or draw a squiggle on the agar surface with the swab, then replace the lid.
5. Label the dish with the date and sample source.
6. Optionally, place a drop of hand sanitizer on part of the streak as a test of antibacterial effectiveness.
7. Seal each dish in a zipper-lock bag and store upside down in a warm, dark place.
8. Observe colonies forming over several days, noting color, shape, and number.
9. Dispose of sealed dishes safely by adding bleach to the bags before throwing them in the trash.

Growing Bacteria - Petri Dish - Sick Science!:

Homemade Petri Dish - Growing Bacteria at Home - The Sci Guys:

📄 Growing Bacteria in Petri Dishes - Steve Spangler: https://stevespangler.com/experiments/growing-bacteria/

📄 Exploring Germs and Bacteria at Home (DIY Agar Petri Dishes) - Astra Zeneca: https://www.astrazeneca.com/content/dam/az/media-centre-docs/article_files/articles-2020/11229%20-%20AZ-USASEF-Agar-Experiment-v7-STEM-Day-Version-GY-FH.pdf

• Test different surfaces such as sinks, desks, toys, or skin.
• Compare the effectiveness of different antibacterial products (hand sanitizers, wipes, soaps).
• Record and compare colony growth over time using photos or drawings.

• Do not open Petri dishes once growth begins—bacteria colonies can be harmful.
• Do not allow students to get samples from anywhere in a toilet or bathroom.
• Wear gloves if handling dishes and keep cuts covered.
• Work under adult supervision, especially when boiling agar.
• Dispose of dishes by sealing in bags with bleach; never reuse dishes for food.
• Wash hands thoroughly after completing the experiment.

• Which surfaces grew the most bacteria? Why? (Frequently touched surfaces harbor more microbes.)
• What differences did you see in colony color, size, or shape? (They indicate different species of bacteria or fungi.)
• How effective was the antibacterial product you tested? (Look for “kill zones” with little or no growth.)
• Why is it important to keep the Petri dishes sealed? (To prevent harmful exposure to growing microbes.)
• What does this experiment reveal about the importance of hand washing? (That our hands spread many invisible microbes.)