Grade Level: 6th - 9th; Type: Biology
Students will learn how to plate samples on Petri dishes, measure bacterial colonies and evaluate the effectiveness of different cleaning agents. They will also consider the behavioral implications of their findings. Do people wash hands in the bathroom in public places?
- Are there more germs on the inside or outside handle of a public restroom? If people wash hands inside, there should theoretically be fewer germs inside.
- Are there more germs on the doors of a public restroom than the bathroom in your home?
- How effectively do Clorox wipes kill bacteria?
- How effectively does rubbing alcohol kill bacteria?
- What can be done to minimize germs in public restrooms?
While we all know that germs are everywhere, people tend to be the most germ-conscious in public bathrooms – and theoretically wash their hands before leaving. By inoculating Petri dishes from swabs of the interior and exterior door handles, students learn whether which is truly cleaner. Clorox is a bacteriocidal agent. With a high pH, it is often recommended for killing germs on surfaces. Rubbing alcohol serves a similar function. Both are bacteriocidal agents, meaning that they kill bacteria. They can be distinguished from bacteriostatic agents that merely inhibit division of the bacteria.
- individually sealed sterile cotton swabs (at least eight)
- Pre-filled Petri dishes (at least eight)
- Clorox wipes
- Rubbing alcohol (optional)
- Paper towels to use for cleaning with rubbing alcohol (optional)
- Hand sanitizer, such as Purel or another comparable brand
Most materials are readily available at home and in the grocery store. Petri dishes can be ordered on-line from vendors such as Edmund Scientific or obtained from educational supply house.
- Mark the outside of the packages of sterile cotton swabs “Inside,” “Outside,” “Inside Clean,” and “Outside Clean” with a pencil, taking care not to break the sterile seal or tear the wrapper. Mark four of your Petri dishes the same way with a Sharpie marker. Keep the Petri dishes sealed.
- Go to a public bathroom in a big store where there is a lot of foot traffic. Bring your Petri dishes, hand cleaner, sterile, individually sealed sterile cotton swabs and Clorox wipes with you. Leave the Petri dishes in the car, but bring the sterile cotton swabs, hand cleaner, gloves and Clorox wipes into the store.
- When you arrive at the bathroom, wash your hands with the hand cleaner.
- Carefully unpeel the two halves of one short end of the sterile sealed swab package labeled “Inside” for about a half inch. Be sure to open the end of the package where the stick, not the swab, is located.
- Remove the swab and thoroughly swab the inside of the door. Carefully slide the used swab back inside the package and fold the packaging shut.
- As you did in step #3, carefully unpeel the two halves of one end of the sterile labeled “Outside” about a half inch.
- Remove the swab and thoroughly swab the outside of the door. Return the swab to the package labeled “Outside” and fold the end of the package shut.
- Scrub the inside and outside handles with Clorox wipes. Wear your cleaning gloves, if needed.
- Repeat steps 3 through 6, using the swabs labeled “Inside Clean” and “Outside Clean.”
- When you get to the car, carefully open one of the Petri dishes. Matching the label on the dish with the label on the swab, stroke the swab over the surface of the agar, make sure you expose most of the agar to the tip of the swab. This is called inoculating the Petri dish. Cover the dish immediately when done. Do not open the dish again.
- Repeat step 10, using all of the swabs and Petri dishes.
- Bring all the Petri dishes home and leave them in a location where they will not be disturbed.
- Repeat the experiment with the bathroom door at home.
- Examine all your Petri dishes every day for a week. Count the colonies in each dish. Which had the most cultures – the inside or outside of the door? What does this say about people’s behavior? How clean was your home bathroom? How effective was the Clorox in removing germs? Take photos of the cultures for your poster board.
- You can also repeat the same experiment using rubbing alcohol to clean the door handle.
Terms/Concepts: Bacteriocidal vs. bacteriostatic, Bacterial growth, Sterile technique, Petri dishes, Growing bacterial colonies
Dyer, Betsey Dexter. A Field Guide to Bacteria. Cornell University Press. (2003)
Wearing, Judy. Bacteria: Staph, Strep, Clostridium, and Other Bacteria (A Class of Their Own). Crabtree Publishing Company. (2010)
Rutala, William, et al. “Antimicrobial Activity of Home Disinfectants and Natural Products against Potential Human Pathogens”. Infection Control and Hospital Epidemiology. 21. January, 2000.