Students will discover that some items in their own kitchens may be contaminated by bacteria. They will be challenged to hypothesize about where bacteria might be found in kitchens and which items might have the most and the least bacteria. Students will develop awareness that bacteria can spread from surfaces to hands, and even to food, and will hypothesize how to control the spread of bacteria. Grades 6-8
For the Class
- Distilled water — 2 gallons
- Disinfectant bleach solution (20 ml of liquid household bleach in 1 L of tap water)
For Each Team of 3 to 4 Students
- 2 to 3 sterile Petri dishes with nutrient agar and covers
- Permanent marker
- Sterile cotton swabs
- Parafilm to seal dishes
- You may want to send home a note to explain why you’re asking students to bring in items from their kitchens.
- Bring in 1-gallon, sealable (zipper-style) plastic bags, at least one for each student.
- Bring in extra kitchen items for students who aren’t able to bring items from home.
- Make one copy of the Lab Report Outline for each student.
Essential File (map, chart, picture, or document)
bacteria: a group of single-celled living things that cannot be seen without a microscope that reproduce rapidly and sometimes cause diseases
Background Agricultural Connections
Sponges, dishcloths, dish towels, can openers, refrigerator and faucet handles, countertops, and cutting boards are among the items in a kitchen that can spread bacteria if they are not cleaned properly.
Bacteria are found everywhere, and under the right conditions, they can multiply fast! Each bacterium contains all the genetic information needed to make copies of itself. Bacteria multiply through binary fission, a process in which the cell’s DNA doubles, the cell splits, and two independent cells are formed. Under the right conditions, a single bacterium will double with each division — 2 become 4, 4 become 8, etc. A single cell can turn into millions of cells in a few hours and billions of cells within one day!
This rapid growth is not usually a problem with good bacteria; however, when it occurs with “bad” bacteria (a.k.a. pathogens), it is “bad” news. As pathogens multiply, some give off harmful toxins or become infectious. If pathogens get into our food and multiply, people can get sick.
Science and our Food Supply
This lesson was developed as a portion of an entire unit of lessons focusing on food safety from farm to table. Use the following links to see the remaining lessons:
Module 1: Bacteria
Module 2: Farm
Module 3: Processing and Transportation
Module 4: Retail and Home
- Supermarket Smarts
- Cooking Right: The Science of Cooking a Hamburger
- A Chilling Investigation
- Crossed Up!
- Hands Off, Bacteria!
Module 5: Outbreak and Future Technology
Evaluation: Lose a Million Bacteria (The Game)
Interest Approach - Engagement
- Explain to students, "Bacteria are everywhere, including in your very own kitchen! In this lab, you’re going to become kitchen inspectors and look in your kitchens for things that may contain bacteria."
Part 1: Introduce the Lab
- Ask students:
- "Where in your kitchen could bacteria be growing?"
- "Could bacteria be on the kitchen items that you or your parents use when preparing food?" Make a list of students’ responses.
- Some probable answers:
- Dish towels
- Pot scrubbers
- Vegetable brushes
- Can-opener blades
- Sink stoppers or disposal covers
- Paper towels
- Cutting boards
- Some probable answers:
- Next, ask students to hypothesize about which kitchen items contain the most bacteria and which contain the least bacteria. Make 2 lists, “Most Bac” and “Least Bac.” Ask them, "Why would/wouldn’t bacteria be found on these items?
- Give each student one copy of the Lab Report Outline to record each step of the lab.
- Then ask students to vote on the items most and least likely to harbor bacteria. List the Top 5 items in each category. Keep the Top 5 lists on the board through the next few labs to allow students to compare their lab results with the lists.
- Group students into teams of 3 to 4. Ask each team to choose at least 3 or 4 kitchen items they want to inspect.
- Students should include items they think will have lots of bacteria as well as those they think will have fewer bacteria. For example, they may want to compare new sponges or just-washed dishcloths with dirty or just-used sponges or dishcloths.
- Items don’t have to be chosen only from the class list. Teams can pick their own items to investigate. Try for as many different items as possible but make sure the important ones, such as sponges, dishcloths and dish towels, are included by at least 2 teams.
- Give each team 1-gallon storage bags to take home. Ask them to bring in kitchen items from home to test. Remind students to ask their parents’ permission to bring the items to class. Ask them to put in 1 item per bag, seal the bag, and bring it to class. Mention that they should bring in some items, like dishcloths, “as is” rather than washing them first.
- Note: This lab should be set up to avoid any comparing and critiquing of items from students’ homes. Put a number on each bag as students give the bags to you. Write each student’s name on a list with their corresponding bag number. The lab should be a general learning experience about how to avoid cross-contamination in the kitchen, not a specific review of individual items. For example, students should look for common items that are most likely to harbor bacteria.
Part 2: Develop, Hypothesize, and Conduct Lab
- Ask each student to label his/her storage bags by the type of item (e.g., sponge, can opener, etc.). Students’ names should not be put on the bags.
- Have each team develop a hypothesis and design a lab to test the hypothesis.
- Pose these questions to guide students through their labs:
- Which type of kitchen items might have the most/least bacteria? Why?
- How can you find out which items have the most/ least bacteria? (Swab the items and inoculate the Petri dishes.)
- What’s a good way to get a sample of the bacteria in these items? (Swab the item directly, or add water to the bags and sample the water.)
- What kind of water would be best to use? (Sterile or distilled water)
- How can you transfer bacteria from the kitchen item to the water? (Squeeze/massage the item in the bag.)
- What does the water look like after you squeeze/massage the item? (The water in some bags will appear very dirty. Water in other bags will appear clean.)
- Is the appearance of the water an indication that bacteria are present? (Not necessarily.)
- How can you be sure you’re not introducing new bacteria to the test samples? (Use sterile dishes and swabs. Use a control bag with distilled or sterile water.)
- Demonstrate how to sample the water and inoculate agar dishes (see the attached Lab Procedures sheet for more details).
- Discuss the importance of a control dish. To test for bacteria in the water, swab half of the control dish with distilled water. Leave the other half untouched.
- Review the important rules of lab safety, especially the handling of inoculated Petri dishes.
- Let each team develop its own hypothesis and design its own experiment.
- Let the students conduct their experiments.
Part 3: Observe and Record Results
- Have teams observe their Petri dishes and record the results on a data table.
- Ask each team to present their findings to the class. List the class results and have the students compare their findings to the “Most Bac” and “Least Bac” lists from Part 1. (Be sure to focus the discussion on the categories of items rather than on specific items.)
- Pose questions to guide students through a discussion. For example,
- How did your original lists compare to the test results? Were there any surprises? What are your conclusions?
- Could bacteria transfer from kitchen items to your food? Your hands? What might happen in these cases?
- Why do certain kitchen items have more bacterial growth than others?
- How do the data you collected relate to the 4 Cs of Food Safety? Which of the 4 Cs?
- How could you reduce the bacteria on the items you tested?
- What are your suggestions for avoiding cross-contamination in the kitchen?
- What advice would you give to family members to help them prevent the spread of foodborne bacteria?
Concept Elaboration and Evaluation
- Did you find any bacteria? Where? Were you surprised with your findings? Why or why not?
- Why is it a problem to find bacteria on a kitchen sponge, dishcloth, dish towel, etc.? (Bacteria from these items can spread to other surfaces and then to food.)
- To what surfaces can the bacteria spread? (Cutting boards, utensils, kitchen equipment, countertops, etc.) How can bacteria spread from your hands to other things? (Bacteria can spread from your hands to other things when contaminated hands touch those surfaces.)
- What can you do to prevent cross-contamination? (Wash your hands thoroughly. Frequently wash sponges, dishcloths, and dish towels. Wash cutting boards, countertops, and cooking utensils with hot, soapy water after each use.)
- What can you personally do to reduce the spread of bacteria in your kitchen at home? (Follow the 4 Cs — especially Clean and Combat Cross-Contamination. See the 4 Cs section in the Food Safety A to Z Reference Guide.)
- Did you discover that you were “guilty” of any unsafe practices in the kitchen? If so, what’s your strategy for correcting those unsafe practices?
Bacteria can spread from kitchen items to hands, and even to food. The spread of bacteria can be controlled through proper cleaning and disinfecting as needed.
Develop a Home Food Safety Survey based on the results of your investigation. Give the survey to at least 5 family members, friends, relatives, or neighbors to survey their kitchens. Tally the answers.
Using the survey results, develop a “kitchen safety” brochure or web page explaining how to prevent cross-contamination in the kitchen.
Use your food safety portfolio to record how your foodborne pathogen relates to your findings from this experiment.
The Science and Our Food Supply Curriculum was brought to you by the Food and Drug Administration Center for Food Safety and Applied Nutrition and the National Science Teachers Association.
- FDA Education Team Leader Food Safety Initiative: Marjorie L. Davidson
- FDA Science and Our Food Supply Project Director: Louise H. Dickerson
- FDA/NSTA Associate Executive Director and Science and Our Food Supply Program Director: Christina Gorski
- FDA/NSTA Science and Our Food Supply Program Assistant: Jill Heywood