Procedures
This lesson is intended to be used as a follow-up lesson to Frankie's Story from EnviroMysteries: Inside Stories, however it can be completed as a stand-alone science lesson.
This lesson focuses on the science behind food-borne illness. For a great lesson on the causes and prevention of food borne illness, see How did that Get Into My Lunch?
On Day 1 students review and discuss food borne illness. They then watch a video that can help them to understand how bacteria live and reproduce; in other words, how a single-celled organism can multiply rapidly, and the environmental conditions which are conducive to this rapid multiplication.
On Day 2, students take samples from various items and locations around the school that they think contain high levels of bacteria. They then culture the bacteria on Petri dishes, predicting the number of colonies bacteria cultures will produce. Following the sampling, students will wait 3-4 days to check their samples, and analyze and reflect on the results.
Student Prerequisites
Students should have a basic understanding of lab safety procedures and cell structure.
Day 1: UNDERSTANDING BACTERIA
Daily Challenge Question: Under what conditions do bacteria thrive, and how can they cause disease?
Set Up Directions:
Prepare enough copies of the Poison Food Video Worksheet.
Have the Teacher Answer Key handy to help guide student responses.
You will need a computer with Real Player or Windows Media, connected to an LCD projector to project the video, Dr. X and the Quest for Food Safety, from the website: http://www.cfsan.fda.gov/~comm/vltdrx.html.
You will watch Module #1, a 15-minute video.
Note: the video is quite small, so be prepared to have the projector as far as possible from the screen in order for the video to be as large as possible. Alternatively, students can watch the video individually or in small groups in a computer lab.
Teacher Presentation and Motivation:
Introduce the topic of Food Poisoning and the role of bacteria in it by asking the students the questions that follow. The answers are provided in parentheses to guide the teacher, but at this point allow the students record any information they (think they) know. Use the Poison Food KWL chart or other graphic organizer to record student answers.
Have you or anyone you know ever become ill from eating food? What are some reasons we become ill from food? (allergic reaction, food poisoning).
What is food poisoning? What causes it? (Food contaminated with microorganisms such as viruses, bacteria, parasites)
What are bacteria? (Single-celled organisms with no true nucleus that often reproduce by binary fission)
How can a single-celled organism make us so sick? Responses will vary based on students’ background knowledge. If you are doing this lesson in conjunction with the Frankie's story from EnviroMysteries: Inside Stories, then students will already know about food borne illnesses caused by the bacteria, Listeria.
Activity Day #1 --Dr. X and the Quest for Food Safety
Tell the students they will now watch a video that will help them better understand bacteria and how they can make us sick.
Show students the Dr. X and the Quest for Food Safety video, Module #1. http://www.cfsan.fda.gov/~comm/vltdrx.html
Focus for Media Interaction:
Students should note any questions or new knowledge about bacteria and food preparation practices as they watch the video. They should also note the characteristics common to bacteria and how some bacteria can make us sick.
Viewing Activities:
Have students watch the video in its entirety. Have students add to the Poison Food KWL chart as they watch the video. In addition, have the students list any food preparation practices they notice and whether they think these practices help prevent food borne illness or foster it.
Distribute the Video Worksheet and discuss the questions. See what students can recall from watching the video.
Rewind the video to about halfway. Replay from the part where there is a drawing of a bacterium, right after the electron microscope image. Students are responsible for answering the questions on the worksheet and for understanding how bacterial cells multiply and how the harmful bacteria can contaminate the food we eat, causing illness. Refer to the Teacher Answer Key to guide the discussion and responses.
Post-Viewing Activity:
Reinforce for students that there are bacteria EVERYWHERE and that food can get contaminated at many points--on the farm, during processing or transport, in a store or at home.
Students will prepare to conduct a lab experiment the next day to see how quickly bacteria can actually grow and to determine the conditions most favorable for bacterial growth.
Wrap-Up:
Summarize what was learned about bacteria, stressing that they are everywhere and that they can multiply rapidly under the right conditions. Ask students where they think high levels of bacteria can be found at home and in their school. Ask students based on what they learned in the video about how quickly bacteria multiply. If they were to collect bacteria samples, what might a Petri dish look like after just 3-4 days? Tell students that tomorrow they will collect samples from surfaces they think have high levels of bacteria.
DAY 2 : WATCH THEM GROW
Daily Challenge Question: Can bacteria be found on common items and foods? What are their relative amounts? Can we control how quickly they reproduce?
Set Up Directions:
1. Have ready the materials described in the materials section.
2. Prepare the Petri dishes ahead of time. Follow the instructions on the agar or bacteria growing kit for sterilization procedures.
3. Pre-assign student pairs.
4. Have enough copies of the Poison Food--Lab Worksheet. Students should fill out one sheet for each sample
5. Have available some food products to test such as uncooked chicken, apples, spinach, strawberries, etc. The experiment can be extended by comparing foods left out overnight with refrigerated foods, or by comparing foods handled in areas that students suspect of having high bacterial counts (floor, bathroom) with foods handled “properly”.
Teacher Presentation and Motivation:
The students will apply what they learned from the video the previous day to make predictions about where they will find bacteria and under what conditions they will grow rapidly. Review the information they learned. Remind students to keep materials as sterile as possible in order to get accurate results.
Activity Day #2--
Watch Them Grow
- Show students the items that you’ve brought in and explain where they’ve been (i.e., refrigerated, left on countertop, etc.)
- Tell students that each pair should choose locations or items that they would like to test. The number of locations depends on the number of Petri dishes. In order to have repeated trials, try to have each area or item tested twice. Pooling data from different classes will allow a greater number of sample locations. Some recommendations for possible items to test:
- door handles
- faucet handles
- desktops
- cafeteria kitchen counter top and/or cutting board
- calculators or computer keyboards
- top of a hand sanitizer pump or soap dispenser
- sample food items
- Review standard lab safety procedures.
- Distribute to each group of students prepared Petri dishes with agar. Have students turn the dish upside down and use a marker to place their initials and kind of sample (item or location; alternatively use sample numbers).
- Review with students the proper way to transfer the sample from the cotton tipped swab to the agar (in a squiggly line, don’t dig into the agar…)
- When students are ready, have them come up and get a sterile cotton tipped swab. Warn them to be very careful to not touch the swab surface to anything or it will become contaminated. Decide in advance if students will dampen the end of the cotton swab with water which has been boiled (so it is sterile).
- Instruct students to roll the clean Q-tip all over the surface of the chosen items. Then, immediately, open the Petri dish and draw a squiggly line onto the agar. Tape the dish closed with two pieces of masking tape.
- Review with students the conditions under which bacteria grow best. Incubate the Petri dishes, lid side down, in a warm, dark place chosen with these conditions in mind.
- If you have enough Petri dishes you can do some comparisons: Have students test the same surface four times. Incubate two plates in the warm dark area, and two in the refrigerator. Alternatively, put hand sanitizer on two of the plates after sampling and then incubate all plates together. Students can think of other comparisons they would be interested in doing.
Wrap-Up:
Have students complete Part I of the lab worksheet at the end of Day 2. Each pair will need one worksheet for each sample they collected. Review with students that approximately one colony will grow on the Petri dish for each bacterium that is on their swab. Review the rapid growth of bacteria discussed when they watched the video clip so students understand that the colony they see on the Petri dish results from many divisions of one, or a small number, of bacterial cells that were on the swab.
Allow students to check the dishes daily (time permitting). In 3-4 days have students complete Part II of each lab worksheet filled out when they set up the experiment. Record data as a class and discuss (consider using data from other classes). Calculate the average number of colonies found on samples taken from the same item Remind students that just because bacteria are present on an item doesn’t mean touching (or, in the case of food, eating) it would make them sick. There are plenty of harmless bacteria as they saw in the video clip at the beginning of the lesson. Have students summarize and analyze the class results on the results and analysis worksheet.
After finishing with the Petri dishes, soak them in a tub of water containing 10% bleach for 30 minutes to insure disinfection. They can then be thrown out in the trash.