National Agricultural Literacy Curriculum Matrix
9 - 12
45 minutes plus additional observation time
This is an advanced level or honors lab. During this investigation, students will perform a coliform analysis of raw hamburger meat. They will collect, organize, and interpret data while practicing safe lab techniques. In the end, they will apply the results of a coliform analysis to food safety.
For the Class
- .25 pound (113 grams) of ground beef (ground chuck or other inexpensive cut of beef is best)
- Safety gloves
- Safety goggles
- 1.5 or 2 L flask
- Violet red bile agar (VRBA)
- Sterile spatula or tongue depressor
- Sterile aluminum foil for weighing the samples
- Burner to heat agar
- 90 ml of sterile saline solution (you can buy it inexpensively at a pharmacy)
- Blender (sterilize bowl, if possible)
- Copy of the Student Lab Sheet: Coliform Counts for each student (attached)
For Each Team
- 5 sterile Petri dishes with violet red bile agar and covers — 4 for the hamburger, and 1 for the control dish
- 3 sterile test tubes
- 27 ml of sterile saline solution
- Test-tube rack
- Permanent marker
- 4 sterile, disposable 1 ml pipettes with pipette bulbs, or 4 sterile 1 ml tuberculin syringes
- Thermal gloves or hot pads to handle hot flasks
- Obtain the violet red bile agar in powder form.
- Make sure that all the materials and equipment are available for the lab.
Essential Files (maps, charts, pictures, or documents)
coliform: a bacteria commonly used as an indicator of sanitary foods and water
food safety: a scientific discipline describing handling, preparation, and storage of food in ways that prevent foodborne illness
Background Agricultural Connections
Interest Approach – Engagement
- Introduce this experiment by asking, "Has anyone ever heard about coliforms and how they might relate to food safety? This advanced-level lab will lead us down a new and intriguing path that scientists take to analyze a food, such as hamburger, to determine if it might be contaminated with pathogens. Let’s get started and see what coliforms are and if they really count!"
- Remind students of good lab techniques and procedures (see attached and Lab Procedures handouts).
- Also remind them that most hamburgers are safe to eat. However, once in a while some bad bacteria show up in hamburger. If you haven’t already done so, discuss with the students why hamburger is so special in terms of bacterial content (see the attached Cooking Right — Temperature Investigation).
- Emphasize that there could be harmful organisms growing in the Petri dishes.
- Introduce students to why detection of coliform bacteria is important (see information found in Background Agricultural Connections).
Part 1: Teacher Demonstration
- Divide the class into teams of 4 students each.
- Distribute Student Lab Sheet: Coliform Counts to each student.
- Have students assist with the preparation of the VRBA and hamburger solution.
Preparation of the Violet Red Bile Agar (VRBA)
- Prepare the VRBA as per instructions on the label.
- In a flask, add 41.5 grams of agar to 1 L of water (use the best water available, e.g., distilled, etc.)
- A general rule is that it takes about 20 ml per Petri dish. This will tell you about how many ml of agar to prepare.
- Once you have the VRBA agar in the flask, bring it to a slow boil. Make sure the agar is at a rolling boil. Be very careful, as it can flash boil over the top very quickly.
- The agar should be translucent with no undissolved granules of agar on the sides of the flask.
- Cool the agar slightly. Caution: It will harden if cooled too long. Monitor the agar temperature as it cools. The best temperature for pouring is 111–115° F (44–46° C). A water bath set at this temperature would be ideal.
Preparation of the Hamburger Solution
- Add 90 ml of sterile saline solution to the blender.
- Weigh out 10 grams of hamburger on sterile aluminum foil. (Wear safety gloves.)
- Add the hamburger to the sterile saline solution in the blender. Blend for about 1 minute on high. The concentration of the hamburger is 1 in 10.
Part 2: Student Activity
Have Each Team of Students:
- Prepare Test Tubes and Petri Dishes
- Label 5 Petri dishes on the bottom: “10,” “100,” “1,000,” “10,000,” and “control.”
- Set up 3 test tubes and label them: “100,” “1,000,” and “10,000.”
- Add 9 ml of sterile saline solution to each of the 3 test tubes.
- Inoculate Petri Dishes (see Student Lab Sheet: Coliform Counts)
- 1 in 10
- Pipette 1 ml of the 1-in-10 hamburger solution directly into the Petri dish marked “10.”
- Carefully swirl the dish to cover the surface. Cover the Petri dish.
- 1 in 100
- Pipette 1 ml of the 1-in-10 hamburger solution into the test tube marked “100.” Now the concentration of the hamburger is 1-in-100.
- Thoroughly mix the solution by holding the test tube by the top and gently striking the bottom with the finger on the other hand for about 5 strikes.
- Pipette 1 ml of this solution into the Petri dish marked “100.”
- Repeat this procedure for the 1-in-1,000 and 1-in-10,000 dilutions.
- Have students draw the dilution and plating scheme in their food safety portfolios.
- 1 in 10
- Add the Agar to the Petri Dishes Containing the Hamburger Solutions
- Help the students learn to properly pour the agar out of the flask into the Petri dishes. Show them how to flame the mouth of the flask.
- Pour about 10 ml of agar into each Petri dish containing the hamburger solution and then swirl the dish to mix and evenly cover the bottom of the dish.
- As soon as the agar is solidified, pour in another 4 to 6 ml of agar and swirl again to spread evenly.
- Pour a control dish to make sure the agar is not contaminated.
- Store the dishes upright until the agar is solid. Then invert the dishes, seal them with Parafilm, and place in the incubator at 95° F (35° C) or let the dishes sit at room temperature (away from the sun) overnight. Examine the Petri dishes for growth the next day and record observations.
- Record Data
- Students should examine the sealed Petri dishes for the presence of colonies. Remind them to be sure that when counting the colonies, they multiply by the dilution factor. This should give relative numbers of coliforms in the hamburger.
- Students should report their findings to the class for analysis and discussion.
Concept Elaboration and Evaluation
- What is the purpose of the control dish?
- Which concentration of the hamburger dishes was the easiest to count? Why?
- What was the purpose of this lab? Write a paragraph about how the lab relates to reducing foodborne illness.
- What should be done to ensure that the hamburger is safe to eat?
- What do you think is the source of coliform bacteria in the meat?
- Do you think that pathogens make you sick every time you eat them? Why? Why not?
- List 10 other foods that you would like to test for coliform bacteria. Explain why you chose each food.
- What do you think the coliform count would be for raw oysters and sushi?
- Do you think fresh strawberries would be high or low in coliforms? Explain.
Summarize by concluding with students that testing for the presence of coliforms is one way food scientists can check for possible contamination of food. The presence of coliformsin food indicates the potential presence of pathogenic microorganisms, and means that proper precautions must be taken to reduce their presence in the food before it is eaten.
We welcome your feedback! Please take a minute to tell us how to make this lesson better or to give us a few gold stars!
Suggested Companion Resources
- How Safe is Your Salad? (Multimedia)
Agricultural Literacy Outcomes
Food, Health, and Lifestyle
- Provide examples of foodborne contaminants, points of contamination, and the policies/agencies responsible for protecting the consumer (T3.9-12.h)
Education Content Standards
Biotechnology Systems Career Pathway
BS.02.01Read, document, evaluate and secure accurate laboratory records of experimental protocols, observations and results.
BS.02.03Apply standard operating procedures for the safe handling of biological and chemical materials in a laboratory.
BS.02.04Safely manage and dispose of biological materials, chemicals and wastes according to standard operating procedures.
Food Products and Processing Systems Career Pathway
FPP.01.01Analyze and manage operational and safety procedures in food products and processing facilities.
FPP.01.02Apply food safety and sanitation procedures in the handling and processing of food products to ensure food quality.
FPP.01.03Apply food safety procedures when storing food products to ensure food quality.
FPP.03.01Implement selection, evaluation and inspection techniques to ensure safe and quality food products.
FPP.03.03Create food distribution plans and procedures to ensure safe delivery of food products.
Health Standard 7: Demonstrate the ability to practice health-enhancing behaviors and avoid or reduce health risks.
7.12.1Analyze the role of individual responsibility for enhancing health.
7.12.2Demonstrate a variety of healthy practices and behaviors that will maintain or improve the health of self and others.
7.12.3Demonstrate a variety of behaviors to avoid or reduce health risks to self and others.
Common Core Connections
Speaking and Listening: Anchor Standards
CCSS.ELA-LITERACY.CCRA.SL.1Prepare for and participate effectively in a range of conversations and collaborations with diverse partners, building on others’ ideas and expressing their own clearly and persuasively.
CCSS.ELA-LITERACY.CCRA.SL.2Integrate and evaluate information presented in diverse media and formats, including visually, quantitatively, and orally.