Good Guys or Bad Guys?
Students will explore and observe microorganisms at work in decomposition as well as in the production and preservation of food. Activities include creating a "decay buffet" and identifying grocery store foods that contain or are made with the help of microorganisms. Grades 6-8
- Fermented Foods image
Activity 1: Microbes in My Food
- TED-Ed video, The Beneficial Bacteria That Make Delicious Food
- Grocery store newspaper advertisement
- Microbe Grocery List handout
- Bread wrapper
- Label from a box of cereal
- Beef jerkey package label
- Yogurt container
- Package from dried fruit
- Food Preservation Techniques
Activity 2: Decay Buffet
- Quart-size Ziplocs, 1 for every 2 students
- Clear tape
- Decay buffet (including grass, vegetable peeling, straw, dry leaves, etc.)
- Soil, 1/2 cup for every 2 students
- Water mister bottle
- Food handlers gloves for each student
- Magnifying glasses
decomposition: the process of breaking down or being broken down into simpler parts or substances especially by the action of living things (as bacteria or fungi)
microorganism: any organism, such as a bacterium, protozoan, or virus, of microscopic size
psychrophile: a psychrophilic organism thrives at a relatively low temperature (compare to mesophile and thermophile)
Did You Know?
- Microorganisms can be both beneficial and harmful.
- Fermentation is a mostly anaerobic process, meaning without oxygen, carried out by microorganisms or cells.1
- Bacteria and yeasts are used in the fermentation of foods.1
- Fermentation of food has been used by humans for thousands of years as a way to preserve foods.1
Background Agricultural Connections
Cells are the basic structure of all living things. They can form single-celled or multicellular organisms. Many of the organisms on earth are single-celled and cannot be seen with the naked eye. Although we can't see them, people interact with microorgansims in many ways. Some microorganisms facilitate decomposition, which is critical to life on earth; others help make and preserve our food; and others can contaminate our food and make us sick. The activities in this lesson explore these microorganism-related phenomena.
Yes, it’s true; decomposition is a fundamental process on which all life depends. We’d all be knee deep in garbage without it. Bacteria, fungi, and other microscopic organisms that live in the soil, air, and water are responsible for turning once living plants, animals and other organisms into nutrients that can be used again and again. Think of them as nature’s recyclers. These tiny creatures have the ability to produce special enzymes, which allow them to break down dead plant and animals and use them as food. No job is too big as they enlist the help of friends and family. As they eat, they grow and multiply at an amazing rate. In just four hours, one bacterial cell can grow to a colony of over 5,000. And at day's end there are millions and billions of them working together. Why, in one teaspoon of soil, there are more bacteria and fungi than all the people on earth!
Molds are probably the best known of the microorganisms. They are widely distributed in nature and grow under a variety of conditions in which air and moisture are present. They are members of the kingdom Fungi. Nearly everyone has seen mold growth on damp clothing and old shoes. The mold we see with the naked eye is actually a colony of millions of mold cells growing together.
Molds vary in appearance. Some are fluffy and filament-like; others are moist and glossy; still others are slimy. They are actually multicellular. Mold cells form a “fruiting body.” The fruiting body produces the spores, which detach and are carried by air currents and deposited to start new mold colonies whenever conditions are favorable. Mold spores are quite abundant in the air. So any food allowed to stand in the open soon becomes contaminated with mold if adequate moisture is present. Some types of molds are also psychrophiles (grow in cool temperatures) and can cause spoilage of refrigerated foods.
Molds are decomposers meaning that they obtain food from non-living organisms. Decomposers help to recycle nutrients from dead organisms and play an important role in creating healthy soil to grow the crops that feed us.
Some microorganisms are harmful while others are benevolent, neutral, or even helpful. Some help us to produce certain foods, some break down toxins in our environment, and others can make us sick or even kill us. For example, protozoa cause amoebic dysentery, fungi cause athlete’s foot and ringworm, bacteria cause pneumonia, legionnaire’s disease, strep throat, tetanus, and other diseases. Contaminants in food like the bacteria E. coli or Salmonella can also make us very sick. The second activity in this lesson will focus on the difference between helpful and harmful microorganisms.
Microorganisms are important to the food industry. Among their many contributions are the flavor and color they add to cheeses, and the making of soy sauce. They also play a role in making chemicals such as citric and lactic acid and many enzymes. Sour cream, buttermilk, yogurt, and hard cheeses (cheddar, Swiss, jack, feta, etc.) are all cultured with a bacteria. Other cheeses such as brie and Roquefort, are cultured by fungi. Processed cheeses, like American cheese, are not cultured with microorganisms.
Many algae are also microorganisms, and some of these are used in our food. Some ice cream contains a thickener made from seaweed. Seaweed, or algae, is found in a variety of foods. Chunks of it float around in Korean soups, paper-thin sheets of it are wrapped around Japanese rice balls, and it lies hidden in the alginates and carrageenans in hamburgers, yogurt, and ice cream. Seaweed-based food additives are now so commonly used in prepared and fast food that virtually everybody in Europe and North America eats some processed seaweed every day.
Sometimes microorganisms spoil food. Most of your students will have seen rotten, spoiled, moldy food in their refrigerators. Food that is spoiled by bacteria may look different to the naked eye, but the food will often smell or taste bad and will probably make you sick. Molds are more visible. Certain kinds of mold can produce poisons called mycotoxins. Mycotoxins have only recently been discovered and little is known about what causes molds to produce them. Probably the best known use of molds is in the drug industry, where they help produce such antibiotics as penicillin.
The old adage for dealing with questionable food is the best advice “when in doubt...throw it out!”
- Display the Fermented Foods image, which shows several food items created with the assistance of microbes.
- Allow students to begin offering ideas about what the foods pictured have in common. Use guided questions to direct them to begin thinking about how the food was processed:
- Are these foods in the same form as they were when they left the farm? (No, they have been processed)
- How are each of these foods made? (Microorganisms play a role in making all these foods; cheese, yogurt, sausage, and pickles are fermented by lactic acid producing bacteria; bread dough rises thanks to yeast, which are single-celled, microscopic fungi)
- Once students are thinking about bacteria and microbes, ask, “Is it safe to have bacteria in our food? Are bacteria always bad?”
Explore and Explain
Activity 1: Microbes in My Food
- Share the TED-Ed video The Beneficial Bacteria That Make Delicious Food (4:39 min) with your students.
- Divide the students into groups of two.
- Provide each group with a grocery store advertisement and the Microbe Grocery List handout.
- Instruct them to find all the foods in the advertisement that have a relationship to microorganisms, and write down the sale price of each item. Remember foods like ice cream may contain thickeners from microscopic algae, and foods containing dough have yeast.
- Ask each group to share the food products they found in their advertisements with the class. Did they miss any? Did other groups find the same products? Are they cheaper? Can these foods be spoiled by other microbes and make us sick?
- Explain to students that virtually all foods can spoil or be contaminated. That is why you find food additives or inhibitors or preservatives in food—to keep them fresher or viable longer.
- Read the labels of the food items listed in the materials list. Ask the following questions:
- Can you identify an ingredient that might be a food additive or preservative? Sometimes sugar, salt, or vinegar is added to a product to inhibit the growth of microorganisms. A chemical preservative may be added to do the same thing; these will have little effect on the flavor of the food. For example, jelly is so sweet that few additives need to be used to preserve freshness; the sugar acts as a “natural” preservative, the same with pickles and vinegar.
- Do all the items have the same food preservative? No, because some food additives or inhibitors only work on certain microbes, see the Food Preservation Techniques page.
- Explain to students that although we don't want our food to spoil, decomposition is an important part of the food chain. When organic materials like food waste, grass clippings, fallen leaves, and animal waste decompose, they release nutrients into the soil. These nutrients are important to growing the crops that we eat.
Activity 2: Decay Buffet
- Divide the class into pairs.
- Provide each pair of students with a Ziploc quart bag and ask them to write their names on some tape and then stick the tape on the bag.
- Set up a “decay buffet” of items like grass clippings, vegetable peelings, weeds, soil, stale bread, straw, dry leaves, etc. to be placed in the bags. In your offering of decay buffet items, try to provide twice as many dried out, high-carbon items as green, fresh, high moisture items. This mimics the recommended ratio for compost of two parts brown (dry) to two parts green (wet).
- Instruct students to place one small piece of each item at the decay buffet into their bags. Have them cut up items if necessary. Explain that it is important not to add any meat or dairy to their bags (or to compost) as potentially harmful bacteria could grow.
- Ask students to record the exact contents that they place in their bags. It may work best if one student places the items in the bag while the other records the bag's contents.
- The recorder should also note predictions as to what will happen to each item over time. Will the item rot? Smell yucky? Remain the same?
- Optional: You may want the students to switch roles and create a second compost bag with a list of contents and predictions.
- Ask the students to add about 1/2 cup of soil to their bags and to lightly mist the contents with a plant mister. (Adding a teaspoon of water and mixing the contents will work the same way.)
- Have the students blow into the bags (to inflate slightly) and carefully seal the bags.
- Once the bags are sealed, leave them for two to eight weeks. You may decide to keep the bags together or place them in various locations with differing conditions. (If you let the students choose their compost bag’s location, be sure to have everyone register their locations on a class master list or you may be unpleasantly surprised when a missing bag finally makes its presence known.)
- Have students create compost bag journals. Ask them to observe their bags periodically and record what they see happening inside. Do they see fuzzy masses? Remind students that they are not to open the bags until the designated date.
- On the designated date, have the students take their bags outside. Distribute plastic gloves to the students to wear while sorting through the contents of their bags with their partners. They may need magnifying glasses to “see” the original items. Caution: students with known allergies to mold or fungus should not participate!
- Ask students to record any items still identifiable and in their present state. Provide misters or water bowls so items can be cleaned off for closer observation and identification.
- Are any items missing? Check the list and note the items missing. How did the results compare with the predictions?
- Define and discuss the process of decomposition or decay.
- You may want to ask your students some questions:
- What are some things you have thrown away over the past couple of days? What happens to these things? Do they disappear? Decompose? Remain in the same form forever?
- Will placing the bags in various conditions have an effect on what occurs in the bags?
- Can you think of any other types of compost containers that would get the decomposition job done?
Share the video Why Can a Cow Eat Grass with students.
Cows are ruminants. Ruminants are herbivores that have a four compartment stomach. The first stomach compartment, called the rumen, is the largest compartment and can hold over 50 gallons of food. The rumen is like a giant fermentation vat, containing billions of bacteria, anaerobic fungi, and protozoa that help to break down proteins and carbohydrates. Without these microorganisms, ruminants would be unable to digest cellulose, the fiber that makes up the bulk of plant cell walls found in grass and hay. Ruminants belch frequently to expel the build up of carbon dioxide produced by the billions of microbes in their stomach.
Make cheese using instructions found in Cheesemaking: A Science, an Art, and a Craft.
One Bad Apple Spoils the Bunch
How did the saying “one bad apple spoils the bunch” come about? To find out take one bad (rotting) apple and one crisp, fresh apple and conduct the following test:
- Dip a sewing needle into rubbing alcohol to sterilize it.
- Pierce the rotten apple with the sterile needle. Immediately pierce the good apple with the same needle.
- Tie a string around the stem of the good apple so you can identify it later. As a control, sterilize a second needle in the same way as the first, and use it to pierce another good apple. (This apple will serve as a control in this experiment to insure that sticking an apple with a sterile needle will have no effect on the results.)
- Place the apples in a warm place and check them daily. Ask students to predict what will happen. (The apple with the string should develop a rotten spot; nothing should happen to the control apple.) The experiment demonstrates how sick people can spread germs by coughing or touching one another. If your friend has the flu, and coughs near you, it’s possible that the cough can pass the microbes (viruses or bacteria) to you and make you sick.
After conducting these activities, review and summarize the following key concepts:
- Cells are the basic structure of living things and many organisms are unicellular and microscopic.
- Microorganisms play an important role in decomposition, which cycles nutrients from plant and animal waste to the soil to the crops that provide us food.
- Microorganisms can be both beneficial and harmful.
- Some microorganisms help make fermented foods like cheese and vinegar. Other microorganisms contaminate food and make people sick.