To understand that many bacteria found in and on humans also are participating in making some of our most flavorful foods and drinks; it isn’t just a coincidence but rather an unavoidable consequence of our relationships with bacteria.
This lesson is part of the Skin Deep Project, which examines the science behind skin. Skin Deep is developed by AAAS and funded by Neutrogena. For more lessons, activities, and interactives that take a closer look at the science behind skin, be sure to check out the Skin Deep Project.
This lesson is the third of a three-part series in which students explore bacteria and our relationship with them.
In The Ecology of Your Skin 1: Bacteria that Live on the Skin, students explore the concept of interdependence of life in the context of our relationship with bacteria. To do this, students explore the physical parameters of the body and how those affect the bacteria that live on the body.
The Ecology of Your Skin 2: The Microbial World Is an Olfactory World introduces students to the olfactory world of our bacterial symbionts. Students examine some ideas about the purposes of body odors and create some hypotheses themselves.
In The Ecology of Your Skin 3: The Body Food Connection, students perform an exploration of bacteria in milk to see how they can get cheese-like results from body bacteria. This is extended to explorations and discussions on traditional and ethnic cuisines and the importance of bacteria in creating the distinctive aromas and flavors.
By the end of the eighth grade, students should have built up a collection of cases based on their own studies of organisms, readings, and film presentations. They should have moved from specific examples of the interdependency of organisms to a more systematic view of the kinds of interactions that take place among organisms. Students also should understand that two types of organisms may interact with one another in several ways: they may be in a producer/consumer, predator/prey, or parasite/host relationship. (Benchmarks for Science Literacy, p. 117.)
Middle-school students also should have had ample opportunity to examine cells from various living things with microscopes and know that all living things are composed of cells. This understanding allows them to examine cells more in depth at the high-school level.
At the high-school level, students are now ready to accept the concept of an ecosystem, which should bring coherence to the complex array of relationships among organisms and environments that students have encountered. (Benchmarks for Science Literacy, p. 117.)
Be sure to read through The Ecology of Your Skin before beginning this lesson. It provides excellent background information on our relationship with the bacteria on our skin.
Get one or more of the following books from the library (school or public or interlibrary loan). These are primarily for your own background reading. However, excerpts may be appropriate for class assignment readings. Also, students who would like to do more with skin bacteria may get ideas from perusing these books:
- Microbial Inhabitants of Humans. Michael Wilson. New York, NY: Cambridge University Press, 2005. (This book has a wonderful chapter on “who is who” of microbial inhabitants of the skin; lots of tables and diagrams.)
- A Field Guide to Bacteria. Betsey Dexter Dyer. Ithaca, NY: Cornell University Press, 2003. (This book has chapters on bacteria as symbionts of animals and as enhancers of foods.)
- The Ecology of the Human Skin. Mary Marples. Springfield, IL: Charles C. Thomas, 1965.
In addition, you might browse the cookbook section of your public library for books on cheese making, traditional sour dough bread, and traditional pickling. If available, look at books on traditional methods in preparing Asian fermented foods such as fermented fish sauces, bean sauces, and kimchi.
Scout out ethnic grocery stores, delis, mail order catalogues, and gourmet food stores in anticipation of buying a few interesting samples for students to try. Be adventurous and try a few yourself ahead of time.
Direct students to their Bacteria, Humans, and Food student esheet where they will learn more about how human cuisine has benefited from microbes. After they have read the information on the esheet, ask them these questions:
- In what ways have microbes contributed to human cuisine?
- (Fermenting microbes can make fresh foods into something different with subtle, interesting, savory smells and flavors.)
- What kind of relationship exists between these microbes and humans?
- (There seems to be a beneficial relationship between the two.)
- Do humans benefit from these microbes? If so, in what ways?
- (Humans can benefit from the microbes, not only by having new foods to eat but also by having some of the microbes end up in our stomachs where they can aid digestion.)
- Do microbes benefit from humans? If so, in what ways?
- (Microbes can benefit too because humans have made great efforts to maintain and domesticate the valuable bacteria and fungi.)
Then have student use their esheet to go to and read Fermentation and Food on the Exploratorium website. Once they’ve read the material, ask them these questions:
- What does it mean to ferment something?
- (When you ferment food, you encourage growth of “good” microorganisms in it while preventing the growth of spoilage-causing microbes.)
- Describe fermentation in these different foods: pickled vegetables, wines, breads, and cheeses
- (Pickled vegetables: soaked in salt brine, allowing the growth of bacteria that eat the vegetable’s sugars; Wines: yeasts eat the grapes’ sugars and produce alcohol; Breads: yeasts digest sugars in the dough, producing carbon dioxide which makes the dough rise; Cheeses: milk bacteria produces lactic acid by digesting milk sugar lactose. This in turn acts with the added enzyme rennet to curdle milk. The whey is drained off and the curd is compacted. The curd is then ripened into cheese by microbes.)
For the main part of this lesson, students will explore bacteria in milk as described in the Cheese-Making Project student sheet. Decide whether you want explicit directions handed out or whether you would like to guide them in more free-form exploration. Have all the materials on hand and let them come up with some procedures with your suggestions.
An important result of the milk experiments is how easy it is to get cheese-like results from body bacteria and from soil bacteria. From a bacterial point of view, we are a convenient continuum. From a human point of view, it’s a good thing that bacteria often tend to make food and drink more delicious. Throughout human history, it has been a necessity that we “share” our food with them.
Have on hand some of these sources of bacteria as well as the other preparatory items found in the student sheet:
- Specks of soil from the school yard or a potted plant
- A little water from a thorough (but soapless) hand-washing
- Some yoghurt or kefir (unflavored)
- Part of a tablet of “Probiotics,” a nutritional supplement of freeze-dried bacteria that can be purchased at a pharmacy
- A little of some fermented food from one of the Asian cultures such as kimchi (Korean) or fermented fish sauce (Thai) or fermented soybeans (Chinese). If you have students who can bring in homemade versions, that would be ideal as those are most likely to be full of active bacteria.
- The air in the classroom. Just leave the container open for 24 hours and then put the lid on and observe over the next few days.
- If you have access to cheese-making supplies, get some packets of freeze-dried bacteria and use tiny amounts.
- Pour milk into the container of choice, keeping the lid off only briefly.
- Add a small amount of “inoculum”—the source of your bacteria. This might be a speck of soil or a small spoonful of water or food.
- With lid either tight or loose (your choice), allow the milk to be fermented by the bacteria. Lactic acid-producing bacteria should be especially active. Leave the cultures at room temperature or any temperature of your choice. Decide whether they are to be in the light or the dark.
- Observe on Day 1, 2, 3, 4, 5 and as long as you like. Students can record their observations on the Cheese-Making Project Record Sheet.
- On the first day of observation, have a class discussion to create a list of what should be observed. (It probably will be necessary to add to that list on days 2-5, as developments occur.) Using that list, each student should add to the information on the record sheet.
Here are some ideas about what to watch for and what to put on the table:
Look for bubbles (small fizzy ones or large trapped ones), curds, complete yoghurt-like thickening, changes in color, and a more acidic pH. If you keep the cultures long enough, there may be fuzzy colonies of blue and white penicillin fungi growing on the surface. You will have created a sort of version of “Blue Cheese.” If there is an odor coming from the containers, especially if lids are loose, it will be cheese-like or yoghurt-like.
Caution: It is never a good idea to breathe or sniff deeply to detect the odors of unknown microbes. It is better to just try to notice whether a general aroma is coming from the slightly opened container. Students with compromised immune systems will have had some advice from their doctor as to whether they should participate in this exercise at all. If they are present, they should not be handling or sniffing the cultures.
A student’s table might look like:
|Feature||Day 0||Day 1||Day 2||Day 3||Day 4||Day 5|
|Blue fuzz||none||none||Tiny blue dot?||Small blue fuzzy spots||____||____|
If you have access to compound microscopes with a minimum 40X objective and of course slides and coverslips, take a small drop of a milk experiment and observe it.
Please see the instructions in the Getting Ready to Do Microscopy resource.
A community of bacteria (not just one type) participates in cheese-making. Some bacteria break down casein (the major protein of milk), some break down lactose (the major sugar in milk), and some do both. In some cases, acids and gasses are produced and curds (solids) are formed. Cheese-makers need to have a good knowledge of microbiology in order to control these activities and produce the desired type of cheese.
Here are some interpretations for some of what is going on. Students could try to make some hypotheses about what happened but some guidance from the teacher is essential:
- Yellow or brown solids: a curd has been formed by bacteria, which coagulate and break down casein. The ability of a cheese to melt smoothly (useful for making grilled cheese sandwiches or macaroni and cheese) is partly due to the break down of casein by microbes.
- There are bubbles in the tube and/or cracks in the curd: some bacteria produce gas such as carbon dioxide (sometimes called a stormy fermentation because of the wild appearance of the milk!)
- You stick in some pH paper to find out how acidic or alkaline the milk is:
- Alkaline (or basic): sometimes this can occur if bacteria release alkaline products from the casein
- Acidic: lactose was fermented to lactic acid (pH 4)
Lactose → Glucose → Pyruvate → Many different products including: lactate, acetate, ethanol, carbon dioxide AND various flavors and aromas
- If you put all of these bacterial reactions together and you do it right, you may have produced cheese with all of its enticing aromas, nuanced flavors, and creamy (or crumbly) texture. If you did it wrong, your cheese will be inedible; it will be decomposing milk. Either way, the bacteria will have a good time.
To wrap up this activity, have students make connections between typical body odors and some especially strong-smelling cheeses and other fermented foods. It is not a coincidence. For example, surface ripened cheeses such as real Munster and Limburger have a surface layer of red-pink bacteria, providing the strong aroma. Those are Brevibacterium, the same ones found between toes contributing to foot odor.
Lots more can be done with investigating artisan and/or ethnic cuisines. Some students may be able to interview older members of their families, write down recipes, and even bring in samples to taste. For example, some Korean families make their own kimchi. Perhaps you or your students have access to an ethnic grocery store where pickles or fermented beans or traditionally made cheeses are available.
You could put together a food-tasting event. Items might include sour dough bread, cheeses, kefir, fermented bean products (used as condiments perhaps), and students could write up informative signage for each food item.
Note for safety reasons, this should take place outside of the lab where you are conducting experiments with bacteria. Also, you should discourage your more adventurous students from inventing in the lab their own versions of fermented foods using bacteria that you have been working with.
A successful outcome would be one in which students really delved into family and neighborhood resources to come up with a rich and well-described assortment of tasting items. Whether this is done just for the class or whether others are invited, each student should be responsible for one or more informative signs or labels, showing an understanding of the human-bacteria-food connection.
An extension topic is to have students look into the extensive literature on immune system development and dirt (bacteria). It appears that it is easier for a child to develop a healthy immune system and a lack of allergies and asthma if a certain amount of “playing in the dirt” occurs. The obsession of some modern humans with disinfecting and sterilizing children’s play environments may actually be detrimental to immune development.