The Ecology of Your Skin 2: The Microbial World Is an Olfactory World

What You Need

The Ecology of Your Skin 2: The Microbial World Is an Olfactory World Photo Credit: Clipart.com


To introduce students to the olfactory world of our bacterial symbionts.


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 second 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. What we want to emphasize is not so much the structure, but the fact that bacteria, like all cells, carry out basic functions and the effects of those functions on us (i.e., body odor).

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 in order to gain a deeper understanding of the interactions between organisms.

In the first lesson of this series, your students were encouraged to brainstorm about their bodies as ecosystem-like habitats for bacteria. This lesson goes into much more depth on the “wet tropical forests” of your armpits and groin.

One of the changes in the human body at puberty is the activation of specialized sweat glands called apocrine glands under the arms and in the pubic area. These glands are different from typical sweat glands (eccrine glands) that keep the body cool. The apocrine glands secrete several chemicals that are olfactory signals of emotions including fear, nervousness, and sexual excitement. Bacteria living in and near the apocrine glands digest the chemicals and release even more different odors.

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.)

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Planning Ahead

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 reading assignments. 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.

Also, try to get from the library (most likely via interlibrary loan) these books:

  • Aroma: The Cultural History of Smell. Constance Classen, David Howes, Anthony Synnott. London/New York: Routledge, 1997. (There are brief synopses of some relevant chapters in the Extensions section. The book is full of interesting ideas for class discussions.)
  • Worlds of Sense: Exploring the Senses in History and Across Cultures. Constance Classen. London: Routledge, 1993.

These two books were informative in forming the four hypotheses discussed in the Development section. Read over these hypotheses in conjunction with browsing the books or even in lieu of browsing the books if they are unavailable. Realize that scientists are still investigating this topic and do not necessarily have all the answers about body odor. Feel free to come up with some of your own hypotheses.

In general, avoid websites that seem derivative, repeating the same information over and over and having some sort of agenda (or commercial product) for preventing bacterial activities. There is enough of that in the popular press. It is more interesting to do something quite different and more exploratory with this topic.


Is it possible to think about body odor in a positive way? This lesson allows students to give it a try. At puberty, specialized odor-producing glands become activated and bacteria begin to participate in modifying those odors. This is typical and normal in most mammals. There must be something more interesting going on than just an opportunity to sell underarm deodorants!

To get students to think about this topic before they move into the Development, ask them questions like these:

  • Why do you think feet smell sometimes?
  • Why does sweat smell?
  • What causes body odors?

Accept all reasonable answers and encourage students to explain their responses. This discussion should help demonstrate current student understanding and also student misconceptions.


When scientists form hypotheses, they are making guesses about answers to questions. The guesses typically are based on whatever information is already known. In this case, you know something about the bacteria that normally inhabit your skin. (See The Ecology of Your Skin 1 for background information and exercises on skin bacteria.)

Divide your class into four groups. Provide each group with a portion of each hypothesis listed below and ask them to develop the hypotheses more fully. (You can do this by handing out the Body Odor student sheet.) Note that the first part is repeated in each question, since assuming body odor is normal and has a purpose is the general starting point for all the further considerations. Ask each group to present their hypotheses. Then have students discuss which one is more likely in their opinion and why.

Note that the following hypotheses are not intended to be the correct answers to the four questions. The students may come up with some of these possible reasons and more. The “answers” below are not answers per se but rather hypotheses or discussion items. Note that browsing the books recommended in the Planning Ahead will broaden your own knowledge base in preparing this topic for your students. Scientists are not finished with hypothesizing about this topic and students should not be either.

  • Assume body odor is normal and that it even has some useful functions. What might those purposes be? Modern humans often live and work in close quarters. How might that affect the functions and perceptions of body odors?
      (When humans began living together peacefully and calmly in close proximity, such as in cities and in enclosed buildings, deodorizers were invented. In tight quarters, humans usually try to control their vocal communications and their visual communications such as body language. This allows other humans a little solitude and privacy as well as choices about with whom to interact. Likewise, it is helpful in close quarters to control aspects of body odor that signal aggression and sexuality.)
  • Assume body odor is normal and that it even has some useful functions. What might those purposes be? Could it be that our olfactory abilities are not sensitive enough to detect information in body odors? Which of our senses are most important in our perception of the world? Try comparing humans to dogs and notice their appreciation of odors.
      (We are no longer olfactory animals as most of our ancestors were. Instead, we are highly visual animals and a considerable part of our brains is devoted to processing what we see. Dogs rely much more on olfactory information. They have about 1000 genes that encode for olfactory receptor proteins. In contrast, humans have only about 400 functional genes for olfactory receptor proteins. In addition, we carry in our genomes about 500 non-functional olfactory genes, a sort of archaeological vestige of our olfactory past. Before those genes became dysfunctional, our sense of smell may have been as acute as a dog’s. We would have experienced a very different sensory world.)
  • Assume body odor is normal and that it even has some useful functions. What might those purposes be? Humans are the only mammals who wear clothing. How does clothing affect body odors and our perceptions of them?
      (We are the only animals who wear clothing. Underarm hair and genital hair probably evolved in part to hold onto important scent information. However, clothing and shoes may capture and hold odors way past their usefulness and freshness as communication devices. For example, in the case of shoes, which are relatively recent inventions, in addition to protecting the feet, they provide yet another dark, moist crevice for bacteria and place for odors to become so complex that they are no longer informative.)
  • Assume body odor is normal and that it even has some useful functions. What might those purposes be? Are manufacturers of deodorants influential in our perceptions of body odors? How are body odors perceived in countries where deodorants are not available? How were they perceived before deodorants were invented?
      (Our perception of body odor is a learned behavior, taught to us by the very aggressive marketing tactics of manufacturers of deodorants. On one hand, the manufacturers of deodorants may have a point: perhaps we are not getting much useful information from smelling each other (unlike dogs). When we are in close proximity, it might be better to not have a chaotic input of various odors. So why not mask the odor and buy a product to do so? On the other hand, it might be difficult to sort this out. Manufacturers have a strong motive to keep us buying deodorants and not much incentive to encourage us to think about the possible benefits of odors. A more objective investigation might be better done with a group of indigenous people with a culture more influenced by traditional practices and who do not have ready access to all of the deodorizing products that fill our stores.)

After (or during) the discussion, you may wish to add some of the following information or hypotheses if they are not brought up by the students:

The function of body odor in almost any mammal, including humans: In a positive sense, your apocrine odor is part of your normal identity as a mature human. Part of that identity is contributed by your own bacterial communities, which process your chemical signals into a variety of distinctive odiferous compounds. In our primate ancestors and relatives (as well as all other mammals), apocrine odors are normal messages to potential rivals and mates. What about in humans? What happened in the history of humans that body odors became so undesirable, especially by modern humans living in developed countries?

After (or during) the discussion, remind the students (to get the topic back to the ecology of skin bacteria) that it is intriguing how much of apocrine odor is a result of bacterial activity. The apocrine chemicals are essentially food for the bacteria and the bacteria create their own distinctive aroma signature that then becomes part of human identities.

Students should follow up on your discussion of the above hypotheses by conducting research or an experiment to test one of the hypotheses. They should then present the results of their research and or experiment to the class.


Our society is full of influential advertising, some of which is the sole source of information to the general public on the topic of bacteria. Bacteria are typically represented as “germs” or pathogens. Our relationships to bacteria are depicted as adversarial. Grocery stores stock many antimicrobial or antibacterial cleaning products for both home and body. We are all encouraged to clean our world of bacteria.

An outcome of this lesson is a more holistic understanding of bacteria and our intimate and positive relationships with them. Ask students to use the Pro-Microbial Advertising Campaign student sheet to set up a sort of advertising campaign for their high school to depict the diversity of bacteria as a normal and positive part of our lives. The pro-microbial advertising might appear in morning announcements to the student body, in student newspapers, on the student radio or cable TV show, or on posters in the hallways. Essays on this topic might take the form of advertising copy. Note that this does not have to be a campaign advocating for body odor. However it should be more informative than the usual advertising campaigns on that topic.

A caveat: An important aspect of the advertising campaign would be not to counteract hype with hype. That is, the goal is not to demand that the reader simply agree that bacteria are great. Rather, the student should include convincing scientific facts and evidence and draw logical conclusions. In that sense, this should not be an advertising campaign in the usual sense but rather an opportunity to spread scientific information in an appealing way to the general public of the school.


Some of the websites and videos suggested in the previous lesson, The Ecology of Your Skin 1, could be reexamined here in the context of thinking about body odors and bacterial contributions to those. Images from the websites may be useful in creating an advertising campaign.

You could assign students to read and do further investigation on particular chapters of:

  • Aroma: The Cultural History of Smell. C. Classen, D. Howes, and A. Synnot. London and New York: Routledge, 1994.

There are many good ideas for classroom discussions—more than just relevant to bacteria. However, you could keep the focus on the role of bacteria in normal body odors.

Here are some highlights of selected chapters:Chapter 1 (Intro):

      Description of what it is like to be anosmic (odor blind). Our unconscious dependence on olfaction and emotional/social bonding, including the famous T-shirt test of wives distinguishing their husband’s T-shirts and mother/baby recognition. Lack of vocabulary in western cultures for smells, compared to vocabulary and classification of colors—high status of “sight.”

Chapter 3:

      Possible greater importance of odor in cultures of deep forests (as opposed to savannahs). The “Desana” people of the Colombian rainforest used as an example throughout.

Chapter 5:

      A reminder that indigenous peoples often consider invading or investigating Westerners to have peculiar body odors.

Chapter 6:

      Odor as a commodity and its commercialization—essentially removal of natural odor to be replaced by artificial (purchased) body odors. The 20th century commercialization and medicalization of body odors by advertisers as in halitosis (bad breath) and bromodosis (smelly feet). Artificial flavors including some attempts to reproduce the nuances or subtle notes of bacterial fermentation.

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This content was created with support from Neutrogena.

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