Microbes 2: Louis Pasteur—a Microbe Discoverer

What You Need


  • One milk carton
Microbes 2: Louis Pasteur—a Microbe Discoverer Photo credit: Clipart.com


To learn about the discovery of pasteurization and Louis Pasteur’s other contributions to science.


This lesson is the second in a two-part series on microbes. In earlier grades, students explored the health of the human body. They learned about deterrents to good health, such as tobacco, alcohol, and drugs. Research suggests that students of all ages may believe that factors important to health are beyond personal control. In addition, students in upper elementary school may think that all illnesses are caused by germs and are contagious. (Benchmarks for Science Literacy, p. 346.)

Microbes 1: What's Bugging You? offers a comprehensive introduction to microorganisms.

Microbes 2: Louis Pasteur—a Microbe Discoverer focuses on Pasteur and his discovery of microorganisms. Students know about germs—for example, they know to wash their hands to prevent spreading germs—and are ready to learn about the discovery of them. Middle-school students may not be able to imagine a world in which people did not know germs existed, because in general, students often have difficulty understanding that the beliefs, values, attitudes, and points of view of people in the past are different from those today. In addition, "Middle-school students show little regard for the thinking of scientists whose theories they know have been superseded." (Benchmarks for Science Literacy, pp. 354-355.)


Hold up a milk carton and read some of the descriptive words aloud. Most cartons say things such as Grade A, pasteurized, homogenized, and so on.

Write "pasteurized" on the board and ask students:

  • What do you think this term means?

Brainstorm with students and write their ideas on the board. Some students may know that the word comes from the last name Pasteur. If students know that, ask what it was that Pasteur did that was so important.

Then, continue with these questions:

  • Does milk come directly from cows?
    (This may seem like a simple question, however, it will give you hints about how much or how little students know of the processing of milk. They probably at least know that some milk is processed.)
  • If students know that milk is processed, ask them what they think the process is, and why. If they don't know that milk is processed and don't have a clue about what pasteurization means, ask them if they think pasteurized might be some sort of "treatment" for milk, and then ask how they think milk might be treated and why.
    (You should be particularly concerned with the "why" of this question. Though students may not be ready to answer this question quite yet, it will be important that they understand that pasteurization is to kill microbes, in order to make milk safer.)
  • Why do you think pasteurization might be important?
    (Even if students still don't completely understand what pasteurization is, they may presume it is important by virtue of the fact that most milk is pasteurized. This is one question that will lead into addressing misconceptions and attitudes. Students may show little regard for Louis Pasteur and his pasteurization achievement because they take for granted the safety of our food. By exploring what people may have had to worry about in the past, they can work on a greater appreciation for Pasteur's achievements. Here, you mostly want to discuss why and how we know milk today is safe. For example, you may want to point out that we have refrigeration and expiration dates as a safety net, but then again, a hundred years ago people may have had ice boxes and their own cows. Try to help them gain perspective.)


Distribute the student sheet Fact Collecting. Tell students to briefly skim the topics on which they are asked to take notes. They should take notes throughout the discussions and readings that are part of this lesson.

Using the Louis Pasteur-a Microbe Discoverer student esheet, students will expand upon the last question of the Motivation (why pasteurization is important) by reading Pressing Risk. This article is about a juice company that created a danger to its consumers because it did not pasteurize its product. Students may be more interested in the history of pasteurization when they realize that it directly affects their lives. Moreover, when they realize the importance of the process, it will help them realize the significance of Pasteur's work.

After students have read the article, ask these questions:

  • What is pasteurization? (This article explicitly states that it is "a process of quickly heating the liquid to kill bacteria and immediately cooling it.")
  • Why is it important? (The article makes it pretty obvious that pasteurization kills microbes that could cause illness.)
  • How do you think this concept and the process of pasteurization were discovered? (This is a lead-in question for the next segment. It is also an opportunity for you to gauge what students think now. It may be interesting to probe and ask if they think one person or many are responsible for pasteurization, or if it happened quickly or over time.)

Refer students back to the Louis Pasteur-a Microbe Discoverer esheet to read the first two pages of Microbes and Disease: Making the Link.

Ask students:

  • What led to the discovery of pasteurization? (Be sure that students understand prior science discoveries that laid the groundwork for Pasteur's discoveries. Discuss Leeuwenhoek's discovery of the magnifying lens and how that led to electron microscopes that magnify objects hundreds of thousands of times, which allow us to see microbes. Also discuss how Koch played a roll.)
  • Pasteur, while studying the microbes that cause wine to ferment, reasoned that microbes might also be responsible for illnesses. How do you think he came to reason this? (Help students to view Pasteur's reasoning in the context of his times and what was then known about microorganisms.)


Refer students back to the Louis Pasteur–a Microbe Discoverer esheet and have students read Louis Pasteur.

Assess student understandings based on their answers to these questions found on the esheet:

  • What animal diseases did Pasteur study in order to prove microorganisms were involved? (Pasteur proved a certain microbe was the cause of a disease that was destroying the eggs of silkworms. When the microbe [germ] was eliminated, so was the disease, making it a process of elimination.)
  • Pasteur lived in the 1800's. How do you think his discoveries were received during this century? (This is an opportunity to measure what your students think of Pasteur and to see if their attitudes show little regard for what scientists achieved in the past, as mentioned in the Research Base—see Context. Given their past readings, students will likely understand that his contributions to science are substantial. You may want to have an extensive discussion with questions such as: How would our lives be right now without some of his discoveries? Where would medicine and something like AIDS research be?)


To extend the ideas in this lesson, you can lead students through Sanitation and Human Health.

At the Protist Image Data Gallery students can look at pictures and descriptions of microorganisms.

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Lesson Details

Grades Themes Project 2061 Benchmarks National Science Standards

Other Lessons in This Series