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Lesson Plans
Backyard Bacteria

Overview Procedures for Teachers Organizers for Students

Procedures for teachers is divided into four sections:

Media Components

  • Computer Resources:

    • Modem: 56.6 Kbps or faster.
    • Browser: Netscape Navigator 4.0 or above or Internet Explorer 4.0 or above. Macintosh computer: System 8.1 or above and at least 32 MB of RAM.
    • Personal computer (Pentium II 350 MHz or Celeron 600 MHz) running Windows® 95 or higher and at least 32 MB of RAM
    • Software: Any presentation software such as Power Point or Hyperstudio (optional), and word processing programs like Microsoft Word, Word Perfect, ClarisWorks, AppleWorks, etc.

  • Materials:

    Teachers will need the following supplies:

    • Marking pens
    • Paper punch
    • Supply of nutrient agar (See above for places to purchase it.)
    • Plastic Petri dishes with compartments (one per group of 2 students)
    • Plastic Petri dishes without compartments (one per group of 2 students)
    • Supply of sterile cotton swabs (at least two per group of 2 students)
    • Large 2-liter beaker
    • Supply of the following:
    • Household bleach
    • Several bars of soap
    • Rubbing alcohol
    • Listerine
    • Liquid antibacterial soap
    • Small cups to distribute the above solutions
    • Hand lenses
    • Handouts of Web resources if computers are not available in the classroom
    • Video of RX FOR SURVIVAL,"Rise of the Superbugs," Episode #101

    Each group of 2 students will need:

    • 1 plastic Petri dish with compartments pre-poured with nutrient agar
    • 1 plastic Petri dish without compartments pre-poured with nutrient agar
    • 2 or more cotton swabs
    • Tweezers
    • Supply of 5 - 10 punched out paper disks
    • Household bleach
    • Several bars of soap
    • Rubbing alcohol
    • Listerine
    • Liquid antibacterial soap
    • Marking pen
    • Safety glasses and lab aprons
    • Hand lens
    • Computers with the capacities indicated above
    • Notebook or journal
    • Pens/pencils/color pencils

  • Bookmarked sites and video resources:

    Tip: Before teaching this lesson, bookmark all of the Web sites used in the lesson on each computer in your classroom, create a word-processing document with all of the Web sites listed as hyperlinks, upload all links to an online bookmarking utility such as, or make paper handouts of necessary Web pages so that students can access the information on these sites. Make sure that your computer has necessary media players, like RealPlayer, to show streaming clips (if applicable).

    Please preview all of the sites and videos before presenting them to your class.


    • Rx for Survival
      Rx for Survival: A Global Health Challenge is a six-part PBS series airing November 1st, 2nd and 3rd, 2005 that discusses the history of public health pioneers and the global health issues we are facing today. The series and a national outreach campaign concerning child survival will strive to create an understanding of global health issues, as well as inspire national dialogue about the United States' role in addressing serious health issues.

    Web sites

    • Rx for Survival
      This is the companion Web site to the groundbreaking PBS series, Rx for Survival. Site content features will include: information about the series along with a preview video clip; an atlas that illuminates key factors influencing global health in countries around the world; stories of global health champions; portraits of killer infectious diseases; first-person accounts of life at the forefront of the battle for global health; the politics of global health; an opportunity to "ask the experts"; and educational materials for middle and high school teachers and students with a focus on both science and social studies.

    • Photo Gallery of Bacterial Pathogens
      This site contains a multitude of photomicrograph pictures of different types of bacteria.

    • Shapes of Bacterial Colonies

      This user-friendly site shows a picture of the common shapes of bacterial colonies and their names.

    • Streak Plate

      This is a photograph of what a streaked agar plate looks like. It should be used to demonstrate how the students should spread the collected colony from their first plate to their second one.

    • Antibiotic Resistant Bacteria
      This site explains what causes bacteria to become antibiotic resistant.

    • Centers for Disease Control and Prevention
      This is the agency of our government that tracks diseases, epidemics, and other health related issues. It is an excellent resource for any study of diseases.

    • National Institute of Allergy and Infectious Diseases
      This is the government's main research institution for the study of infectious diseases. It has excellent resources on diseases. Be sure to visit the general NIH education page at for educational comprehensive resources available free of charge to teachers.

    • National Institutes of Health Office of Science Education
      This site is an excellent resource for science teachers. It contains many multimedia modules on a host of interesting and timely science topics. The module on Infectious Diseases is appropriate for this unit.

  • Tips

    • Petri dishes and nutrient agar can be purchased from any science supply company. The catalogue numbers from Flinn Scientific are as follows: (Tel) 1-800-452-1261; Plain Petri Dish -- AB1470 ($5.00/pack of 20); Partitioned Petri Dishes -- AB1471 ($6.65/pack of 20).

    • In a pinch you can use unflavored gelatin mixed with beef broth instead of agar, but it will take some trial and error on your part to get the gelatin to set properly, and it doesn't work quite as well as true nutrient agar.

    • If you don't have access to an incubator, all is not lost. You can also make an incubator by placing a small desk lamp inside a wooden or metal box. Or you may put a Styrofoam cooler upside down over a desk lamp. A small lamp (15 watts) should be able to create enough heat to warm up a small space. Prepare your incubator in advance and use a thermometer to test it the day before starting your experiment.

    • Solid agar can safely be melted in a microwave oven or by placing the bottle of solid agar in a pan of boiling water.

    • Petri dishes should be incubated with the agar side up to prevent moisture from collecting on the agar.

Prep prior to class:

  • Pour nutrient agar into Petri dishes. There should be enough dishes so each group of two students has two dishes. Ideally, purchase Petri dishes that have two or three compartments. One supply house with on-line ordering is Science Kit and Boreal Laboratories or contact them at 777 E. Park Drive, PO Box 5003, Tonawanda, NY 14150; (Tel) 800-828-7777; (Fax) 800-828-3299.
  • Set up an incubator with the temperature at 37 degrees Celsius.
  • Fill a 2-liter beaker with about 1500 ml of tap water, and add about 50 ml of household bleach to it. This will be used to sterilize items used to handle the bacteria.
  • Have several bars of soap on hand so the students can wash their hands often.
  • Using a one-hole punch, punch out several hundred pieces of filter paper or coarse paper towel. These will be used in the second part of this experiment (Day #4).


Introductory Activity

  • Ask the students what they know about bacteria. Record their responses on the board. Remind the class that 99% of all bacteria in the world are "good" bacteria.

  • Ask the students if they know some good uses for bacteria. Record their responses on the board. Bacteria are used to make yogurt, to make some cheeses, to clean up oil spills, and to help digest food in your intestines. Bacteria also decompose things that have died, returning their substances to the environment.

  • Show the class pictures of different kinds of bacteria.

  • Tell the class that there are bacteria all around and that it is easy to grow colonies of them. All bacteria need is some food and a nice warm place to grow. Tell the class that the food they like comes from seaweed and is called agar. It is mixed with water and some beef broth, heated up, and then poured into sterile dishes called Petri dishes. Show the class an empty Petri dish.

  • Tell the class that bacteria multiply rapidly. If they put one bacterial cell on some agar, the bacteria will double every 20 minutes. Ask the class how many bacteria there would be after 24 hours. The answer is 272 or 2 with 72 zeros after it.

  • Ask the class if one tiny, lonesome bacterium cell could harm them if it was the "bad" kind of bacteria. Explain to them that their immune system would easily kill that one bacterium off. Ask the class if the body would have such an easy time fighting off 272 of the same kind. Explain to the class that this is why certain safety precautions must be taken when culturing bacteria. List the following rules on the board and insist that they be followed:

    1. Do not take samples from bathrooms.
    2. Once the cotton swab has touched the agar, do not touch it with your hands.
    3. Dispose of all used cotton swabs in the large beaker of bleach and water.
    4. Never open an exposed agar plate unless instructed to do so.
    5. Wash your hands with soap and water after handling any bacteria, agar, or even if you just think you might have.
    6. Wash your hands with soap and water anyway before leaving the classroom.

  • Ask the students if they know what a controlled experiment is. A controlled experiment is an experiment where all variables are held constant except for the one under study. In this case, the controlled variables would include the size of the Petri dish, the type of agar used, the temperature of incubation, and other such factors. The experimental variable in this case would be the location chosen to sample.
  • Have the class design an experiment where all variables are controlled except the one under study. Record their procedure on the board and either have them copy it into their lab notebooks or prepare it as a handout for the next day. (See STUDENT ORGANIZER #1 for a suggested procedure.)
Learning Activities:

  • Day #1

    1. When the students get to class, review with them the procedure they decided upon the day before. Remind them of the safety precautions needed for this experiment.
    2. Ask the class to suggest locations within the school building that might be sampled for bacteria. List the locations on the board.
      Note: Devise a way to assign locations to different groups.
    3. After reminding the class once more of the safety precautions, send them off to sample their assigned location for bacteria. Tell them to be back in 10 minutes.
    4. When the groups return, remind them to dispose of the cotton swab in the beaker with bleach.
    5. Give each group a marking pen and have them label the bottom of their Petri dish with the name of the location sampled, the date and their own initials.
    6. Have the groups seal the Petri dishes by wrapping masking tape around them.
    7. Put the sealed and labeled dishes in the incubator and leave them for at least 24 hours. 48 hours is better.
    8. Have the students make predictions about how much bacterial growth they expect on their plates. Have the class list the locations chosen from "most growth" to "least growth." Keep this list until the next day.
  • Questions for Day #1:

    1. Why was one side of the dish not swabbed? (It is the control side. It shows what happens if nothing is done to the dish.)
    2. Why was the experimental side swabbed twice in an "s" pattern? (To make certain that any bacteria are evenly spread across the surface.)
    3. Why is the cotton swab placed in the bleach solution? (To kill any bacteria on it.)
  • Day #2

    1. When the students arrive, hand them their Petri dish from the incubator. Remind the class, once again, not to open their dishes.
    2. Have each group sketch the growth found on their plate. (See STUDENT ORGANIZER #2.) They should sketch using colored pencils, or they could simply use different shading for colonies that appear to be different.
    3. Hand out STUDENT ORGANIZER #3 - A Guide to Bacterial Colony Shapes.
    4. Have the students report how many colonies of each shape they've found on their plate. Record this information on the board.
    5. Have the students make two lists. The first should be of the locations ordered according to the number of total colonies found. The second list should be organized by location according to the number of different kinds of colonies found.
    6. Have the students compare these two lists to the one they came up with the day before. Discuss with the class why there is such variety of bacterial types in their school.
    7. Return the Petri dishes to the incubator until the next class.
  • Questions for Day #2

    1. Were there any colonies on your control side? (There may very well be colonies there. This is due to bacteria in the air settling on the agar. It is common and shows that bacteria are all around us.)
    2. How many different colony shapes did you find? (Answers will vary.) How many different types of bacteria does this represent? (Answers will vary, but the number of different colony shapes is the lowest possible number.) Can two different types of bacteria have the same shaped colony? (Yes.)
  • Day #3

    1. Have the class summarize what they'd learned from the day before. Ask the students if they'd ever heard of antibiotic resistant bacteria.
    2. Ask the class if they know how bacteria might become resistant to antibiotics. Tell them that they'll be using the cultures they grew to help answer this question.
    3. Return the cultures in the incubator to each group. Remind them to leave the Petri dish sealed. Give each group a second poured Petri dish, but this one should have no dividers in the bottom.
    4. Have each group identify one colony they'd like to study more. Have them circle this colony on the underside of the dish using a marking pen.
    5. Tell the class to very, very carefully unseal the dish but not to open it.
    6. Demonstrate to the class how to open the dish and sample a targeted colony. Simply lift one edge of the dish top high enough so the cotton swab can be brought into contact with the desired colony. Once the colony has been collected by simply rolling the swab across it, remove the swab and replace the cover. Reseal the plate with tape.
    7. Open the second dish and streak the collected colony across the surface of the agar being careful not to puncture the surface. The streaked plate should have this pattern:
    8. Cover the newly streaked plate, and set it aside.
    9. Tell the class that they're gong to expose the sampled colony to several different cleaning agents.
    10. Give each group a pair of tweezers and several filter paper punch-outs. Also give each group a small amount of the following substances:
      • Rubbing alcohol
      • Anti-bacterial soap
      • A weak bleach solution
      • Listerine
      • Tap water (as a control)
    11. Have each group dip each punch out in different solutions and place it on the agar plate near the edge. The different punch-outs should be spaced equally apart. Tell the students not to have the paper dripping wet, but that they should tap the paper on the inside of the container to knock some of the excess liquid off.
    12. After all punch-outs have been placed, have the groups cover the plates and put them in the incubator for at least 24 hours, although 48 hours would be better.
  • Questions for Day #3

    1. What were some experimental controls in this experiment? (size of dish, size of paper disks, colony used. Many controls can be identified. These are just a few.)
    2. What was your experimental variable? (The liquid on the disks.)
  • Day #4

    1. Before the students come to class, dispose of their first plates by immersing them in a bleach solution for about 15 minutes. Wrap the plates in a plastic bag, double bag them, and dispose of the plates in the regular trash. Having been soaked in bleach, they pose no hazard to the environment.
    2. When the students arrive in class, give them their plate from the incubator. Have them sketch what they see. (STUDENT ORGANIZER #4) Around some paper disks there may be sparse growth, but around others there may be heavy growth.
    3. Ask the class to report what they see on their plates. Ask them what sparse growth indicates. Ask them what heavy growth indicates. Sparse growth indicates that most of the bacteria in that region were killed off by the substance on the paper disc. Heavy growth indicates that the bacteria was unaffected by the substance.
    4. Ask the students to look at the sparse area very closely. Ask them if they see any indication at all that some bacteria might have survived. It is very likely that some did. Ask the class why these bacteria survived. They're somehow resistant to the substance on the paper disc.
    5. Ask the class what would happen if they collected these resistant colonies and spread them on a plate with the other liquids on disks. Ask the class if they'd see the same thing: some areas of sparse growth and some areas of heavy growth. It is very likely that this would happen again. If you have any poured plates remaining, you might try this experiment, too.
    6. Ask the class if they're getting the idea that we could create a strain of their bacteria that is resistant to all of the cleaning agents. Ask them if this would be a good thing or a bad thing.
    7. When you're finished observing the agar plates, dispose of them the same way you did for the first set of plates. Be sure to have the students wash their hands with soap and water, and you should do the same. You might even want to wash your hands off with some rubbing alcohol before using the soap and water.
  • Questions for Day #4

    1. What effect did each liquid have on bacterial growth? (Answers will vary.)
    2. Why did we use water on one of the disks? (It was the control.)
    3. What about this part of the experiment surprised you? (Answers will vary.)

Culminating Activity/Assessment:

    Option #1
      Each student is to write a story using one of the following genres:
      • Fairy tale based on a familiar one or of the students' invention
      • Horror story
      • Science fiction
      • TV sit-com
      • Shakespearean play

      Story Requirements:
      1. Each story must be based on the life of one bacterial cell's journey through this unit's series of experiments.
      2. Each story must be illustrated using either original drawings or pictures that are not copyright protected from the Internet.
      3. Each story must contain the following information:
        • The names of several different bacterial colony shapes
        • The temperature of the incubator
        • The names of the shapes of different bacteria (This will require additional research on the part of the student)
        • The elements of a controlled experiment
        • Information about antibiotic resistant bacteria

    Option #2


      Have each student select a developing country and research a disease that's straining the public health system in that country (Note: Students may find out no public health system exists in the country they chose. Encourage them to discuss how that situation would impact the spread of disease.) Then students will write a 2-page paper discussing the following:
      1. What causes the disease?
      2. What are the symptoms of the disease?
      3. Where else in the world is the disease is commonly found?
      4. What treatments are available for this disease?
      5. What are the obstacles involved in obtaining treatment?
      6. Has this disease become resistant to medicines?

      Suggested Countries:
      • Peru
      • India
      • Mali
      • Nepal
      • Bangladesh
      • Sudan
      • Indonesia
      • Gambia
      • Chad
      • Thailand

Extension Activities

Antibiotic Resistant Bacteria
    1. Ask the class if they've ever heard of antibiotic resistant bacteria. This site has excellent background information:
    2. Divide the class into small groups.
    3. Ask each group to discuss how bacteria might become resistant to an antibiotic.
    4. After the groups report their ideas to the class, have them look at the Web site referenced above.
    5. Discuss with the class the common misuse of antibiotics.
    6. Have students develop an advertising campaign to inform the public about the dangers of and how to prevent the rise of antibiotic resistant bacteria.

Public Health Campaign
  1. Record the Rx for Survival episode "Back to the Basics" to show the class.
  2. Have them list some of the most basic methods that can help prevent disease. Examples include: washing hands, drinking clean water, and eating nutritional foods.
  3. Divide the class into interest groups and select one of the methods of disease prevention. Each group will then prepare a presentation on how they'd implement an education campaign in a developing country to teach the residents about disease prevention. Ideas can include:
    1. Making a simple water filter. Guidelines can be found on the Rx for Survival Web site below.
    2. Using the techniques learned in the first experiment in this lesson, the students could show how washing one's hands can reduce the amount of bacteria. This would require the students to use the skill of designing a controlled experiment studied earlier in the lesson.
    3. Using the techniques learned in the first experiment in this lesson, the students can show how boiling water can reduce the amount of bacteria. This, too, would require the students to use the skill of designing a controlled experiment studied earlier in the lesson.
    4. Making a poster display of how a septic system and/or sewage treatment plant works.
  4. The National Institutes of Health has produced some excellent classroom materials. You can find them at this site: The module titled Emerging and Re-emerging Infectious Diseases is outstanding. Even though it is listed as a resource for grades 9 - 12, it can be easily adapted for use in middle school classrooms. The materials are free, extensive, and highly motivating. This module is an excellent way to extend and expand a study of bacteria and diseases.

Community Connections:

Have some members of the class research the following organizations to see what they have to say about antibiotic resistant bacteria:
Have other students conduct a survey asking family members, friends, community members, and student peers about their viewpoints on antibiotic resistance. Then both groups can share their findings during a class discussion on the similarities and differences in perspective between professional organizations and the public.

Overview | Procedures for Teachers | Organizers for Students