What lives on our skin? Dr. Martin Blaser, the chairman of the department of medicine at New York University (NYU) School of Medicine, is trying to find out. After swabbing the arms of six people, Blaser and his lab found 182 different species of bacteria--thirty of which had never been seen before.
Overview
Bacteria are one-celled organisms that can only be seen under a microscope. There are thousands of kinds of bacteria, and they are found everywhere - in the air, in the depths of the ocean, in the human body and on human skin. Under favorable conditions, bacteria can multiply rapidly and form colonies (millions of bacterial cells grouped together) that can be observed with the naked eye.
In this activity, students will formulate a hypothesis about which area of skin on their bodies may have the most or least amount or kinds of bacteria. To test their hypothesis, students will learn the scientific process and basic lab procedure for creating a bacterial culture. They will observe and record the physical changes of their bacterial cultures in a journal and compare and contrast the bacteria found on various skin areas. Students will use the information collected to confirm or reevaluate their hypothesis.
Activity Materials
Agar prepared Petri dishes (Recommended: Tryptic Soy Agar Plates 10 Pack from www.carolina.com for $18.75)
Clean cotton swabs
Distilled water
Sterile latex gloves
Permanent markers
Masking tape
Ziploc bags
Clean cotton swabs
Distilled water
Sterile latex gloves
Permanent markers
Masking tape
Ziploc bags
Vocabulary
Bacterium (plural: bacteria) – a single-celled microorganism that does not have a nucleus.
Agar - a gelatinous substance (usually derived from seaweed extract) used as a nutrient for growing microorganisms.
Microorganism - a tiny organism such as a virus or bacterium that can only be seen under a microscope.
Bacterial Culture - the growing of bacteria in a nutrient substance in specially controlled conditions for scientific, medical or commercial purposes.
Agar - a gelatinous substance (usually derived from seaweed extract) used as a nutrient for growing microorganisms.
Microorganism - a tiny organism such as a virus or bacterium that can only be seen under a microscope.
Bacterial Culture - the growing of bacteria in a nutrient substance in specially controlled conditions for scientific, medical or commercial purposes.
What To Do
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Start the lesson by having the students watch the SciFri Video, Life On Our Skin. Use the video to review the method for growing bacteria and to explain to students that they will conduct a similar experiment to test for bacteria living on their own skin.
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Have students formulate a hypothesis: On which area of skin on their bodies do they think they will find the most bacteria? The least bacteria? Why would one skin area have more bacteria than another area? If possible, try to have students test a variety of skin areas, for a wider range of comparisons at the end of the activity.
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Hand out the following materials to each student: two agar prepared Petri dishes, two clean cotton swabs, distilled water, a permanent marker, and a pair of sterile latex gloves. (Make sure to wear sterile latex gloves when handing out materials to avoid cross-contamination). N.B., If any of your students are allergic to latex, have them label the Petri dishes record observations rather than handle the gloves.
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Have students label their Petri dishes with their initials and the part of the body that they are planning to swab. Inform students that they should write in small letters to avoid blocking their view of the growing bacteria.
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Instruct students to put the latex gloves on before handling any of the materials. Ask students why do they think that it is important to wear sterile latex gloves. Discuss the importance of maintaining a clean and sterile environment, and different ways to avoid contaminating the experiment.
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Instruct students to moisten a cotton swab in distilled water before gently rubbing it on the first skin area that they selected to test. Then students should remove the Petri dish cover and lightly rub the swab across the surface of the agar (without tearing it) in a zigzag pattern. Ask students why the agar is an important factor in promoting bacteria growth.
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Have students cover the dish immediately and place their used swab in a Ziploc bag. Ask students why the dish needs to be covered immediately. And why is the used swab sealed in a Ziploc bag?
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Tape the sides of the Petri dish to ensure that it will not open if dropped.
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Have students repeat the swabbing procedure with clean, unused materials for the second skin area that they will be testing.
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Once all the Petri dishes have been sealed, place them in a secure and dark place that will remain consistently at room temperature for up to seven days.
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Have students examine the bacteria growth on a daily basis. Students should observe the growth without removing the Petri dish’s cover. Have students maintain a journal in which they sketch the growth of their bacteria on each day, and write their observations, including changes in color, texture, size and shape. Ask students what factors they think can affect the rate of growth.
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On the seventh day, have the students compare and contrast the bacteria growth from all the dishes. Why did some bacteria grow more than others? Was there more than one type of bacteria colony in one dish?
N.B.: Most bacteria collected will not be harmful. However, some of the dishes may contain pathogens (bacteria or viruses that can cause disease) and can be a potential hazard after the bacterial colonies begin to grow. Keep Petri dishes sealed and avoid ingesting or breathing in bacteria. After the experiment is over, the recommended procedure to have an adult pour a small amount of bleach into every dish, and seal the dishes in a bag before disposal.
What’s Happening?
Bacteria are single-celled microorganisms that can only be seen through a microscope. There are many different kinds of bacteria, and they are found everywhere, in almost any type of environment, including extreme environments like hot springs. Just like any other organism, bacteria need nutrients to survive. The agar in the Petri dish is composed of nutrients usually derived from seaweed extract and other possible components, depending on the source. The nutrients are converted into energy and used to help the bacterium grow and reproduce. Under favorable conditions, a single cell can turn into millions of cells within a few hours, and into a billion cells within a few days. The grouping of millions of bacterial cells is called a colony. Often a colony can be seen without a microscope.
Samples collected from human skin may contain other types of microorganisms. Most bacteria have one of three basic shapes – rod, spherical, or spiral. Colonies that exhibit fuzzy hair-like growth are most likely fungus or mold.
Topics for Science Class Discussion
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What variables could have caused different types of bacteria to grow?
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How can you identify the bacteria or other microorganisms growing in the Petri dish?
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Why do you think that bacteria can become resistant to antibiotics or antibacterial agents such as household cleaners?
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How do bacteria reproduce?
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Why would scientists want to grow and study bacteria?
Extended Activities and Links
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Try swabbing various surfaces in your school or home. Compare bacteria collected from surfaces that have been recently cleaned with detergent (kitchen counter, window pane, glass cups, etc.) and surfaces that are constantly used (doorknobs, telephones, computer keyboards, etc). Are there clear differences in the amount or types of bacteria found on each surface?
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Design an experiment to test which natural substances such tea tree oil, red pepper, curry, or vinegar, and which antibacterial household cleaners have the best antibacterial properties.
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Only a tiny fraction of known bacteria causes disease while a significant number of bacteria is helpful to humans and the environment. Have students research and create a scrapbook on different types of beneficial bacteria. Collect articles, photos or draw pictures of various bacteria and explain how they are beneficial to humans or the environment.
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View videos and images of various kinds of bacteria:
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Investigate how small a microbe really is with this online experiment:
Discussion