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PDF Files for PrintingLesson 4-Explain: What Lives Inside Your Mouth?



Exee looking through a magnifying glassTo begin Lesson 4, students review the environment of the mouth from the mouth trek in Lesson 2. They discuss what an environment is and describe the characteristics of their mouths' environment. They create and draw an animal that they think could live in their mouths. Next they listen to a story that introduces the concept that everyone's mouth is full of oral bacteria that eat, reproduce, and produce waste products inside the mouth. The waste products of certain types of oral bacteria cause tooth decay.


In this lesson, students will


After completing this activity, students will


Oral Bacteria

The oral cavity hosts an environment full of life. The teeth, tongue, lining of the mouth, gums, and bone are all living tissue. The mouth also plays host to many microscopic living organisms. Oral bacteria are everywhere in the mouth; certain kinds prefer certain areas on the tooth, while others prefer the gums, tongue, or tissues inside the mouth. These oral bacteria eat, reproduce, and excrete wastes just as people and all living things do. Oral bacteria may be spherical (coccus) or rod-shaped (bacillus) or even corkscrew-shaped.

Did You Know?
There are more than 500 species of bacteria in a healthy mouth.1

Bacteria are approximately 1 micrometer, or one-millionth of a meter, in size.

If a tooth were as tall as the Empire State Building (412 meters), a bacterium would be about the size of a thumb (4 centimeters).1

Some oral bacteria can be neutral or helpful, while others can be harmful if their numbers are not controlled. For example, Streptococcus mutans (S. mutans) and related species are mainly responsible for dental caries (tooth decay). Other bacteria such as Porphyromonas gingivalis are associated with periodontal (gum) diseases.1

Visuals Unlimited/David M. Phillips

In this curriculum, we use the term oral bacteria to refer to microorganisms in the mouth. Bacteria is a more accurate term than those that children often use, such as germs or bugs.


Dental plaque is a soft, sticky, colorless combination of bacteria, food debris, and bacterial products. It is also known as a biofilm. Plaque forms and builds in the mouth constantly.

How does plaque form? Even a freshly cleaned tooth surface is coated with a thin layer of salivary proteins where bacteria can attach. To help them completely adhere to the tooth, the bacteria produce a "bacterial glue" called dextran (a chain of glucose molecules).2 Dextran acts to hold the bacteria onto the tooth. Bacteria can digest dextran for nourishment, a form of bacterial "snack."2

Some oral bacteria, such as S. mutans, have properties that make them very resilient. These bacteria not only attach to tooth surfaces, but they can attach to one another, secrete waste products such as lactic acid, and survive in a highly acidic environment. If the acids produced by oral bacteria are held onto the tooth for extended periods of time, the tooth can lose minerals. If acid exposure continues, the tooth will eventually decay.

Not only does plaque cause dental caries, it is also a major cause of gum diseases. When plaque builds up along and under the gum line, the bacteria release enzymes and toxins that cause the gums to bleed and to become swollen, tender, and red and, if not corrected, may destroy the supporting structures of the tooth.2

Susceptibility to plaque varies greatly among individuals because people have different types of bacteria in their mouths and their bodies have different abilities to protect themselves from these bacteria. Controlling plaque is a lifelong component of oral care. Brushing at least two times a day, particularly after breakfast and before bed, helps reduce plaque formation.2

Dental Caries (Tooth Decay)

sugars, starches and bacteriaDental caries is a bacterial disease that primarily affects children and adolescents, but still is a problem for many adults. Dental caries begin when certain bacteria, mainly S. mutans, digest sugars and starches and produce acids that dissolve the enamel of teeth. Sugars and other carbohydrates provide nutrients for oral bacteria to survive and reproduce. Our saliva, lips, and tongue help combat tooth decay by removing food and some bacteria from the teeth. Saliva also helps prevent tooth decay by neutralizing some of the acids produced by the oral bacteria. However, once plaque is formed, it cannot be washed away by saliva, by rinsing with water, or by eating fibrous foods such as apples. The decay process will occur more quickly if plaque is not removed every day and when fluoride is not used daily.

Did You Know?
Dental caries, or tooth decay, is the single most common chronic childhood disease—it is five times more common than asthma and seven times more common than hay fever.3

Sometimes, the earliest sign of tooth decay can be seen as an opaque "white spot" on the surface of the tooth. This white spot indicates an area that acids have attacked by leaching minerals from the tooth's enamel. The loss of minerals, or demineralization, usually starts on the chewing surface of the tooth or in between adjacent teeth. If the demineralization continues, the enamel and eventually the dentin are destroyed. If the bacterial infection goes unchecked, it can reach the pulp (the inner part of the tooth containing nerves and blood vessels), causing significant destruction to the tooth, pain, and infection. This is why recognizing tooth decay at very early stages is extremely important; scientists have now shown that it is possible to stop or even reverse the early damage of demineralization with techniques that help the tooth's enamel remineralize. (See Lesson 5 for more information on demineralization and remineralization.)

A particularly damaging and painful type of tooth decay is called early childhood caries. This severe pattern of tooth decay, baby bottle tooth decay, may occur in infants and very young children who are put to bed or are allowed to suck at will during the day from a bottle filled with juice or sugar-containing drinks. Luckily, with proper care by parents and care givers and good feeding practices, this condition is entirely preventable.

Children who experience tooth decay at an early age are at higher risk for decay later in life. Therefore, it is very important to prevent tooth decay through good oral hygiene, healthy diets, and the use of fluorides and sealants (see Lesson 5 for more information).


Activities that include the Web site
Activity Number Web Version
Activity 1 no
Activity 2 yes

Activity Number Master Number Number of Copies
Activity 1 none none
Activity 2 Master 4.1, Animals Do More than Eat and Drink 1 book for the class

Activity 1
For the class:
  • chart from What the Inside of My Mouth Feels Like, Lesson 2
For each student:
  • 1 sheet of drawing paper
  • crayons or markers
Activity 2
For the class:
  • Web site address
  • computers with Internet access
  • Master 4.1, Animals Do More Than Eat and Drink book



Activity 1: What Lives in Warm, Wet Places?

The purpose of this activity is to help students recognize the mouth is an environment containing organisms that share characteristics with other living organisms.

1. Remind students of the mouth trek from Lesson 2 and post the flip chart, What the Inside of My Mouth Feels Like, from that activity. Review the words that students used to describe the inside of their mouths.

assessment icon Assessment:
This is a good time to assess your students' prior knowledge of the concept of environment. Be sure they understand that an environment is made up of the total surroundings of a given place. For example, their school environment includes all the students, teachers, and staff, as well as the desks, chalkboards, chairs, hallways, and building. It also includes such things as air and sunlight, temperature, and sound. A pond environment, in contrast, would include some of the same surroundings but would have some living and nonliving structures unique to it.

What the inside of my mouth feels like: warm, wet, soft (cheeks, lips, roof of mouth - soft palate, tongue), hard (teeth, jaw, roof of mouth - hard palate), bumpy (teeth, tongue), slippery (gums, cheeks, tongue, floor of mouth), smooth (gums, lips, floor of mouth), rough (tongue).

2. Focus on the mouth as a warm, wet environment. Ask students if they know of other places that also are warm and wet. Encourage students to think about different environments on earth.

The students might answer a pond, a jungle, inside a volcano, inside a cave, a rain forest, or a steamy bathroom after someone takes a hot shower.

Then ask, Can you name some animals that live in those places?

Students should list a variety of animals, from large to small, such as whale, alligator, monkey, deer, sea otter, turtle, fish, and germs.

National Science Education Standards iconContent Standard C:
Characteristics of living organisms.

Next, ask students to list the things animals need to survive in their wet, warm environment.

Animals need food, water, shelter, and space.

3. Ask students, What if an animal—a very tiny animal—lived in your mouth? How could that animal survive? How could the animal get food? Water? Where could it live? Where could it find shelter?

Students might suggest that the animal eats whatever they eat, or drinks their saliva. They might think that the animal lives under their tongue or on the roof of their mouth.

student drawing
 Example of a student drawing

4. Invite students to draw a picture of an animal that could live in the environment of the mouth. Ask students to imagine that they are looking through a microscope at the animal in the mouth. Encourage them to show what the animal would eat and drink and where the animal could live.

Activity 2: Animals Do More than Eat and Drink

The purpose of this activity is to make students aware of oral bacteria and their actions in the environment of the mouth.

1. Gather students in a whole class area and read the story from the book Animals Do More than Eat and Drink. As you read, stop periodically and ask students to explain the story in their own words. Allow students time to process all the information.

There is a lot of information in this story. We do not expect students to remember all of it at this time. The story sets the stage for the next lesson, which reinforces the concepts.

National Science Education Standards iconContent Standard C:
Organisms and their environment.

Help students relate the actions of oral bacteria to those of other animals. All animals take in nutrients (they eat); all animals reproduce (they produce offspring, they have babies); all animals eliminate waste (they go to the bathroom, they poop!). We want students to understand that the waste products from some of the oral bacteria (the acids and toxins) are what cause tooth decay. If the students understand that they can influence the amount of acids that the bacteria produce by what they eat and drink and by brushing through oral hygiene practices, they can greatly reduce (or eliminate) their risk for tooth decay.

2. Ask students to tell you how they feel about what they have learned in the story.

Students might tell you that they think the information they learned in the story is "icky." Many will suggest that they want to brush their teeth right now! Some may be worried because they eat sugary foods or don't brush their teeth regularly. Let students know that the bacteria in their mouths are normal, but that they can avoid tooth decay caused by the bacteria by taking good care of their mouth. In Lesson 5, students will dramatize how good oral hygiene can control the growth of oral bacteria and reduce the amount of acid that can lead to tooth decay.

assessment iconAssessment:
You can collect the Mouth Journals and assess your students' individual understanding of the concepts presented in the lesson: that the mouth is an environment that contains bacteria, living organisms that share characteristics with other living organisms.

3. Allow time for students to view the animated movie on the Web site about microorganisms living in the mouth.

Web activity iconOpen the Web site in your browser. From the main page, click on Web Portion of Student Activities and choose either English, Español, or one of the accessible versions of the activities. The Student Activities window will open and the Exee Movie will play automatically. You can skip the animation by clicking the skip button. From the main menu in the Student Activities window, select What Lives Inside Your Mouth? This animation will help students understand more about the microorganisms in the mouth. Students may also benefit from reviewing this segment during Lesson 5.

Exee writingWrap-up

Instruct students to use their Mouth Journals to write or draw about what they learned in this lesson. Help them decide what to include by suggesting that they answer the question, What would you tell Exee about the mouth now?

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