Bacteria and Basic Experimental Design

Summer Research Program for Science Teachers

Jared Fox

Frederick Douglass Academy III, Bronx

August 2008

Audience:  Secondary Life Science

Lesson Objectives: 

Students will be able design a controlled experiment.

            Students will be able to quantify and graph data.

            Students will be able to begin to understand where and how bacteria grow

Materials Needed

Petri Dishes (2 – 3 per group)

Gloves

Parafilm

Scissors

Transparencies (1 per group)

Pipettes or graduated cylinders (25 mL)

Incubator (optional)

Bacterial Growth Medium (LB)

Hand Sanitizer

Soap

Water

Before the activity

Prepare plates 1 day ahead of the activity.  Prepare plates by pouring 25 mL of LB growth medium and letting stand until solid.  Once settled store plates face down until needed (storage in a refrigerator is best).

Teacher Steps

Do Now:  What are bacteria?  Where can you find them?

Elicit student responses.  Accept all answers.  Push students thinking further by asking leading questions.  Are all bacteria harmful?  When might bacteria be a good thing?  Where would you most likely find the most bacteria?

The Activity:  Working in pairs students will design a controlled experiment to test where bacteria are commonly found. 

1.  Encourage students to create a testable question.  The key to proper questioning is that they are quantifiable.   Possible testable questions include… 

• Is hand sanitizer better at limiting bacteria growth than soap?

• Are there more bacteria on the door knob or door frame?

• Do gloves stop bacteria from contaminating a surface?

2.  Have students create a hypothesis for their testable question

3.  Students set up the experiment. 

Things to remember…

• Explain the importance of having a control group in all experiments.
• It is o.k. for students to touch the agar plates directly, just be sure they wash their hands when they are finished.
• Be sure to seal plates with a strip of parafilm. 
  • Plates should remain sealed at all times
• Place plates in an incubator at 37 degrees Celsius if available.

Data Collection

Using a transparency create a grid that can be placed on top of the petri dish.  To quantify bacterial growth have student count the number of grids that contain bacteria. 

A great way to collect data over a number of days is to leave the plates out of the incubator on day 1 and have students record bacterial growth over a 4 or 5 day period.  Carrying out the activity this way will allow students to create a graph of their results.

Conclusion

Have students share their findings and answers to their testable questions in a whole class discussion.  Be sure to emphasize how the controlled experiment was created.

Extensions

As a class or in groups have students read the NY times article exploring the concept that the faster you pick something up off the floor the safer it is to eat.  Also known as the “5 second rule”  http://www.nytimes.com/2007/05/09/dining/09curi.html?scp=1&sq=5 second rule&st=cse

National Science Standards: 

Content Standard A:  As a result of activities in grades 9-12, all students should develop abilities necessary to do scientific inquiry and understanding about scientific inquiry

Content Standard C:  As a result of their activities in grades 9-12, all students should develop understanding of the cell, interdependence of organisms, and behavior of organisms.

Content Standard G:  As a result of activities in grades 9-12, all students should develop understanding of the nature of scientific knowledge