Summer Research Program for Science Teachers

Melinda Pittis

BOCES Southern Westchester Hearing Impaired Program

Rye Brook High School, Rye Brook, NY

2000

Which Household Cleaners or Soaps Work Best Against Bacteria?

This lab activity is to test the effectiveness of various household cleaning products or soaps for their antibacterial ability.  Any non-pathogenic strain of a common bacteria, such as E. Coli can be used.  This lab will require teamwork, research and problem-solving skills from the students. 

 Teachers can choose to use some or all of the parts of this lab.  You may wish to ask your students to research which species of bacteria are commonly found in the home.  Part 2 of the activity includes a demonstration and practice of sterile handling techniques. Additionally, students will work in groups to write the procedures for how they plan to execute the lab.  After working in small groups, the class will assemble to offer ideas to determine the most appropriate, controlled procedures.  As a whole, the class must agree on a set of procedures which incorporate the collective best ideas from the small groups.  Part 3 will be conducting the actual experiment, collecting data, showing results and formulating a conclusion.  Part 4 provides suggestions for assessment of the activity and ideas for extensions. 

Implementation problems might include choosing an appropriate representative household strain of bacteria.  Additionally, since bacteria are often shipped freeze-dried in pellet form, the titre of the bacteria in its media will need to be determined prior to the experiment.  Advanced students may want to try this themselves with teacher guidance.  It may also be difficult to determine exactly how much of a household chemical should be used.  This should be addressed during the procedures discussion with the entire class.  I will need only a portion of my enhancement funds for this experiment to cover the costs of an incubator, pipettes, agar plates, the spinner to spread plates evenly and the bacteria. 

Introduction

Bacteria are part of the monera kingdom, which by definition means they are one-celled and lack a nucleus.[9-12 Content Standard C- The cell] They live in mixed populations almost everywhere.  Observing one isolated bacterial specimen not contaminated with other microorganisms can be problematic.  Just like all living things, each species of bacteria exists best under specific temperature and food conditions. [9-12 Content Standard C- Interdependence of organisms] Under the right conditions, bacteria will reproduce very rapidly, sometimes doubling their numbers every 20 minutes.  To combat the spread of bacteria and viruses, many companies now market antibacterial or disinfecting cleaners and soaps.  Disinfectants often use bleach or ammonia products and antiseptics add triclosan.  [9-12 Content Standard E- Understandings about science and technology]

The goals of this lab include:

·        Learn sterile handling techniques for bacteria

·        Compare the effects of different cleaners on bacterial growth

·        Design a controlled experiment

·        Culture bacteria under the appropriate growth conditions

Materials

Bacterial freeze-dried culture of non-pathogenic E. Coli

Sterile agar plates

Sterile disposable pipettes

Incubator

Various cleaning products or soaps

Plate spreader tools

[Teaching Standard D- Make accessible science materials]

Part 1: Sterile Handling Techniques

It is important while handling bacteria that your cultures do not become contaminated with other monerans or fungi that exist normally in our environment. The agar plates that support the growth of your selected bacteria provide a wonderful habitat for other bacteria and fungi.  It is important that you “protect” your plates and cultures from other organisms in the room.  Additionally, many bacteria can cause harm to you.  You must understand and demonstrate how to handle these organisms as safely as possible. [Teaching Standard D- Ensure a safe working environment]

Several simple techniques will be demonstrated.  For each one, write a brief summary of the method and its significance to keeping your experiment as free as possible from contaminants. 

Flaming the caps to all bottles or tubes:

Keep your agar dish closed as much as possible:

Hold caps with the same hand you hold test tubes (don’t put caps on the table):

Change pipettes (or tips) when appropriate:

When are your gloves on or off?  Can you contaminate your classroom?

How can you avoid endangering yourself with any bacterial cultures?

Part 2: Procedures

An overview of the experiment:

·        the freeze-dried pellet must be reconstituted in media (follow directions)

·        some bacteria must get put onto to plate and spread evenly

·        some of your cleaner needs to be sprayed/spread onto the plate

·        the cultures will be incubated for a set time and the colonies counted

With your lab partner or group, brainstorm your ideas for how you will conduct this lab.  You must include and identify your dependent and independent  controls as well as specific quantities of each cleaner you use.  Write clearly so that another student not in your group would understand your instructions.  [9-12 Content Standard A- Design and conduct a scientific investigation]

Class Procedures

Post-discussion, these are the agreed-upon procedures that everyone will follow: [Teaching Standard B- Orchestrate scientific discourse]

Part 3: Results/Conclusions

1.      Describe the appearance of your plates after the incubation time:

2.      How can you tell if your bacteria were spread evenly or not?

3.      Which cleaner resulted in the most and the fewest colonies growing? [9-12 Content Standard A- Formulate explanations using evidence]

Analysis Questions

1.      Describe your controls and why they are important to the outcome of this experiment:

2.      Do some students have different numbers of colonies for the same bacteria?  What could be some possible reasons for this? [9-12 Content Standard A- Analyze alternative explanations/models]

3.      How do you know which cleaner was the most effective against the bacterial growth?

4.      What is the difference between an antiseptic and a disinfectant?

5.      What were your sources of error and how could you improve the lab? [Teaching Standard D- Engage students in designing the learning environment]

6.      How did the bacteria survive in its prior freeze-dried state?

7.      What is the purpose of the agar plate?  Could you do this experiment without the agar?  Without the incubator?

Part 4: Assessment Ideas

[Teaching Standard C- Use multiple methods to gather data about student understanding]

A)    With your research team, make a poster showing the results of your experiment.  It should include the following: introduction, materials, procedures, observations and data (visually represented), conclusion and errors/improvements.

B)    Using your lab report guidelines, write a formal lab report of your experiment.

C)    Write the script for a consumer commercial advertising your anti-bacterial product.  Construct a data chart and graph from your experiment as evidence for why your product is superior to others on the market.