Bacteria are convenient for laboratory studies of populations because large numbers of bacteria live in a very small space and bacteria reproduce rapidly. In this investigation, you will examine the growth of a bacterial culture.

Problem 

What happens to a population that depends on limited resources?

Materials 

• 2 lima beans

• 2 dropper pipettes

• 100-mL beaker

• coverslips

• microscope slides

• 10-mL graduated cylinder

• 100-mL graduated cylinder

• methylene blue stain

• microscope

• test-tube rack

• 4 test tubes

• aluminum foil

Skills 

Calculating, Using Tables and Graphs

Procedure 

1. Before you begin, review the rules for sterile procedure with your teacher.

2. Wash your hands. Put on your plastic gloves. Then, to start a bacterial culture, put 2 lima beans into a 100-mL beaker. Add 50 mL of water. Allow this mixture to sit for 48 hours.

3. Construct a data table with four columns and five blank rows. At the top of the table, label the columns “Day,” “Bacteria Observed,” “Dilution Factor,” and “Bacteria Present.”

4. After 48 hours, use a dropper pipette to place a drop of the culture on a microscope slide. Add a coverslip. Place a drop of methylene blue stain on the slide next to the coverslip. Lightly touch a paper towel on the opposite side of the coverslip to draw the stain under the coverslip.

5. Use the high-power objective of a microscope to locate some bacteria. If you can count the bacteria in your field of view, go to step 7. If there are too many bacteria to count, go to step 6.

6. Use a dropper pipette to put 1 mL of the culture into a 10-mL graduated cylinder. Add 9 mL of water to the graduated cylinder. Empty the graduated cylinder into a test tube. This procedure dilutes the culture by a factor of 10. Examine the diluted sample under the microscope as in step 5. If there are still too many bacteria to count, dilute the sample again in the same way. Stop diluting when you can count the bacteria. Each time you dilute, multiply the dilution by 10.

7. Record the number of bacteria and the dilution factor in your data table. If you did not dilute, the dilution factor is 1. To determine the number of bacteria present, multiply the number of bacteria you observed by the dilution factor.

8. Cover the beaker with aluminum foil and set it aside overnight. Wash your hands thoroughly with soap and warm water when you are finished.

9. Predicting Record a prediction of how the population of bacteria will change.

10. Repeat steps 5 through 8 every day for 5 days.

Analyze and Conclude 

1. Using Tables and Graphs Make a graph of the data from your data table. When did the population grow most quickly? Most slowly?

2. Drawing Conclusions How can you explain the changes in population growth?

3. Inferring What caused the changes in the population growth rate?