Bacteria in a Flask -- Spreadsheeting Population Density vs. Time

Loretta Sharma
Delta College
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project

Summary

Over a period of 40 days following the injection of a bacterial inoculum into a sterile nutrient broth, the bacteria density in the culture increases from 40 cells/ML to 600 million cells/ML, and then declines to 1 cell/ML. The students tabulate and graph the data and identify the four stages: lag phase, exponential-growth phase, plateau phase, and death phase. They use both arithmetic and logarithmic ordinates to explore the rates of change in the various phases and see that each type of graph has its advantages and disadvantages. The module includes numerous prompts for students with little experience with Excel. The module can be used with a laboratory activity in which students gather their own microbiological data.


Learning Goals

Students will:
  • Create a spreadsheet showing bacteria density (cells/mL) as a function of time and log bacteria density as a function of time.
  • Create graphs of bacteria density vs. time and log bacteria density vs. time on XY scatter plots.
  • Use the graphs to identify the lag phase, exponential-growth phase, plateau phase and death phase for data that show the growth of the bacteria culture in a space-limited environment.
In the process the students will:
  • Gain experience with data that span several orders of magnitude.
  • Find that the log scale is necessary to see the changes while N is small.
  • Find that the log scale complicates the interpretation of rate of change from the slope of the graph and thus distinguish between dN/dt (the rate of change of N) and dlogN/dt (the rate of change in the order of magnitude of N).
  • Learn that exponential growth, which is so widely discussed as a model of bacteria populations, is only part of the story. What do the data look like when the population feels the boundaries of its environment?

Context for Use

This module is designed for microbiology students, public health students, or students who are preparing for other careers in the health professions. The module assumes that students have minimal (or no) skills using Excel spreadsheets and graphs. Students should have access individually to computers, that are linked to the internet. Students may perform this activity as a lab exercise under instructor supervision, but may also have this module assigned as homework. Students may also work in groups to master this module and answer end-of-module questions.
While it would be helpful for students to have some familiarity with concepts relating to the growth of bacterial cultures, it is not absolutely necessary. Instructors may wish to have students use populatin data obtained through optional laboratory investigation.

Description and Teaching Materials


SSAC2005.QR75.LS1.1-stdnt (PowerPoint 194kB Jul6 07)

The module is a PowerPoint presentation with embedded spreadsheets. If the embedded spreadsheets are not visible, save the PowerPoint file to disk and open it from there.

This PowerPoint file is the student version of the module. An instructor version is available by request. The instructor version includes the completed spreadsheet. Send your request to Len Vacher (vacher@usf.edu) by filling out and submitting the Instructor Module Request Form.

Teaching Notes and Tips

Ideally, each student should have a computer. Alternatively, students can work in pairs. Assistance by one or more instructors would be helpful.

Assessment

Post-test ("End of Module Questions") assessments are included in both the Instructor and Student versions. The assessment is designed to evaluate Internet research skills, Excel graphing and tabulation skills, specific quantitative literacy skills, and skills relating to understanding key concepts in the exponential growth of populations. Identification of variables affecting population growth is addressed. Critical thinking skills, application of concepts, and evaluation of pertinent information are also assessed.

References and Resources