Name _______________________________
Date______________________
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Bacteria Growth Rate
Graphing Growth Rate as an Exponential Function
Background Information
In this experiment, you will examine the nature of biological population growth.
Biological population growth generally follows a specific pattern. It begins with an initial ‘lag’ or
slow phase where there is little increase
increas in the number of individuals. After a fairly short ‘lag’
phase (depending upon the species population) there is a very ra
rapid
pid ‘exponential’ phase, where
growth tends to be logarithmic. For most (if not all populations) the duration of this
exponential growth phase is fairly short. Most populations then reach a ‘stable’ phase, which
as long as nutrients or other limiting resources
resources are available, the population will fluctuate
around some mean number of individuals.
The curve that will represent this growth pattern for most populations (plotted X axis is
Time and Y axis is Number of Individuals) is described as being Sigmoid or “S” shaped. The
human population at this time in history does not have or has not yet reached the “stable”
point. Historically, human populations have been “J” shaped with rapid logarithmic
logarithm growth,
then a sudden crash, for
or example the p
plague of the Middle Ages, or the population dies off, and
then a rebuilding.
MATERIALS
Document Camera
Graph paper
Petri dish
Agar or nutrient medium appropriate for bacteria
Bacteriological loop
Bunsen Burner
Bacteria culture
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Name _______________________________
Date______________________
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PROCEDURE
Activity 1
Setting Up and Recording Bacteria Growth Rate
1. Wear safety goggles and gloves at all times.
2. Prepare sterile Petri dish(es) with proper agar or nutrient source according to
instructions usually provided by bacterial provider using aseptic techniques.
3. Sterilize the bacteriological loop, and then inoculate the agar with the bacteria or use
the prepared agar provided by your teacher
teacher.
4. Note the time on the bottom of the Petri dish.
5. Focus with the document camera on the Petri dish and take an initial image.
image
6. Place the cover on the Petri dish, but do not turn upside down as usually suggested.
Condensation may form on the plates, but usually not at room temperature.
7. Use the document camera and set for recording a time lapse sequence of pictures.
8. Focus the camera, so the entire radius of the Petri Dish is in view.
9. Have the camera take a picture every 30 minutes of the Petri dish
dish..
10. Stop recording after a 48 hour period and record your observations in your science
notebook.
Activity 2
Measuring the Growth Rate and Graphing
1. In each image of the growing bacteria, measure the diameter or approximate area of 33
5 colonies of bacteria on the plate. Average these for each time point.
a. Measure the approximate diameter
b. Find the radius of the diameter you measured.
c. Find the area of the circle of the diameter you measured.
2. This measurement can be done with a calibration tool on your computer or with a
metric ruler.
3. Record the average diameter or area of the bacterial colonies per time elapsed.
4. Label the graph paper with time on the X axis and Colony size on the Y axis.
5. Plot the average diameter or area of colonies in each frame of the time lapse sequence.
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Name _______________________________
Date______________________
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6. With the recorded data, find an equation that describes the line or curve on the graph in
the growth portion of the graph. Express the equation in both change in measurement
(numbers of bacteria) over change in time and also in the natural log function.
QUESTIONS
1. Does the plotted curve obtained match the predicted sigmoid curve
curve of the normal
growth model?
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about
2. What does a best fit curve look like? Does it provide any new information abo
growth?_________________________________________________________________
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3. Are the normal “lag’, ‘log” and ‘stable”
‘st
phases
evident?_________________________________________________________________
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4. What factors might cause all living things to basically follow this kind of growth curve, be
they bacteria or elephants?
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Name _______________________________
Date______________________
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5. Can you suggest an experiment which might change your results either in terms of
number of organisms produced in each colony, or the rate at which the colony grows?
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phase If humans are truly
6. Can you specifically suggest why populations reach a ‘stable’ phase?
in a ‘J’ shaped curve, how or why are they the exception to the ‘S’ shaped curve?
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