Grade Levels Grades
5 and 6
Estimated Teaching Time
50 minutes
Interdisciplinary Connections
- Learning about bacterial growth (Science)
- Investigating exponential growth rates (Math)
- Graphing and doing arithmetic (Math)
Objectives
Students will:
- Apply math skills to learn about optimal conditions for
bacterial growth.
- Learn about the exponential speed at which bacteria can
multiply.
- Learn about the role of bacteria in promoting decay.
What Students Do
Students will use clay to illustrate bacterial multiplication
rates while calculating the astronomical potential reproductive
growth of just one bacterium in a single day.
Materials Required
- Modeling clay, one or two colors
- Gridded chart paper
- Graph sheets, one per student
- Magnified images of common germs (blackline
master 1.1 PDF from Lesson 1.1)
- Optional Overhead
projector and transparency film
Advanced Preparation
- Duplicate blackline
master 1.1 (PDF), one per student, or transfer onto
an overhead transparency.
- Divide clay into a fist-sized piece for each group of
four students.
Suggested Sequence
- Ask students for examples of decay they have seen, such
as food left in the refrigerator too long or a dead animal
in the yard. Explain that bacteria and fungi cause most
of the decay.
- Explain that an individual bacterium is far too small
to be seen by our eyes alone; most are about 1/1000 of a
millimeter in diameter. Pass out copies of blackline
master 1.1 PDF (or show transparency) and review the
magnified images of germs. (NOTE: These images have been
taken from a variety of sources and do not necessarily reflect
what a student would see looking through an optical microscope.)
- Divide the class into groups of four. Give each group
a fist-sized piece of clay that represents a single bacterium.
Every 30 to 60 seconds, have each group divide its “bacteria”:
first two, then four, then eight, then 16, then 32. Track
the bacterial growth on the class graph sheet.
- Explain that real bacteria — including strains that
make us sick — divide every 20 minutes under optimal
conditions. The real bacterium would have gone from one
to 32 in 100 minutes. Now ask them to calculate how many
bacteria there would be after two hours, three hours and
four hours at this fission rate.
- Ask them to consider why such unchecked growth does not
actually happen. [Finite food supply, limits of suitable
living space, propensity for crowded bacteria to poison
themselves with their own waste, antibiotics that are created
by competing fungi, ability of humans and many animals to
produce antibodies.]
Interesting Fact In just 12 hours, one bacterium
could multiply to over 8.5 billion under perfect conditions.
After three days, with no bacteria dying, there would be enough
of them to cover the entire earth.
Check for Understanding
Have students graph an exponential multiplication rate
with a specified time period and rate at which that number
doubles, then redoubles again and again. Example: if an organism
doubles every hour, how many hours must pass for there to
be over one million of them? [20]
Ask students how their model bacteria are different from
real life. [Size, structure, dividing bacteria do not
get smaller and smaller with each generation and growth rates
are not limitless.]
Extensions
In essay form, have students answer the following questions:
- What are other examples of rapid species growth in the
natural world?
- Does this sort of growth apply to humans?
- What sort of environmental and biological factors limit
that growth?
Words to Share
- Antibiotic
- Antibodies
- Bacteria
- Exponential growth
- Fungi
- Microbe
- Microorganism
- Optical microscope
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