Yakima WATERS Mini Lesson
Bacteria Ecosystem Exploration
Targets and Assessment
WA Science Standards Addressed:
 4-5 INQA Scientific investigations involve asking and
answering questions and comparing the answers with
evidence form the real world
 4-5 INQC An experiment involves a comparison. For
an experiment to be valid and fair, all of the things
that can possibly change the outcome of the
experiment should be kept the same, if possible.
 4- INQE Repeated trials are necessary for reliability
Assessments:
 Proper notes should be kept in the student’s science
notebooks with proper definitions of experiment
terms.
Lesson Parameters
Content Area: Biology, microbiology
Overview: This is a classroom experiment exploring the
diversity of bacterial life and how to properly conduct an
experiment.
Grade Level: 5
Suggested Time: 15-30 minutes collecting, few days to weeks for
bacterial growth
Special Materials:
 Petri-dishes with agar, parafilm, sterile cotton swabs
Learning Outcomes:
Knowledge: The student will know that an experiment answers a question by exploring the real world.
Students will know that as many things as possible must be kept the same for the experiment in order for it to
be valid. Students will know that in order for an experiment to be reliable they need to repeat the trials.
Skill: Students will know how to experiment using the scientific method.
Science Concept Background: Bacteria are very cells that live everywhere in the environment. Most are
harmless to humans eating organic matter they find, reproducing, and dying just like any other living thing.
We can only see them when they are masses together in very large numbers. By sampling a very small area
we can see what types of bacteria and other microorganisms live there.
Wildlife biologists use several methods to determine the species richness in the locations they study.
This method can be used on bacteria as well by counting the bacterial masses or colonies. If the bacteria
colonies can’t be accurately counted area covered can be determined with a simple grid placed over the
bacteria and counting the number of squares they occupy. This is used by other scientist on larger scales.
Materials: Sterile petri-dishes with agar. Sterile cotton swabs, parafilm, wax pencils, and notebooks.
Procedure: 1.) Give each science group two-three sterile petri-dishes with agar gel, two-three strerile cotton
swabs, and their notebooks. The petri-dishes must be kept closed to ensure that no bacteria other than where
you sampled can get in. Using a wax pencil the students will put their name on the bottom of the petri-dish,
this is the part that holds the agar. 2.) After the students have chosen a location to sample and written in
down in the science notebook they will be given a sterile cotton swab. These cotton swabs are opened from
the end that doesn’t have the cotton. The tip must not touch anything other than the location the students
chose to sample or their data will be corrupted. 3.) The petri-dish can now be flipped so the bottom is
touching the table and very slightly opened so that the top still covers the bottom. The cotton tip is very
carefully put into the dish and very lightly rubbed over the agar while slowly spinning the swab. It is important
not to dig the cotton tip into the agar, it will be much harder to determine area cover or to count colonies if
this happens. The cotton swabs can be thrown away as nothing that isn’t already found in the classroom is on
them. 4.) A thin strip of parafilm is then stretched around the petri-dish where the two halves meet then
flipped back over. Have the students chose a location they would like to put their petri-dishes and write that
location or trial down in their notebooks. Let the petri-dishes sit in their location for a few days to few weeks
depending on the temperature, colder temperatures mean longer growing times. 5.) After the bacteria have
grown have the students look at each petri-dish. If there are many circles on them use the Worksheet to
construct a “species” chart. If there is a large mass use graph paper to determine the area covered.
Key questions: Why is it important to keep everything sterile?
Why do we have more than one petri-dish at each location? Explain the importance of replication and
repeated trials. If something is contaminated or one petri-dish breaks we still have another one to use in our
experiment.
Scientific terms to look for in the students’ notebook: Control, replication, repeated trial.
Extension(s): The students can compare different bacteria from different locations after they have determined
what location is the best for growing them.
Teaching Tips: The fifth grade is not expected to know about cells, a short explanation of what cells are and
how they relate to the visible world may be needed.
Supplements: http://www.waksmanfoundation.org/labs/washstat/elemexp2000.html
Author: Dale Jansons, Yakima WATERS Project, CWU, Winter 2011
Colony shape
Colony color
Number of that type of
colony
Directions: Find all the different types of colonies on your petri-dishes. In the first column describe the shape
on each different type of colony, in the second describe the colony’s color, and in the last column count the
number colonies that look like this.