NAME: ________________________
Culturing Microbes
Purpose: To sample different environments where microbes might exist & to investigate what
might promote/inhibit their growth.
Introduction: Fomites are inanimate objects that carry microbial (sometimes disease-causing)
organisms. A door knob, the sponge in the kitchen sink, a pen, a coin or monetary bill—all of
theses are fomites. In this activity you will look at microbes from fomites and areas around
campus, then attempt to grow them “in culture” so that they will reproduce and grow enough
that we can observe them with the naked eye as well as a stereomicroscope.
Background Questions to Consider/Research:
What roles do microbes play in our biosphere?
What are some of the different kinds of microbes we have studied?
What do they need to survive and thrive? (Metabolism)
What are some of their unique physical characteristics used to identify them?
How do they grow and reproduce?
What is a Petri dish & LB (nutrient) agar and how they promote microbial growth?
What are antiseptics and how do they inhibit microbial growth?
Procedure:
Get rid of any gum you are chewing. Put away all drinks/food.
Listen to your teacher for any modifications and note them below.
Your teacher will put you into groups of 3 (or 4). Wash your hands before leaving class.
1. Obtain a sterile Petri dish (aka, culture plate) with LB agar. The Petri dish should be
kept upside down at all times unless samples are being added to it or being observed
later on. Check to see if the dish has been sectioned off into thirds/quadrants. If not, use
a marker to so. Be sure each sector can be identified (1, 2, 3, or 4) not conspicuously.
2. Now it’s time to sample different potential sites of unseen microbes. You must obtain
three different samples from:
a. A fomite where people have come into contact with it (door handle, for ex.).
b. Outdoors (a sidewalk, wall, the ground, or goose poop are possibilities).
c. Personal microbe (cough, spit, skin from palm). Be careful here—You will
need teacher approval—if you think it’s inappropriate, it probably is.
Predict which (a, b, or c) will have the most microbes and circle the letter above.
Each student should obtain half of a clean Q-tip+ one of you an extra for the prediction.
3. Now swab the sample surface by rubbing and rotating the Q-tip. If you are leaving the
classroom, be polite and appropriate—do not disturb any classes. If you are doing the
personal microbe (c.) you should not be leaving class.
4. Open the lid of the Petri dish and GENTLY streak /rotate the Q-tip across one sector.
Be careful to not press too firmly and gouge your agar surface. It should be a light stroke
back and forth. Replace the lid when not streaking the sample onto the agar.
5. Record what sample was placed in each sector.
Sector #
Sample
1
2
3
4
Antiseptic
6. Find the lab counter with different antiseptics. Take your covered Petri dish there.
7. Choose one antiseptic for your group to test. Using forceps, remove a circular piece
of filter paper from the empty glass dish. Using the plastic pipette (dropper) and holding
the filter paper over the beaker/container, add 1-2 drops of the antiseptic to the filter
paper. Carefully place the filter paper on top of the agar in the sector you sampled. Be
very careful not to gouge the agar surface. If having trouble getting the filter paper off the
forceps, use a metal probe to help. Replace the top to your Petri dish after all in your
group have added filter paper. Flip the dish over.
8. Record in the chart above which antiseptic was added to each sample.
9. With two small pieces of tape, secure opposite sides of the Petri dish, top to bottom.
Using a marker or pen, put one person’s initials & the class period number on the tape so
you can find it.
10. Make initial observations to include the smell, the color, texture of the Petri dish
surface and specifically for your sector. Rank which sector you believe will have the
fastest/the most growth and which antiseptic will be most successful at preventing
growth. This hypothesizing probably should be done in some kind of table form.
11. After making initial observations and predictions, put your Petri dish upside down in
the incubator on your class’s assigned shelf. Record the temperature here ___________.
(There is a thermometer sticking out of the top of the incubator.)
12. Each day you have class, retrieve your Petri dish from the incubator and make
detailed observations. Note what day of the experiment each set of observations are for.
A minimum of one drawing will be required for the final lab report. Observations should
include qualitative and quantitative ones. These can include growth pattern, size,
shape, color, texture, location (esp. around filter paper), and smell (do not open, though).
Be sure to look at least once under the stereomicroscope. NEVER open up the dish! On
the final day, compare and contrast to one other group’s Petri dish results.
13. When finished with the lab, follow the instructions of the teacher as to how to
properly dispose of the Petri dish.