Bacteria Introduction Kit
This kit contains four plastic petri dishes, dry agar mixture, sterile swabs, a pipet, and ten paper dots. You will need to
provide some household items such as rubbing alcohol and hand soap. Adult supervision is required for these
projects.
Introduction
Bacteria are everywhere—you have millions living on your skin alone, and lots
more inside you! Bacteria reproduce when one cell splits into two cells in a
process called binary fission. Fission occurs rapidly in as little as 20 minutes.
Under perfect conditions a single bacterium could grow into over one billion
bacteria in only 10 hours! (It's a good thing natural conditions are rarely perfect, or
the earth would be buried in bacteria!) Individual bacteria can only be seen with a
microscope, but they reproduce so rapidly that they often form colonies that we
can see. With this kit you can grow bacteria colonies and experiment with them.
Safety
While most environmental bacteria are not harmful to healthy individuals, once
concentrated in colonies, they can be hazardous. To minimize risk, wear
disposable gloves while handling bacteria, and thoroughly wash your hands before
and after. Never eat or drink during bacteria studies, nor inhale or ingest growing
cultures. Work in a draft-free room and reduce airflow as much as possible. Keep
petri dishes with cultured mediums closed—preferably taped shut—unless
sampling or disinfecting. Even then, remove the petri dish only enough to insert
your implement or cover medium with bleach or 70% isopropyl alcohol. When
finished experimenting, seal dishes in a plastic bag and dispose. Cover accidental
breaks or spills with bleach or alcohol for 10 minutes, then carefully sweep up,
seal in a plastic bag, and discard.
Do Bacteria Like It Warm or Cold?
1. Use a permanent marker to draw a line down the middle of two of your petri dishes. Number each half (1,2,3,4). Label
another dish as “5”. This will make it easier to keep track of your experiments later.
2. Wash the petri dishes with hot water and soap. (Usually sterile petri dishes are used in the lab—ones without outside
air or germs in them. For this experiment, however, the dishes only need to be clean.)
3. Mix one level teaspoon of agar powder with 3/8 cup (100ml) of room-temperature water.
Bring the mixture to a boil while stirring to ensure the agar is dissolved.
4. Pour the agar solution into the three labeled petri dishes and the fourth extra dish, using
enough to cover the bottom part of the dish. Dispose of the leftover agar.
5. Cover the petri dishes with their lids and let them sit until the agar has solidified. Then set
the extra, unlabeled dish and the ones marked 1/2 and 3/4 in the refrigerator. (You’ll use the
labeled dishes for the experiments in this guide. Use the extra dish to make up your own
experiment later.) Place the #5 dish in a warm, dark place, like a closet.
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6. After a couple days, compare what is going on in the labeled, refrigerated petri dishes and the one you left out. Do not
open the petri dishes. It may take a few days before anything is visible in the un-refrigerated dish, so give it more time
if necessary. After you have made your observations, put the same two petri dishes back in the refrigerator and the #5
dish wherever you are storing it. Continue to observe the specimens for at least a week.
7. Write a brief description of what you observe and/or draw pictures of what, if anything, you saw growing in the three
petri dishes.
Bacteria, as well as mold and fungi spores, are transferred through the air; this is how they “appeared” in the petri dish
even through you did not put anything dirty in the agar. Nutrients in the agar encourage growth of these members of the
Kingdoms Monera and Fungi. As observed, however, cold temperatures slow down their reproduction. This is why we
keep perishable foods in the refrigerator instead of leaving them out in the warm air.
Are There Bacteria on My Hands?
1. When your hands are dirty from playing, working in the yard, etc., have someone with clean hands bring you the
refrigerated #1/2 petri dish and open the lid for you. There should be no bacteria growing in this dish. Place your
thumb on the #1 side of the dish and push hard enough to leave a thumbprint. Quickly replace the lid and then store
the dish in a warm, dark place.
2. After a couple days, look at the petri dish—don’t remove the lid—and see if you can find little dots in the agar. These
dots are bacteria colonies! If the bacteria aren’t visible yet, check again in the next day or so.
3. Remove a sterile swab from its package, being careful not to touch the tip. Remove the lid from the petri dish and
touch one end of the swab to a bacteria colony that is slightly separated from the others.
4. Rub the swab in a zigzag pattern over the agar on the #2 side of the petri dish. Go over it several times, but don’t dig
a hole in the agar.
5. Put the lid back on and place the dish in the warm, dark place where you were storing it.
6. After a day or so, observe the petri dish. You should be able to see bacteria growth on both sides of the dish.
Every time that we touch something, our skin collects bacteria and leaves some behind! This is one of the ways
contagious diseases spread. If we wash our hands well we can help prevent the spread of bacteria. In this experiment,
bacteria from your thumbprint grew inside #1 of the petri dish. Each different type of bacteria grew in its own colony. When
you transferred one type of bacteria from its colony to side #2, you isolated that type of bacteria. This is the same method
scientists use to isolate different strains of bacteria for lab study.
Do Disinfectants & Soaps Stop Bacteria?
1. You will need to use the mold and bacteria specimens from the #5 and #1/2 petri dishes used in the previous
experiments—or else grow more specimens. You will also need the #3/4 petri dish from the refrigerator.
2. Take five of the paper dots and lay them out on a paper towel. Find different solutions around your house that you
think would destroy bacteria. Use the pipet to soak each dot with a different one of the solutions that you selected.
Rinse the pipet between each solution. Some examples of solutions to use are rubbing alcohol, liquid soaps, cleaning
spray, or other “germ-killing” products. Wipe off any excess liquid on the paper towel—you don’t want the dots to be
dripping.
3. Make a hypothesis—an “educated guess”—about which solutions you think will work best. For example, based on
what you know about hand soap and alcohol, which do you think will kill germs more effectively? Which appears to be
a more potent solution? Make sure that you keep track of which dot has which solution on it!
4. Use the remaining sterile swabs to move mold and bacteria from the #5 and #1/2 petri dishes to sides 3 & 4 of the
fresh petri dish. Use a different swab for each transfer, and rub the agar in a zigzag pattern.
5. Use tweezers to place the paper dots onto different parts of the agar on side #3, leaving side #4 as a control to show
how many bacteria colonies grow without disinfectants present.
6. Let your bacteria grow for one or two days in a warm, dark place.
The bacteria and mold probably did not grow very well in the area where the dots were placed. Were there more bacteria
around some dots than others? What does this tell you about how well each solution kills bacteria? Use the extra
materials in this kit to try variations of these experiments or devise your own bacteria! For further study, research “good”
bacteria—there are kinds that are essential to our bodily health.
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