Design an experiment
to test each hypothesis. Make a step-by-step list of what you
will do to answer each question. This list is called an experimental
procedure. For an experiment to give answers you can trust, it
must have a "control." A control is an additional experimental
trial or run. It is a separate experiment, done exactly like
the others. The only difference is that no experimental variables
are changed. A control is a neutral "reference point"
for comparison that allows you to see what changing a variable
does by comparing it to not changing anything. Dependable controls
are sometimes very hard to develop. They can be the hardest part
of a project. Without a control you cannot be sure that changing
the variable causes your observations. A series of experiments
that includes a control is called a "controlled experiment."
|Below you will see
how to grow bacteria for the purpose of counting bacteria.
You need to perform the bacteria
growth experiment from the same sample and using the same
nutrients in two different groups. One group will be exposed to
the UV light for one hour and the other will not.
Place both groups of plates in
an incubator and compare the results after 36 hours.
In order to grow bacteria, you will
need culture media, plates or petri-dishes and some laboratory supplies
Culture Media: Culture media is
a moist or liquid matter that contains nutrients for bacteria. Almost
any nutrient food may be considered a culture media for general
bacteria, however if you want to grow a specific bacteria or prevent
growing some other bacteria, you will need to use a fine tuned recipe
for your culture media.
Chicken broth and beef broth are among
nutrients that most bacteria like. In some recipes you may also add some
mushroom extract. Sugar can also be added to most culture media. Small
amounts of some minerals such as potassium phosphate and calcium
carbonate may also be added to the culture media. Note that there are
many foods that are good for growing bacteria, but they are not good as
culture media. For example bacteria can easily grow on milk, but milk is
not a good culture media because it will change by the activity of
bacteria. Part of milk will solidifies when bacteria produce acids. A
good culture media must be clear and must remain liquid and should not
easily change pH. If we need to solidify our culture media, we use agar
to do that. Agar is a gelatinous substance that is extracted from sea
weeds. If we need to grow bacteria for the purpose of identification or
counting, we need to grow bacteria in nutrient agar plates. These are
petri-dishes containing a mixture of agar and nutrients.
Peteri-dishes: Petri-dishes are
disposable clear plastic dishes with a cap that are used for many
science experiments and bacteria growth. A thin layer of nutrient
agar in a petri-dish is enough for growing bacteria. You can see the
bacteria colony shapes and count them without opening the lead of the
petri-dish. Since petri-dishes are clear, you can see the bacteria from
either side of the dish.
Incubator: Incubator is a warm
cabinet that you can set it's temperature to a proper temperature for
bacteria growth. About 35º C is a good temperature for most bacteria.
This is close the body temperature. If you be able to create such a
temperature in any other way, it is as good as an incubator. You may
find warm places behind your refrigerator, next to the radiator or
inside an oven that is off.
You may also make an incubator by
placing a small desk lamp inside a wooden or metal box. Or you may put a
Styrofoam cooler upside down over a desk lamp. A small lamp (15 watts)
should be able to create enough heat to warm up a small space. Prepare
your incubator in advance and use a thermometer to test it a day before
starting your experiment.
- Agar (dry powder)*
- 10 petri-dishes (100 mmx 15mm)*
- 1 ml Pipette*
- 10 ml pipette*
- 2 transfer pipette*
- 2 test tubes with cap*
- Glass beaker or steel pan *
- Chicken broth or beef broth *
- Filter Paper *
* These material
are included in a kit
from MiniScience.com or you may buy them individually from a local
* Chicken broth
or beef broth can be purchased from supermarkets and health food stores
or you may make them at home. (It must be fat free). Filter paper is
coffee filter or you may substitute it with any clean cotton
If you are making your own chicken
broth, one small chicken can give you enough broth for this experiment.
(Half a pound beef can be used instead). Boil the chicken for one hour. Separate
the broth by transferring it to another pot. remove any fat from the top
of the broth. Filter the broth using a clean piece of cloth or coffee
filter. If you are buying chicken or beef broth from a super market, it
comes in small bags of about 4 grams each. Use two bags in about 300 ml
water and boil it. Then filter it and transfer the filtered broth to
another clean pan.
Add water to the broth to bring it to
about 650 ml and boil it again. Start adding the Agar powder while stirring.
(If you are using a MiniScience kit, entire agar tube must be used,
otherwise use 8 grams of dry agar powder). You will add agar gradually
while stirring. Adding agar will take 30 seconds to one minute. If you
don't stir it good, agar solidifies at the bottom of the pan and burns.
Continue stirring for another two minutes or until the agar solution is
fully dissolved and the solution is clear.
At this time you may optionally add a
calcium carbonate tablet and some mushroom extract. (You could also boil
mushroom with beef or chicken broth).
Your nutrient agar is ready at this
time. Turn off the heat and let it cool down. While it is still warm set
your petri-dishes on a table. Open the lead of each petri dish and pour
some nutrient agar in each dish (enough to cover the bottom of the dish)
and immediately put the lead back. Repeat this for all petri-dishes.
Leave the dishes where they are until they solidify.
Note: The nutrient agar should
be free from all bacteria, however boiling usually is not enough to kill
all the environmental bacteria that have entered our nutrient agar. For
school experiments, it may not matter but in real biology laboratories,
nutrient agar plates are transferred to an autoclave for sterilization.
Hot, under pressure steam of autoclave can kill all bacteria in about 30
minutes. Advanced students may use a pressure cooker to do the same.
Use a 1 ml pipette to place 0.1 ml of
polluted water or any other bacteria infested liquid on a dish. Spread
the liquid all over the plate surface. Cover the plate. Turn it upside
down and place it in an incubator. Nutrient agar will remain moist when
the plate is upside down. Check back within 36 hours to see the result
of bacteria growth.