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."
In order to grow
bacteria, you will need culture media, plates or
petri-dishes and some laboratory supplies and
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.
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.
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.
- 2 test tubes
with screw cap*
- 1 ml Pipette*
- 10 ml pipette*
- 2 transfer
- 2 test tubes
- 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 laboratory supplier.
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 cloth.
chicken broth or beef broth and boil it with some
mushroom. Let it cool off to the room temperature
and filter it to be clear. Use a transfer pipette
to collect samples such as water from a polluter
source or saliva from your mouth in the morning.
Pipette 10 ml of filtered broth and 2 ml of the
polluted sample in a test tube and close the cap.
Place the tube in an incubator or other warm place
for the bacteria to grow. As bacteria population
increases, the sample will start to have a bad
odor and becomes more cloudy. Use a separate test
tube for each bacteria sample. Within 3 days you
should have enough bacteria in your test tube.
bacteria is ready, you can do other experiments
such as pH test to see ho do bacteria affect the
pH of a solution.