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
you will see how to grow bacteria for
the purpose of counting bacteria.
need to make different agar plates with
different nutrients. Sugar, chicken
broth, beef broth, blood, filtered beans
soup and mushroom extract are among the
nutrients that you can try.
0.1 of some bacteria infected water on
each nutrient agar and place the plates
in an incubator. compare the results
after 36 hours.
In order to
grow bacteria, you will need culture media,
plates or petri-dishes and some laboratory
supplies and incubator.
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.
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 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
- Agar (dry
- 10 petri-dishes
(100 mmx 15mm)*
- 1 ml
- 10 ml
- 2 transfer
- 2 test
tubes with cap*
beaker or steel pan *
broth or beef broth *
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.
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).
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.
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
your test sample:
bacteria, you will prepare different dilutions
of your original sample. For example you will
make 1:10, 1:100, 1:1000 and 1:10000 dilutions.
You will then grow each dilution on a different
nutrient agar plate.
Use a 1 ml
pipette to place 0.1 ml of each dilution in each
dish. Spread the liquid all over the plate
surface using a sterile spoon. 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.
bacteria grow, each bacteria will become a
bacteria colony that can be seen as a small spot
on the petri dish. By counting bacteria
colonies, you will know how many bacteria
existed in the sample.