I am working with my daughter on her Science Fair
Project. We are testing daily items that we come
in contact with to see how many germs and bacteria it has. How can we
differentiate between the types of bacteria? How can we decide which one
has the most? We are using the growth medium Agar in petri dishes.
Where can I find more scientific info as to why this happens so we can
write up the project?
These are complex questions. First, the agar medium is
used as a solid phase so that one can see colonies
formed. These are round mounds of growth because
bacteria multiply in all directions, but they cannot
normally move in or on a solid phase so they remain at
the site of multiplication. Every bacterial cell can
multiply into a colony. Thus, the number of colonies
is a measure for the number of cells present, if you
have taken quantitative samples. If you want to
quantitate, you should try to standardize your samples
(for example, use 1 ml liquid to wash surfaces, food
particles, 1 ml of liquids, etc. and add of this one
drop (with a micropipette would be more accurate) per
agar plate and let the drop form a tear on the plate.
The number of colonies that grow in this tear are a
measure for the original number of bacteria present in
the drop, because each colony is derived from a single
You should realize that the agar you use may not be
optimal to allow growth for all kinds of bacteria that
are present. However you may notice differences in
morphology, such as colony size, shape, color etc.
This indicates that there are different kinds of
bacteria growing. Try to count the colonies per
species and count the number of different species you
I strongly recommend you spend some time browsing in
the Virtual Museum of Bacteria, in which you will find
basic information on what bacteria are, how they grow,
how they are contaminating our food, what
bacteriologists do, etc. Have a look at
If you have further questions, or difficulties getting
around in the musuem, please contact me.
Dr. Trudy Wassenaar
There are many types of agar. What specific kind are you using? What exactly
is the question you are trying to answer and what is the hypothesis of this
experiment? Are you just counting bacteria in general or are you looking
for a specific kind of bacteria? If you are just trying to count numbers,
you can just count the colonies (dots) of bacteria. If you are looking for a
particular type, you can look on the web for a picture if you know the name
and spelling. There are many microbiology books out there but they don't
always have pictures in them. Good luck.
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Update: June 2012