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Method for Collecting Samples
Name: Carole
Status: Educator
Age: 50s
Location: N/A
Country: N/A
Date: 2002
Question:
I have several students who are planning on doing
microbiology science projects. One wants to find if there is bacteria in
used make-up, and the other is testing places around the school (faucets,
handles, etc.) for bacteria. My question is: How do they collect and
spread the bacteria on the agar plates? Is it acceptable to just swab
from the object onto the plate, or should it first be cultured in
broth? If cultured in broth, could you please send instructions for doing so?
Replies:
It really depends on the number of bacteria you expect
to be present in your sample. If the number if
sufficient to pick up directly, it is easier than
culture in broth. I suggest to use sterile cotton
swabs, moistured with sterile 0.9% saline solution
prior to use. After sampling/swabbing whatever surface
or material, the cotton swab is spread completely over
an agar plate. That way the agar is less easily
damaged.
Trudy Wassenaar
You can do both and see which gives the best recovery. It also depends what
your goal is. If you want to see WHAT is growing, you can subculture in
broth. If you want to know HOW MUCH is growing, you can swab it directly
onto agar. Growing in broth will give you increased numbers because the
bacteria will multiply before you put it on the agar. You will not get an
accurate idea of how much is there. If you just want to see what kinds of
organisms, you might get better recovery by growing in broth first. Also,
organisms that are recovered from the environment are usually the kind that
grow at environmental temperatures and may be inhibited by growth at
incubator temperatures. However, they grow more slowly. So incubate them at
room temperature, but for a longer time. Or you can have two plates, one at
room temperature and another in the incubator.
Good Luck
vanhoeck
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Update: June 2012
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