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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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