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Method to detect E. coli

Postby Califday » Wed Nov 07, 2007 8:21 pm

Hiya; I would like to find out if there is any E. coli in the beach sand where the frequent contaminated streams meet the ocean. Would it make sense to collect some sand in a test tube or other container, add distilled water, and then swab the liquid on an agar dish? Also, has anyone heard of the Coliscan/Easygel dishes that help you to detect E. coli? Is this cheating to use these kinds of dishes? What would be another way to go about detecting or identifying this kind of bacteria? Thanks for any information.

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Postby EmilyDolson » Wed Nov 07, 2007 10:27 pm


Great project idea! This is a really important topic to study if we are going to preserve our environment.

As with any real-world experiment, there are many factors that can affect your results, so you have to be careful how you set up your project. According to this study, (which is incredibly dense, but relevant to your project if you are planning on going really in-depth) sand tends to have a higher concentration of E. coli, than the water beside it. Testing the sand itself sounds like a very interesting idea, but keep in mind that it is very different than testing the water. Where exactly are you planning to collect the samples from, sand on the stream-bed/ocean floor, or along the shoreline? You will probabky need to find some way to make sure that all of your test sites have the same amount of contact with the water nearby. Your idea of collecting sand in a test tube and adding distilled water is great, but make sure to shake it really thouroughly. You will also need to make sure that you are shaking it in the same way each time, and adding the same amount of water.

Once you have your sample in liquid form, you could swab it directly onto an agar dish, but membrane filtration would give you more accurate results. It requires special equpiment, so it may not be practical for you. However, you may want to check with science teachers, especially if any of them teach environmental science, at your school before you discount it, as they may have the neccesary equipment for labs. For more information on membrane filtration, you can look it up on google. There is a fairly clear explanation starting on page 8 of this web page:

Whichever technique you decide to use, it would probably be best to have both a positive and negative control every time you test samples. To do this, set up a petri dish with a sample you know is uncontaminated, and one with a sample you know has E. coli in it. This proves that your equipment was sterile, and your conditions made bacteria growth possible. Also, be careful when culturing bacteria, as harmful pathogens could also grow. Wash your hands well after handling samples, and check the rules regarding bacteria for any science competitions that you are entering, because some require extra approval for projects that involve culturing it, to make sure safe lab technique is used.

As for Coliscan dishes, it depends on who you are doing the project for. If it is a school project, check with your teacher. If it is for a science fair, you may want to check the rules just to be sure, but I would imagine that it would be fine to use them. They don't seem to detract from the level of scientific reasoning that goes into your project, just the amount of tedious, time-consuming work you have to do. Coliscan dishes are a scientifically accepted part of E. coli testing, and I am sure the judges would recognize that you have many better uses for your time.

For other methods of detecting E. coli check out this web page:

Whew! Sorry that turned out to be so long. I hope it helps, and don't hesitate to ask if you have any more questions!

- Emily
Reach for the stars and, if you miss, grab the moon!

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Agar Colony counting

Postby vtrip » Tue Nov 13, 2007 12:36 pm

You have a very interesting project idea, and I hope everything works out. I also have a few ideas for your project.

1.) As mentioned before, BE CAREFUL with bacteria and try to maintain as sterile a technique as possible. Follow the posted guidelines for using bacteria, and if you have access you a sterile hood, definitely use it.

2.) I think a good pre-test for your experiment would be just to use plain agar plates, because they are cheap and will tell you what parts of your project you need to further work on in regards to experimental design. Collect your samples as you describe in your protocol and then plate them onto the agar. Grow them overnight and see what happens.

-if you get a bacterial lawn, then you have way to much sample or your sterile technique isn't very sterile and you may need to dilute your samples further and work in a hood.
-if you get countable bacterial colonies, then you can proceed with just counting the number of colonies you get, and plot your data and analyze it.

I hope this helps, if you have any questions post another reply.

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