|MadSci Network: Microbiology|
Dear Krisitn: Escherichia coli is said to be found in the feces of all warm-blooded animals (i.e., mammals and birds), so its presence in water generally indicates some level of fecal contamination. Of course, in most instances the feces are not of human origin — could be birds (e.g., wild ducks) as well as wild animals or food animals such as cattle, chickens, etc. Most surface water Lakes and rivers) will have some E. coli in it, from birds or whatever, unless it's so terribly polluted that the animals avoid it. E. coli is a bacterial species; if E. coli is detected, it is very difficult to tell from what animal host species it originated. Although E. coli may multiply in the environment under some circumstances, most of what you may find in water is whatever was shed from an animal's (including human's) intestines. E. coli are short, rod-shaped bacteria that are negative (red) in the gram stain and do not form spores, so they are fairly easy to kill with heat, ultraviolet light, chlorine, etc. Older methods of testing for E. coli in water were pretty laborious; you can probably find a copy of Standard Methods for the Examination of Water and Wastewater, published by the American Public Health Association, in your local library and learn about these. In more recent years, attention has focused on an enzyme called "beta-glucuronidase" that is produced by almost all E. coli and very few other bacteria. Media have been developed that contain a substance that either changes color or becomes fluorescent (when lit with the right kind of ultraviolet light) after being acted upon by beta-glucuranidase. The enzyme splits the colorless molecule so that one of the products has a color or will fluoresce. These tests are not 100% accurate, but they are ever so much quicker and more convenient than the old ones. Some of the new test media are broths (liquids) — you put the water sample into a bottle or tube with the medium and incubate it at ~98F for a day or two, then look for the color or fluorescence that tells you E. coli was present in the sample. Other media have agar in them, so that they gel; the sample is spread on the surface of the agar in a petri plate and incubated. Colonies that grow and show the right color or fluorescence can be counted, so you not only know that E. coli was present in the sample, but you can tell how much was there. Commercial E. coli test media usually also test for a wider group of bacteria called total coliforms, based on the production by these bacteria (including E. coli) of another enzyme called "beta-galactosidase." Two brand names are Colilert and Colitaq; they tend to be rather pricey. If you have the necessary facilities and adult guidance, you may wish to produce your own medium. Very explicit directions for how to concoct the medium and how to perform the test are given at the EPA web site . Tell them I sent you! Good luck!
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