MadSci Network: Microbiology

Re: How can I test for E-coli in local water resouces?

Date: Fri Dec 21 12:46:09 2001
Posted By: Dean Cliver, Faculty, Food Safety Unit, Uiversity of California, Davis
Area of science: Microbiology
ID: 1002498897.Mi

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) 

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 

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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