Indicator Organisms


Michigan Environmental Education Curriculum
Water Quality - Pollutant Sources and Impacts

ColiformHow do we KNOW that there aren’t any pathogens in our drinking water or in the water down at the old swimming hole? There are so many different kinds of organisms that cause diseases that it would be too expensive and time consuming to test for each one.

Diseases are spread when water is contaminated with fecal matter. We can test for that pollution by using indicator organisms. These are microbes whose presence in water signals the presence of fecal matter, and potentially, pathogens. One such indicator organism is Escherichia coli, a bacterium that is normally found in the guts of humans and other warm-blooded animals. Water that is highly polluted with fecal matter may have E. coli counts in the tens of millions of bacteria per liter. In the State of Michigan, beaches are ordered closed if the E. coli count exceeds a monthly average of 130 or a maximum of 300 bacteria per 100 milliliters. A drinking water source is declared safe only if these bacteria are not detected in the sample.

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