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For decades, coliform bacteria have been used as an "indicator organism" in the evaluation and monitoring of public and private drinking water sources and distribution systems. The ultimate goal is to assure the water is free from pathogenic (disease causing) microorganisms.

Since most pathogens in drinking waters, if detected, are usually in very low concentrations, it would be difficult and ineffective to test water specifically for E. coli or other specific organisms. For this reason, coliforms were selected to be indicator organisms. It is always present in water contaminated with fecal material, is easy to detect, is generally more hardy than true pathogens, and will be found in greater numbers in contaminated water. Its absence is a good bacteriological indicator of safe water, and it grows readily on relatively simple media. Unfortunately, there are many groups of microorganisms such as enteric virus and protozoan pathogens that are relatively more resistant to chlorine and other disinfectants than coliforms, and could possibly be present when coliforms are absent. Total coliforms continue to be the basic microbiological standard in the United States because their absence assures the absence of E. coli in a conservative manner.

Positive test results for coliforms can be caused both by the presence of coliforms, as well as the presence of some environmental bacteria, such as Klebsiella pneumoniae, which may be from a soil source. However, positive coliform results should be regarded as in indicator of possible fecal contamination. Compliance with public health laws is based on the presence or absence of total coliform. In public water systems, repeat samples are required for each coliform positive sample, including same tap, and upstream and downstream connections. In addition, each positive must be tested for the presence of E. coli or fecal coliforms. If the repeat sample is fecal coliform positive, or if the original fecal or E. coli positive is followed by a total coliform positive, state regulators must be notified on the same business day. Positive coliform results, with negative E. coli or fecal results may not require any immediate action, as the maximum contaminant level (MCL) is based on the number of coliform positives in relation to the number of samples taken.

There are pathogens in water whose presence will not be detected by testing for coliforms, including many types of virus and protozoan organisms. Not only is the presence of these organisms not indicated through routine coliform testing, they may be present in such small concentrations, that routine samples pulled for testing are not large enough to capture them. Other methods of sampling for microorganisms, including filtration of thousands of gallons of water, are normally required for the detection of some pathogens.