There are dozens of types of waterborne pathogens which can be spread in drinking water. The goal of the water treatment plant operator is to ensure that none of these pathogens are present in the finished water to infect the customers. However, testing the water for the presence of each type of waterborne pathogen is too time consuming and expensive to be feasible. Instead, operators test for the presence of one or a few indicator microorganisms which are usually present when pathogens are present.
The most commonly used indicator microorganisms are coliform bacteria which are found in large numbers living beside pathogenic microorganisms in the intestinal tracts of warm-blooded animals. Masses of coliform bacteria are discharged from the intestinal tract with these animals' feces. If sewage enters water, the coliform bacteria in the sewage will be carried along and will survive in the water for long periods of time. Thus, the presence of coliform bacteria provides evidence that water has been polluted with sewage, and also indicates the possible presence of pathogenic microbes.