They are used as an indicator organism for pathogenic bacteria.
Explanation:
Coliforms are bacteria found in digestive tract of animals and humans. They are non-pathogenic, easy to culture and are used as an alternative indirect testing for presence of other microorganisms. Coliforms thrive in exact environmental conditions like other pathogenic bacteria, thus often used to indicate whether they are bacteria found in drinking water and swimming pools that might end up causing infections.
Therefore the existence of coliforms in water easily indicates feacal contamination.
