Coliforms are a family of bacteria that are commonly found in soil, plants, and the intestinal tracts of humans and animals. They are used as an indicator of the potential presence of disease-causing bacteria in water. There are three types of coliform bacteria that are commonly tested for:

• Total coliforms: This group includes bacteria that are found in the soil, in water that has been influenced by surface water, and in human or animal waste. Testing for total coliform bacteria gives a general indication of the sanitary condition of a water supply.

• Fecal coliforms: This group of bacteria is considered to be present specifically in the gut and feces of warm-blooded animals. Because the origins of fecal coliforms are more specific than the origins of the more general total coliform group of bacteria, fecal coliforms are considered a more accurate indication of animal or human waste than the total coliforms.

• Escherichia coli (E. coli): This is the major species in the fecal coliform group. Of the five general groups of bacteria that comprise the total coliforms, only E. coli is generally not found growing and reproducing in the environment. Consequently, E. coli is considered to be the species of coliform bacteria that is the best indicator of fecal pollution and the possible presence of pathogens.

While coliform bacteria are not normally causes of serious illness, their presence in drinking water indicates that disease-causing organisms (pathogens) could be in the water system. Testing drinking water for all possible pathogens is complex, time-consuming, and expensive. Therefore, testing for coliform bacteria can be a reasonable indication of whether other pathogenic bacteria are present.