Coliform bacteria are a group of bacteria that are commonly found in the environment, including in soil and surface water, and in the digestive tracts of animals, including humans, and are found in their wastes. They are often referred to as "indicator organisms" because their presence indicates the potential presence of disease-causing bacteria in water. Total coliform, fecal coliform, and E. coli are all indicators of drinking water quality. Total coliform counts give a general indication of the sanitary condition of a water supply, while fecal coliforms are considered a more accurate indication of animal or human waste than the total coliforms. E. coli is a subgroup of fecal coliform and is found in greater quantities than total coliform in animal fecal matter. If E. coli is detected along with total coliform in drinking water, there is strong evidence that sewage is present, and therefore, a greater potential for pathogenic organisms exists.

The presence of coliform bacteria in drinking water does not guarantee that drinking the water will cause an illness. However, their presence indicates that a contamination pathway exists between a source of bacteria (surface water, septic system, animal waste, etc.) and the water supply, and disease-causing bacteria may use this pathway to enter the water supply. If coliform bacteria are present in drinking water, the risk of contracting a water-borne illness is increased. Fecal coliform bacteria are a sub-group of total coliform bacteria and their presence in a drinking water sample often indicates recent fecal contamination, meaning that there is a greater risk that pathogens are present than if only total coliform bacteria is detected.

In summary, coliform bacteria are a group of bacteria that are commonly found in the environment and in the digestive tracts of animals, including humans, and are found in their wastes. They are often used as an indicator of the potential presence of disease-causing bacteria in water. Total coliform, fecal coliform, and E. coli are all indicators of drinking water quality, and their presence in drinking water indicates that disease-causing organisms (pathogens) could be in the water system.