Protecting Public Health and the Environment.

Coliform Bacteria in Drinking Water

Coliform bacteria are organisms that are present in the environment and in the feces of all warm-blooded animals and humans. Coliform bacteria will not likely cause illness. However, their presence in drinking water indicates that disease-causing organisms (pathogens) could be in the water system. Most pathogens that can contaminate water supplies come from the feces of humans or animals.

Role of Coliforms in Detecting Contamination

Testing drinking water for all possible pathogens is complex, time-consuming, and expensive. It is relatively easy and inexpensive to test for coliform bacteria. Total coliform testing is used as an indicator of potential contamination.

Total Coliform, Fecal Coliform, and E. coli

Total coliform bacteria are commonly found in the environment (e.g., soil or vegetation) and are generally not harmful. However, if environmental contamination can enter the system, there may also be a way for pathogens to enter the system.

Fecal coliform bacteria are a subgroup of total coliform bacteria. They appear in great quantities in the intestines and feces of people and animals. The presence of fecal coliform 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.

E. coli is a subgroup of the fecal coliform group. The presence of E. coli in a drinking water sample almost always indicates recent fecal contamination, meaning there is a greater risk that pathogens are present.

Total Coliform, Fecal Coliform, and E. coli

Response to Coliform Detection

When coliform bacteria are found, water systems investigate to find out how the contamination got into the water. They collect additional, or "repeat," water samples for testing, and often inspect the entire system. Taking repeat samples helps determine whether an actual problem exists in the system. If any of the repeat samples detect coliform bacteria, the initial findings are considered confirmed. If total coliform bacteria are confirmed in your drinking water, your water system should be inspected to find and eliminate any possible sources of contamination. Once the source is identified, it can usually be resolved by making system repairs, flushing, and adding chlorine for a short period of time.

Confirmation of fecal coliform bacteria or E. coli in a water system indicates recent fecal contamination, which may pose an immediate health risk to anyone consuming the water. Public notification, in the form of a boil advisory or potentially a do not drink advisory, will be issued within 24 hours to alert all water users that there is a health risk associated with the water supply. The notice will inform customers of actions being taken to correct the problem, and when the problem will likely be resolved.

The system will be required to identify the source of contamination, correct the problem, and thoroughly disinfect and flush its system. Once the contamination issue is verified to be resolved the advisory will be lifted.

Health Effects

Symptoms of bacterial waterborne diseases may include gastrointestinal illnesses such as severe diarrhea, nausea, and possibly jaundice as well as minor symptoms like headaches and fatigue. These symptoms are not associated only with disease-causing organisms in drinking water and may be caused by several other factors. Young children and the elderly are usually more susceptible.



Staff Contacts

Decision Support Coordinator
Monica Van Bussum
DEQ State Office
Water Quality Division
1410 N. Hilton
Boise, ID 83706
(208) 373-0111

EPA Safe Drinking Water Hotline


More Information

Revised Total Coliform Rule and Total Coliform Rule