Coliform Bacteria in Drinking Water

Public water systems are required to deliver safe and reliable drinking water to their customers 24 hours a day, 365 days a year. If the water supply becomes contaminated, consumers can become seriously ill. Fortunately, public water systems take many steps to ensure that the public has safe, reliable drinking water. One of the most important steps is to regularly test the water for coliform bacteria.

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About Coliform Bacteria

Coliform bacteria are organisms 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. Testing drinking water for all possible pathogens is complex, time-consuming, and expensive. It is relatively easy and inexpensive to test for coliform bacteria. If coliform bacteria are found in a water sample, water system operators work to find the source of contamination and restore safe drinking water. There are three different groups of coliform bacteria; each has a different level of risk.

Total Coliform, Fecal Coliform, and E. Coli

Total coliform, fecal coliform, and E. coli are all indicators of drinking water quality. The total coliform group is a large collection of different kinds of bacteria. Fecal coliforms are types of total coliform that mostly exist in feces. E. coli is a sub-group of fecal coliform. When a water sample is sent to a lab, it is tested for total coliform. If total coliform is present, the sample will also be tested for either fecal coliform or E. coli, depending on the lab testing method.

Image of the relationship between Total Coliform, Fecal Coliform and E.coli.


Total coliform bacteria are commonly found in the environment (e.g., soil or vegetation) and are generally harmless. If only total coliform bacteria are detected in drinking water, the source is probably environmental. Fecal contamination is not likely. However, if environmental contamination can enter the system, there may also be a way for pathogens to enter the system. Therefore, it is important to find the source and resolve the problem.

Fecal coliform bacteria are a sub-group 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 sub-group of the fecal coliform group. Most E. coli bacteria are harmless and are found in great quantities in the intestines of people and warm-blooded animals. Some strains, however, can cause illness. 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.

A note about E. coli: E. coli outbreaks receive much media coverage. Most outbreaks have been caused by a specific strain of E. coli bacteria known as E. coli O157:H7. When a drinking water sample is reported as "E. coli present," it does not mean that this dangerous strain is present and in fact, it is probably not present. However, it does indicate recent fecal contamination. Boiling or treating contaminated drinking water with a disinfectant destroys all forms of E. coli, including O157:H7.

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What Happens When Coliform Bacteria are Found in My Water

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.

What Happens When Total Coliform Bacteria are Confirmed in My Water

If total coliform bacteria are confirmed (at least 2 samples with coliform bacteria present) 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. The state Health Department works with water systems and utility managers to help resolve such problems.

What Happens When Fecal Coliform Bacteria or E. Coli are Confirmed in My Water

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. Responding to health emergencies is our highest priority. We will issue a Health Advisory within 24 hours to alert all water users that there is a health risk associated with the water supply. In most cases, we will recommend the use of boiled or bottled water for drinking and cooking. The notice informs customers of actions being taken to correct the problem, and when the problem will likely be resolved. We will inspect the system as soon as possible to assist the water system in resolving the problem. The system will take more water samples to find and eliminate potential contamination sources, and chlorination and flushing of the system will most likely occur. The Health Advisory remains in effect until the situation is resolved and the water is safe to drink.

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Information for Water Systems

More publications are available through our online publication and forms database.

For More Information

Northwest Regional Office

Coliform Program; Main Office: 253-395-6750

Southwest Regional Office

Coliform Program 360-236-3045; Main Office: 360-236-3030

Eastern Regional Office

Coliform Program; Main Office: 509-329-2100

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