Protecting Public Health and the Environment.

Coliform Bacteria in Drinking Water

Biological organisms are among the oldest health threats to drinking water quality and the agents currently responsible for most waterborne diseases. They are the most common contamination incidents water operators will encounter. Organisms known to cause disease include bacteria, protozoa, and viruses; some algae and helminths (worms) may also be capable of producing disease. These disease-causing organisms thrive in the intestines of warm-blooded animals. They are easily transmitted to drinking water if the feces of an animal contaminates a water supply for which there is no suitable disinfection. Potential sources of contamination include sewers, septic systems, and animal feedlots.

Role of Coliforms in Detecting Contamination

Unfortunately, specific disease-producing (pathogenic) organisms present in water are not easily identified. It would be very difficult, expensive, and time-consuming to monitor for them. For this reason, it is necessary to select an easily measured indicator organism, whose presence indicates that pathogenic organisms may be present. A group of closely related bacteria (i.e., total coliform) has been selected as an indicator of harmful organisms in drinking water.

Sources of Coliform Bacteria

Total coliform (TC) bacteria are common in the environment (such as soil) and the intestines of animals and are generally not harmful. Fecal coliform (FC) and Escherichia coli (E. coli) bacteria are found in greater quantities than TC in animal fecal matter. If FC or E. coli is detected along with TC in drinking water, there is strong evidence that sewage is present; therefore, a greater potential for pathogenic organisms exists.

Responses to Coliform Detection

Public drinking water systems must have monitoring conducted on a routine basis. If FC, or E. coli is detected in the distribution system of a public water system, the system must be disinfected. In most cases, this includes emergency chlorination, which can last for 2 to 5 days. At the same time a system is being disinfected, customers of the public water system are advised to vigorously boil their drinking and cooking water for 1 minute before using it. Once the system has been disinfected and flushed, the system tests the water again for coliform bacteria. If none are detected, the boil advisory is lifted. If only TC is detected (without the presence of fecal coliform or E. coli), the source is most likely from contamination from the environment, introduced during construction or while repairs to plumbing or a water main were underway. The system will identify the source of contamination, correct the problem, and thoroughly disinfect its system. The public will also be notified of the situation; however, unless unusual circumstances exist to cause particular concern about the safety of the water, a boil advisory will not be issued.


Total coliforms are not perfect indicators of the actual or potential presence of harmful organisms. Some disease-producing organisms, especially protozoa such as Giardia and Cryptosporidium, are able to withstand treatment that kills total coliforms. These two protozoa are often found in surface waters (the principal carriers of these organisms) contaminated by human sewage or wildlife. However, for the majority of public water systems, this is not a significant threat since most systems obtain their water from wells rather than surface water sources such as rivers and lakes. For those public water systems that use surface water, a combination of coagulation, filtration, and disinfection has been successful and is recommended to reduce the risk of Giardia or Cryptosporidium contamination.

Health Effects

Symptoms of waterborne diseases may include gastrointestinal illnesses such as severe diarrhea, nausea, and possibly jaundice as well as associated headaches and fatigue. It is important to note, however, that these symptoms are not associated only with disease-causing organisms in drinking water. They may also be caused by a number of other factors. In addition, not all people will be affected to the same degree; 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

Safe Drinking Water Hotline


More Information

Basic Information about Pathogens and Indicators in Drinking Water