Total coliform bacteria are a collection of microorganisms that live in large numbers in the intestines of humans and animals, as well as in most soils and surface water. A sub-group of these microorganisms is the fecal coliform bacteria, the most common member being E. coli. These bacteria occur naturally in lakes and streams, but indicate that the water is contaminated with human or animal waste and therefore may pose a health risk to people who drink it.
The water treatment process removes these bacteria from the water, but events such as a water main break or a loss of pressure in the water distribution system may allow these bacteria to enter water lines through cracks in pipes or back-siphoning from a residential plumbing system. Boiling water vigorously for one minute will kill these bacteria and make water safe to drink.