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 1 minute will kill these bacteria and make water safe to drink.