The coliform index is a rating of the purity of water based on a count of fecal bacteria.
Coliform bacteria are microorganisms that primarily originate in the intestines of warm-blooded animals.
It is important to know this because many disease-causing organisms are transferred from human and animal feces to water, from where they can be ingested by people and infect them.
Thermotolerant coliforms are present in higher numbers than individual types of pathogenic bacteria and they can be tested relatively easily.
Thermotolerant coliforms can survive in water on their own, especially in tropical regions, so they do not always indicate fecal contamination.