E. coli as an indicator

Monitoring the microbiological quality of drinking water relies largely on examination of indicator bacteria such as coliforms, Escherichia coli, and Pseudomonas aeruginosa. E. coli is a member of the fecal coliform group and is a more specific indicator of fecal pollution than other fecal coliforms.

https://www.pagepress.org/journals/index.php/mr/article/view/mr.2013.e2/6220

Coliform bacteria are organisms that are 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 pathogens could be in the water system.

https://www.doh.wa.gov/CommunityandEnvironment/DrinkingWater/Contaminants/Coliform