Bacteria are common single-celled organisms and are a natural component of lakes, rivers and streams. Escherichia coli (E. coli) bacteria, is commonly found in the digestive tract of animals, and is easily transferred into the environment. When humans drink contaminated river water the bacteria can cause sickness including diarrhea and gastroenteritis. It is measured in the number of colony forming units per 100 mL of water sampled (CFU/100 mL). For drinking purposes, E. Coli should not be detected in a 100 mL sample (i.e. 0 CFU/100 mL).

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Sources:

(1) https://www.usgs.gov/special-topic/water-science-school/science/bacteria-and-e-coli-water?qtscience_center_objects=0#qt-science_center_objects

(2) World Health Organization, 1997. Guidelines for drinking-water quality Second Edition - Volume 3 Surveillance and control of community supplies, WHO Library Cataloguing in Publishing Data, Malta.