E. coli bacteria: microscopic organism indicates fecal contamination.

Detailed Description
E. coli bacteria
Escherichia coli or E. coli is a type fecal coliform bacteria that is commonly found in the intestines of animals and humans. E. coli in water is a strong indicator of sewage or animal waste contamination. Sewage and animal waste can contain many types of disease causing organisms. Consumption may result in severe illness; children under five years of age, those with compromised immune systems, and the elderly are particularly susceptible.
Sources/Usage
Public Domain.
Coliform
EPA