Why can E. coli in water indicate fecal pollution?

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Bacteria A scanning electron micrograph of gram-positive *Mycobacterium tuberculosis* bacteria, the cause of tuberculosis and scrofula.

Escherichia coli (E. coli) is a bacterium that naturally inhabits the lower intestinal tract of humans and other animals. Because it is a normal resident of the gut, its presence in water serves as a reliable indicator that the water has been contaminated with intestinal waste.123

In public health and microbiology, E. coli is used as a standard marker for fecal pollution. While the bacterium itself does not always cause illness, its detection in a water supply signals that the water may also contain other dangerous, disease-causing organisms that are commonly found in feces. Consequently, testing for E. coli is a critical, routine procedure used to determine whether water is potable and safe for human consumption.435