Which of the following pathogens is commonly found in untreated water?

E. coli is commonly found in untreated water due to its presence in the intestines of warm-blooded organisms, including humans. This bacterium serves as an indicator of fecal contamination in water sources. When water is untreated or inadequately treated, it can contain high levels of E. coli, signaling a potential risk for waterborne diseases.

Other pathogens listed, while they also can be present in water, do not serve the same prominent role as reliable indicators of fecal contamination. For instance, amoebae exist in various environments, but the specific species that cause disease are typically not as prevalent in untreated water as E. coli. Staphylococcus is usually found on human skin or in nasal passages rather than in water. Listeria is often associated with food products rather than untreated water supplies. Thus, E. coli’s clear association with fecal matter and its widespread occurrence in untreated water make it the most relevant choice.