Salmonella is an enteric bacterium. Ditto for Escherichia coli. Sanitation "engineers" look for evidence of "coliform" bacteria in water supplies as evidence of fecal contamination. Bird droppings in to open wells, runoff of water from animal grazing areas and rodent droppings are common sources.
Everyone has E. coli in their intestinal tract. We depend upon E. coli to manufacture vitamin K to help our blot clot properly. The "bad boys" in the E. coli world are E. coli O157:H7 and E. coli O129:H19. They produce exotoxins that damage the kidneys. This toxin is virtually identical to the one produced by another enteric bacterium, Shigella dysenteria. It typically only takes 50 viable E. coli organisms to "set up housekeeping" in your intestinal tract.
Bon appetit!
More than I needed to know!