Escherichia coli is a coliform bacterium that is a normal component of the human intestinal flora, and is ordinarily not a dangerous organism. The K12 and related strains of E. coli used in molecular cloning have been modified so that they are able to survive in culture only under very specific conditions, and unable to survive at all in the human gut. 

    Municipal water sources are routinely tested for the presence of "coliform" (rod-shaped) bacteria, which would indicate possible contamination with human or animal faecal material. Recent cases of E. coli contamination stem from "natural" pathogenic strains of E. coli that have found their way into public water sources through improper sterilization or filtration.