The use of an organism that can serve as a surrogate for
another is called an indicator organism. Trying to detect
disease-causing bacteria and other pathogens in water is
expensive and may pose potential health hazards.
Further, testing for pathogens requires large volumes of
water, and the pathogens can often be difficult to grow
in the laboratory and isolate. E. coli bacteria are good
indicator organisms of fecal contamination because they
generally live longer than pathogens, are found in
greater numbers, and are less risky to collect or culture in a laboratory than pathogens. However, their
presence does not necessarily mean that pathogens
are present, but rather indicates a potential health
hazard.