Which type of bacteria occurs naturally in the intestines and indicates the presence of fecal contamination in water?

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Indicator organisms in water quality testing are used to flag fecal contamination. Coliform bacteria form a broad group that includes many species commonly found in the intestinal tracts of humans and animals as well as in the environment. Their presence in a water sample signals that waste from fecal sources may have entered the supply, so further testing for fecal contamination is warranted. Escherichia coli is a specific member of this group and serves as a more precise indicator of recent fecal pollution, but the general indicator role is fulfilled by coliform bacteria as a whole. Pseudomonas aeruginosa is an environmental organism that can indicate health risks but isn’t used as a routine fecal contamination indicator, and Bacillus subtilis is a soil bacterium not tied to fecal contamination.