Indicator organism

Indicator organisms are used to measure potential fecal contamination of environmental samples. The presence of coliform bacteria, such as E. coli, in surface water is a common indicator of fecal contamination. Coliform bacteria in water samples may be quantified using the most probable number (MPN) method, a probabilistic test which assumes cultivable bacteria meet certain growth and biochemical criteria. If preliminary tests suggest that coliform bacteria are present at numbers in excess of an established cut-off (the Coliform Index), fecal contamination is suspected and confirmatory assays such as the Eijckman test are conducted.

Coliform bacteria selected as indicators of fecal contamination must not persist in the environment for long periods of time following efflux from the intestine, and their presence must be closely correlated with contamination by other fecal organisms. Indicator organisms need not be pathogenic.

Non-coliform bacteria, such as Streptococcus bovis and certain clostridia may also be used as an index of fecal contamination.