Indicator organism
Encyclopedia
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.
Coliform bacteria
Coliform bacteria are a commonly used bacterial indicator of sanitary quality of foods and water. They are defined as rod-shaped Gram-negative non-spore forming bacteria which can ferment lactose with the production of acid and gas when incubated at 35-37°C...
, such as E. coli, in surface water
Surface water
Surface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric 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
Coliform Index
The coliform index is a rating of the purity of water based on a count of fecal bacteria. Coliform bacteria are microorganisms that primarily originate in the intestines of warm-blooded animals...
), 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
Streptococcus bovis
Streptococcus bovis is a catalase- and oxidase-negative, non-motile, non-sporulating, Gram-positive lactic acid bacterium that grows as pairs or chains of cocci. It is a member of the Lancefield group D streptococci. Most strains are non- or gamma-hemolytic, but some also display alpha-hemolytic...
and certain clostridia
Clostridia
The Clostridia are a class of Firmicutes, including Clostridium and other similar genera. They are distinguished from the Bacilli by lacking aerobic respiration. They are obligate anaerobes and oxygen is toxic to them. Species of the genus Clostridium are all Gram-positive and have the ability to...
may also be used as an index of fecal contamination.