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Escherichia coli (Coliform)

Escherichia coli is a member of the coliform group, part of the family Enterobacteriaceae and is described as an anaerobic, Gram-negative, non-spore-forming, rod-shaped bacterium.

As a member of the Enterobacteriaceae family, E. coli is naturally found in the intestines of humans and other warm-blooded animals. Unlike other bacteria in this family, E. coli does not usually occur naturally on plants or in soil and water. Within human and animal faeces, E. coli is present in large volumes and comprises about 1% of the total biomass in the large intestine. Although E. coli are part of the natural faecal matter, some strains of this bacterium can cause gastrointestinal illness along with other, more serious health problems.

Of all the coliforms, E. coli is generally the most sensitive to environmental stresses. Its survival time in the environment is dependent on many factors, including temperature, exposure to sunlight, presence and types of other microflora, and the type of water involved. In general terms, E. coli can survive for about 4-12 weeks in water at a temperature of 15-18C. Regrowth of E. coli in water distribution systems is not a concern, since E. coli rarely grows outside the human or animal gut. The inability of E. coli to grow in water, combined with its short survival time in water environments, means that the detection of E. coli in a water system is a useful indicator of recent faecal contamination.

Although modern microbiological techniques have made the detection of pathogenic bacteria, viruses, and protozoa possible, it is currently not practical to attempt to routinely isolate them from drinking water. Reasons for this include the large number of possible pathogens, the lack of inclusion of previously unknown pathogens, and the time and expense associated with routine monitoring of all pathogens. Instead, microbial indicators are used, since it is less difficult, less expensive, and less time consuming to monitor indicators than to monitor individual pathogens. Simple, inexpensive techniques encourage a higher number of samples to be tested, giving a better overall picture of the water quality and therefore better protection of public health. Newer molecular methods may provide an easy, inexpensive, and quick method for the detection of pathogens or indicators; to date, however, this is not the case.

An appropriate health-based indicator of microbial pathogens should possess several ideal qualities. The indicator should always be present when the pathogen is present and should not be detected when the pathogen is absent; it should have a life span similar to that of the pathogen of concern; it should be present in large numbers and should be readily detected by simple and inexpensive methods; and it should not multiply in the environment once it has been shed by the host. Based on these qualities, if the indicator is isolated from the water supply, this infers that pathogenic organisms could be present; if the indicator is absent, pathogenic organisms are probably also absent.

Of the contaminants that may be found in drinking water, those present in human and animal faeces pose the greatest danger to public health. For this reason, the ability to detect faecal contamination in drinking water is a necessity for ensuring public safety. As early as the 19th century, E. coli was recognized as a good indicator of faecal contamination. It was identified as the only species in the coliform group found exclusively in the intestinal tract of humans and other warm-blooded animals and subsequently excreted in large numbers in faeces. In addition to being faecal specific, E. coli do not usually multiply in the environment and have a life span on the same order of magnitude as those of other enteric bacterial pathogens, both of which are qualities of an ideal indicator. As mentioned previously, they are also excreted in the faeces in high numbers, making detection possible even when greatly diluted.