Escherichia coli

4.0 Significance of E. coli in drinking water

4.1 Description

Escherichia coli is a member of the coliform group, part of the family Enterobacteriaceae, and is described as a facultative anaerobic, Gram-negative, non-spore-forming, rod-shaped bacterium that possesses the enzyme β-glucuronidase.

4.2 Sources

As a member of the Enterobacteriaceae family, E. coli is naturally found in the intestines of humans and 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 at a concentration of approximately 10⁹ per gram (Edberg et al., 2000) and comprises about 1% of the total biomass in the large intestine (Leclerc et al., 2001). Although E. coli are part of the natural faecal flora, some strains of this bacterium can cause gastrointestinal illness along with other, more serious health problems. Further information on illness-associated E. coli strains can be found in Bacterial Waterborne Pathogens: Current and Emerging Organisms of Concern (Health Canada, 2006a). It should be noted that faecal concentrations of the typical non-pathogenic E. coli, used to indicate recent faecal contamination, will always be greater than those of the pathogenic strains, even during outbreaks.

Of 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 (e.g., groundwater, surface water, or treated distribution water). In general terms, E. coli survives for about 4-12 weeks in water containing a moderate microflora at a temperature of 15-18°C (Kudryavtseva, 1972; Filip et al., 1987; Edberg et al., 2000). Regrowth of E. coli in water distribution systems is not a concern, since E. coli rarely grows outside the human or animal gut (Geldreich, 1996). 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 good indicator of recent faecal contamination.

5.0 Role of E. coli as an indicator of microbiological safety

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 (approximately 10⁹ per gram) (Department of National Health and Welfare, 1977). 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.

In contrast to the situation with E. coli, it was recognized that most genera in the total coliform group occur naturally in soil, vegetation, and water in addition to faeces, making them unsuitable indicators of faecal contamination. Nevertheless, total coliforms were used as a surrogate for E. coli, primarily because routine methods to distinguish E. coli from other coliform bacteria were not available. It was not until the mid-20th century that more specific methods for the thermotolerant coliforms (previously referred to as faecal coliforms), which include E. coli and members of the genera Klebsiella, Enterobacter, and Citrobacter, were developed. Although thermotolerant coliforms were more specific than total coliforms for E. coli, the former group contained other species that were capable of surviving and growing in water and were not faecal specific. With the advent of enzyme substrate tests, it is now possible to routinely monitor for E. coli.

Today, greater attention is being paid to viral and protozoan pathogens in water systems. Although E. coli is a good indicator for vegetative bacterial pathogens commonly found in treated drinking water, such as Salmonella species (Mitchell and Starzyk, 1975), it has proven to be a less effective indicator of viral and protozoan presence. In general, compared with protozoans and some viruses, E. coli and members of the coliform group do not survive as long in the environment (Edberg et al., 2000) and are more susceptible to many of the disinfectants commonly used in the drinking water industry. In contrast to the above, there is evidence that, although viruses may innately survive longer than bacteria in the environment, they may actually have a shorter life span. This is the result of their being smaller than bacteria and without defence mechanisms, so that they are more readily phagocytized by the other microflora. Consequently, some studies suggest that under certain conditions, bacterial indicators may still be good indicators for viral presence. For example, in a study by Craun et al. (1997) on groundwater consumption, it was found that the presence of coliforms correlated very well with the presence of viral gastroenteritis. An additional study conducted in southern Ontario, which looked at the quality of rural well water in terms of the presence of E. coli and any associated illness in the families, found that the occurrence of E. coli in the well was statistically associated with gastrointestinal illness in an individual, although the causative agents were not identified (Raina et al., 1999). Studies have also shown that bacterial indicators under specific conditions can be used to indicate protozoan pathogens. For example, one study of raw water quality showed that at high levels of thermotolerant coliforms (including E. coli), the probability of finding enteric protozoa and viruses was also very high (LeChevallier et al., 1991; Payment et al., 2000). In an additional study, an outbreak of waterborne giardiasis in a town in northern Ontario was characterized by the presence of abnormally high levels of cysts and thermotolerant coliforms in the raw water supply (Wallis et al., 1998).

As indicated above, indicators of faecal contamination, such as E. coli, are good indicators of pathogens that are present and transmitted through faeces. There are other waterborne illnesses that are the result of pathogens that are not transmitted by the faecal-oral route and therefore are not found through detection of faecal indicators. No indicators are currently known for such pathogens. Implementation of a multi-barrier approach will minimize their impact.

It should be emphasized that no bacteriological analysis can replace a complete knowledge of the quality of the water at the source of supply, during treatment, and throughout a distribution system. Contamination is often intermittent and may not be revealed by the examination of a single sample. A bacteriological water analysis shows only that at the time of examination, bacteria indicating faecal pollution did or did not grow under laboratory conditions from the sample of water tested. Therefore, if a sanitary inspection shows that an untreated supply is subject to faecal contamination or that treated water is subject to faecal contamination during storage or distribution or is inadequately treated, the water should be considered unsafe, irrespective of the results of bacteriological examination.