Total Coliforms

4.0 Significance of total coliforms in drinking water

4.1 Description

Total coliforms belong within the family Enterobacteriaceae and have been defined in the 20th edition of Standard Methods for the Examination of Water and Wastewater (APHA et al., 1998) as follows:

These definitions are not to be regarded as identical; rather, they refer to three groups that are roughly equivalent. All three groups contain various species of the genera Escherichia, Klebsiella, Enterobacter, Citrobacter, Serratia, and many others (Leclerc et al., 2001). Some members of these groups are naturally occurring in the environment and are of faecal origin, while others are found exclusively in the environment (Table 1).

Although not included in the coliform group, members of the genus Aeromonas can ferment lactose and possess β-galactosidase; therefore, they can yield false-positive total coliform reactions. Aeromonas species are ubiquitous in the environment, having been found in lakes, rivers, marine waters, sewage effluents, and drinking waters, among other places (Allen et al., 1983; Nakano et al., 1990; Poffe and Op de Beeck, 1991; Payment et al., 1993; Ashbolt et al., 1995; Bernagozzi et al., 1995; Chauret et al., 2001; El-Taweel and Shaban, 2001). False positives resulting from the presence of Aeromonas can be excluded using the cytochrome oxidase test.

Table 1: Selected coliform bacteria in the family Enterobacteriaceae^a

	ONPGb	Faecal origin	Non-faecal origin
Budvicia	+	-	+
Citrobacter	+	+	+
Enterobacter	+	+	+
Erwinia	+	-	+
Escherichia	+	+	-
Klebsiella	+	+	+
Leclercia	+	-	+
Serratia	+	-	+

^a Adapted from Leclerc et al. (2001).
^b ortho-Nitrophenyl-β-D-galactopyranoside.

A subset of the total coliform group, known as the thermotolerant coliforms (previously referred to as faecal coliforms), has been used as a surrogate for E. coli in water quality testing. Thermotolerant coliforms were considered more faecal specific than total coliforms, and, given that E. coli testing was difficult, thermotolerant coliform detection was used routinely. Thermotolerant coliforms are distinguished from total coliforms by their ability to tolerate elevated incubation temperatures during culturing. By definition, thermotolerant coliforms include the portion of the total coliform group capable of forming gas within 24 hours at 44.5°C or that produce a blue colony on m-FC broth within 24 hours at 44.5°C (APHA et al., 1998). This group includes members of the genera Escherichia, Klebsiella, Enterobacter, and Citrobacter. Recent advances in E. coli detection methods have made the need for thermotolerant coliform testing in drinking water quality management redundant.

4.2 Sources

As mentioned previously, the total coliform group is composed of various genera with similar characteristics. The natural niches for members of this group range from being faecal specific, such as E. coli, to being widely distributed in the water, soil, and vegetation (Leclerc et al., 2001; Rompré et al., 2002). Many total coliforms are not specific to any one source and are present in both faecal and non-faecal environments. Comparison of total coliforms within a specific environment has shown that some members of the coliform group can consistently be found in higher concentrations in that source. For example, analysis of the coliform complement of faecal matter found Klebsiella, Citrobacter, and Enterobacter present in small numbers compared with E. coli (Edberg et al., 2000). In contrast, the majority of thermotolerant coliforms isolated from a distribution system were found to be Klebsiella (Edberg et al., 2000).

The presence of total coliforms in a distribution system, as opposed to the natural environment, results from inadequately treated source water, allowing total coliforms to pass through the treatment system into the distribution system; subsequent regrowth; or intrusion of the organisms into the water post-treatment. A study done by Kirmeyer et al. (1999) showed that coliforms could be detected surrounding distribution system pipelines; therefore, post-treatment contamination could result from numerous problems, such as pipe leaks with negative pressure events, pipe breaks, inadequate cleaning and disinfection after repairs, and cross-connections, including backflow, with non-potable water. In addition, surges in water mains from activities such as hydrant tests and fire-fighting may result in the sloughing of biofilm and a subsequent rise in total coliform bacterial counts.

After the introduction of total coliforms into the distribution system, their survival and possible growth depend on many factors, including (but not limited to) water temperature, retention time of the water, type and concentration of disinfectant (if present), presence of nutrients, specifically the assimilable organic carbon and the biodegradable dissolved organic carbon concentrations, pipe material characteristics, and presence of sediments. Not all members of the coliform group persist in water for the same length of time under identical conditions (APHA et al., 1998). E. coli, for example, is generally the most sensitive to environmental stresses and does not usually grow outside the human or animal gut (Geldreich, 1996). Klebsiella, Citrobacter, and Enterobacter, on the other hand, are more likely to persist in the environment and, under favourable conditions, can multiply in water. For example, in water distribution systems, Klebsiella was able to survive and even grow in drinking water biofilms on the interior surface of water mains and in storage tanks (LeChevallier et al., 1987; LeChevallier and McFeters, 1990; Edberg et al., 1994). Total coliform presence in biofilms may result in resistance to disinfection and other eradication measures (Martin et al., 1982; Geldreich and Rice, 1987).