Health Stream Literature Summary - Issue 50 - June 2008

E. coli as a public health indicator of drinking water quality.
Standridge, J. (2008) Journal / American Water Works Association, 100(2); 65-75.

E. coli is one of the major bacterial inhabitants of the gut of healthy humans and other warm-blooded animals, and has long been used as an indicator of faecal material. Only certain variants of E. coli are capable of causing disease, the most significant of these is E. coli 0157 which has been responsible for several waterborne disease outbreaks. A dilemma for the public health community is the fact that E. coli has both pathogenic and harmless variants. Routine testing of water for E. coli is not designed nor intended to detect the pathogenic strains. Therefore the presence of E. coli detected as the result of regulatory testing indicates faecal contamination but not imminent E. coli disease. Following such detections it is often incorrectly believed that the pathogenic strain has been detected and therefore action is required to deal with disease threats.

Most of the literature overwhelmingly supports the use of E. coli as the indicator of choice for protecting drinking water. This support for E. coli may in part be attributed to the inadequacies of total coliforms as ideal indicators. There has been international acceptance of E. coli as a drinking water indicator organism with Australia, the World Health Organization and the European Union all having undertaken comprehensive reviews in the past five years of the status of microbial drinking water indicators and concluding that total coliform testing should be abandoned and only E. coli should be used.

The US Department of Agriculture (USDA) and the US Food and Drug Administration (USFDA) have both promulgated rules that identify E. coli as the chosen indicator of faecal contamination. The US Environmental Protection Agency (USEPA) has conflicting statements in its rules on contamination testing. There are sections strongly promoting the value of E. coli testing along with sections strongly discounting its value. A negotiated rule-making process to amend the USEPA Total Coliform Rule (TCR) is now underway and one of the important debates will be whether to replace total coliform testing with E. coli testing for monitoring of public water systems.

One of the goals in choosing indicator organism for drinking water monitoring is that the indicator predicts not only the presence of faecal contamination but also the presence of other waterborne disease organisms. One of the reasons to continue total coliform testing is that it may indicate other nonfaecal microorganisms of potential health concern. As most nonfaecal pathogens found in water distribution systems come from biofilms and as coliforms can be associated with biofilms, it has been suggested that the presence of total coliforms may be a good indicatory of nonfaecal pathogens. This premise requires careful consideration however. Coliforms may be present in a biofilm community however they are not often the predominant organism. Biofilms may have few or no coliforms associated with them and therefore reliance on total coliform testing to detect biofilm occurrence would be risky in terms of public health and create a false sense of security. Specific testing for free-living pathogens of concern may be more appropriate.

E. coli generally fulfils the criteria for an ideal indicator for drinking water monitoring but it is not without flaws. If E. coli is used as the main indicator of drinking water quality then a commitment to educate the public to allay any concerns over the confusion of the indicator function and the rare occurrence of pathogenic strains is also needed.

Comment The issues discussed in this paper highlight the difficulty in trying to define any single microorganism as an “ideal” all-purpose indicator organism for drinking water. E.coli is an excellent indicator of recent faecal contamination in undisinfected water. Its presence in (supposedly) disinfected tap water indicates either disinfection failure or recontamination, however its absence does not guarantee absence of non-bacterial faecal pathogens (viruses and protozoa) nor absence of non-faecal pathogens.