Review of Coliforms - National Health and Medical Research Council

MICROBIAL INDICATORS OF DRINKING WATER QUALITY
6.3 ESCHERICHIA COLI AND ENTEROCOCCI – KEY FAECAL INDICATORS
It is widely acknowledged that the major threat to public health from drinking water is from
microbial contamination with human, and to a lesser degree, animal faeces. One detailed risk
assessment of pathogens and chemicals in drinking water concluded (Regli et al., 1993):
“risk of death from known pathogens in untreated water is 100 to 1000 times greater
than risk of cancer from known disinfection by-products in chlorinated drinking water
and
the risk of illness from pathogens in untreated surface water is 10 000 to 1 000 000 times
greater than risk of cancer from disinfection by-products in chlorinated drinking water”
As a component of the assessment of public health risk through monitoring of water quality
at consumer’s taps, E. coli is regarded as the most sensitive indicator of faecal pollution. The
large numbers of E. coli present in the gut of humans and other warm-blooded animals and
the fact that they are not generally present in other environments support their continued use
as the most sensitive indicator of faecal pollution available (Edberg et al., 2000).
To increase the confidence of water quality results, especially when monitoring for faecal
pollution, analysis for enterococci has been used (eg. EU guidelines, Section 3, Box 3). The
enterococci are the group of bacteria most often suggested as alternatives to coliforms, and
interest in their use as a water quality indicator date back to 1900 when they were found to be
common commensal bacteria in the gut of warm-blooded animals (Gleeson and Gray, 1997).
The enterococci were included in the functional group of bacteria known as “faecal
streptococci” and now largely belong in the genus Enterococcus which was formed by
the splitting of Streptococcus faecalis and Streptococcus faecium, along with less important
streptococci, from the genus Streptococcus (Schleifer and Klipper-Balz, 1984). There are
now 19 species that are included as enterococci (Topley, 1997). The predominant intestinal
enterococci are Enterococcus faecalis, E. faecium, E. durans and E. hirae. In addition, other
Enterococcus species and some species of Streptococcus (namely S. bovis, and S. equinus)
may occasionally be detected. Generally, for water examination purposes enterococci can
be regarded as indicators of faecal pollution, although some can occasionally originate
from other habitats.
Enterococci have a number of advantages as indicators over total coliforms and even E. coli,
including that they generally do not grow in the environment (WHO, 1993) and they have
been shown to survive longer (McFeters et al., 1974). Despite being approximately an order
of magnitude less numerous than faecal coliforms and E. coli in human faeces (Feacham et
al., 1983), they are still numerous enough to be detected after significant dilution. Rapid and
simple methods, based on defined substrate technology, are available for the detection and
enumeration of enterococci and routinely employed in many laboratories (see Appendix A
for description of methods).
There is some concern that enterococci are a diverse group of bacteria, and that the group
contains species that are environmental and their presence in water is not necessarily
indicative of faecal pollution. This concern is driven by the problems associated with the use
of total coliforms as an indicator of faecal pollution. An early research report showed that
Enterococcus faecalis var liquefaciens was common in good quality water and its relevance
was not considered clear if recovered in waters in concentrations of less than 100 organisms/
100 mL (Geldreich, 1970). More recent research on the relevance of faecal streptococci as
indicators of pollution, showed that the majority of enterococci (84%) isolated from a variety
of polluted water sources were “true faecal species” (Pinto et al., 1999).
23