Intestinal enterococci
(endorsed 2011)
Guideline
Intestinal enterococci should not be present in drinking water. If used as an indicator and detected in drinking water, immediate action should be taken, including investigation of potential sources of faecal contamination.
General description
Intestinal enterococci are a functional group of organisms from the Enterococcus and Streptococcus genera that are excreted in human and animal waste. Species include Enterococcus faecalis, E. faecium, E. durans, E. hirae, E. cecorum, E. columbae, E. avium and E. gallinarum together with Streptococcus bovis and S. equinus (ISO 1998, Ashbolt et al. 2001, WHO 2003).
Use of the terms intestinal enterococci, faecal streptococci, enterococci and streptococci has been a source of some confusion. This has been exacerbated by revisions of taxonomy. The older taxonomy described faecal streptococci as a subgroup of the genus Streptococcus. The group including S. faecalis, S. faecium, S. bovis, S. equinus, S. avium, and S. gallinarum all possess the Lancefield group D antigen. The enterococci S. faecalis, S. faecium, S .avium and S. gallinarum represented a smaller subgroup of the faecal streptococci, which was differentiated by an ability to grow in 6.5% sodium chloride, at pH 9.6 and at both 10°C and 45°C (APHA et al. 2005). The nomenclature of this subgroup has been changed and they are now identified as Enterococcus spp.; E. faecalis, E. faecium, E. avium and E. gallinarum (LeClerc et al. 1996). These species are principal members of the intestinal enterococci, together with other enterococci of faecal origin.
The similar membership of the various groups means that tests for intestinal enterococci, faecal streptococci, enterococci and streptococci often provide the same results.
Source and occurrence
Intestinal enterococci are excreted in the faeces of humans and other warm-blooded animals, including livestock, domestic animals and birds (Ashbolt et al. 2001). Most species do not grow in water but the standard test for intestinal enterococci can detect environmental species such as E. casseliflavus and E. mundtii (ISO 1998).
Intestinal enterococci are present in large numbers in sewage and can be present in water environments polluted by sewage or wastes from humans and animals.
Method of identification and detection
Intestinal enterococci can be detected by a liquid- or agar-based culture method requiring standard microbiology laboratory facilities.
A standardised test for intestinal enterococci in water has been described by the International Organization for Standardization (ISO 1998). The test is based on the capability of growing at 44°C and of hydrolysing 4-methylumbelliferyl- -D-glucoside (MUD) in the presence of thallium acetate, naladixic acid and 2,3,5-triphenyltetrazolium chloride (TTC). The test is performed in a liquid medium. It detects enterococci of faecal origin and may occasionally detect strains of S. bovis and S. equinius. It also detects non-faecal species such as E. casselifalvus and E. mundtii.
A membrane filtration method is also available (AS/NZS 4276.9, 2007) using initial isolation on m-Enterococcus agar (36 ± 2°C for 44 ± 4 hrs) followed by confirmation on Bile Aesulin Azide Agar (44 ± 0.5°C for 2 hrs).
Indicator value and application in practice
Numbers of intestinal enterococci in sewage are generally about an order of magnitude lower than those of E. coli. An important advantage of this group is that they tend to survive longer in water environments than E. coli (Ashbolt et al. 2001, WHO 2004), and they have been used as an index of faecal pathogens that survive longer in water than E. coli. While they are more resistant to disinfection than E. coli, they are readily removed by disinfectants used to treat drinking water.
It has been suggested that intestinal enterococci (faecal streptococci) to E. coli ratios could be used to indicate whether faecal waste is of human or animal origin, but this is confounded by a number of factors including different rates of die-off for the various species in water environments, and it is not considered reliable (Sinton and Donnison 1994, Ashbolt et al. 2001).
The presence of intestinal enterococci provides evidence of recent faecal contamination. They can be used to assess:
source water quality and potential impacts of human and animal waste;
the adequacy of treatment;
whether there is post-treatment ingress of human and animal waste into distribution systems; and
the effectiveness of risk management plans in assuring delivery of safe drinking water at consumer taps (verification).
Although the intestinal enterococci as a group survive longer than E. coli they are not effective indicators for the presence of enteric protozoa or viruses.
The detection of intestinal enterococci in drinking water should always lead to investigation of the cause, which could include inadequate treatment or ingress of contamination. Investigation will generally require further testing.
NOTE: Important general information is contained in PART II, Chapter 5
References
APHA, AWWA, WEF (American Public Health Association, American Water Works Association, Water Environment Federation (2005). Standard methods for the examination of water and wastewater, 21st edition. American Public Health Association, Washington DC.
AS/NZS 4276.9 (2007). Water Microbiology Method 9: Enterococci – membrane filtration method (ISO 7899-2:2000, MOD). Standards Australia, Standards New Zealand.
Ashbolt NJ, Grabow WOK, Snozzi M (2001). Indicators of microbial water quality. In: Fewtrell L, Bartram J, eds. Water quality: Guidelines, standards and health — Assessment of risk and risk management for water-related infectious disease. WHO Water Series. London, IWA Publishing, pp. 289-315.
ISO 7899-1:1998 (1998). Water Quality – Detection and Enumeration of Intestinal Enterococci in Surface and Wastewater – Part 1 Minituarized method (most probable number) by inoculation in liquid medium. International Organization for Standardization, Geneva, Switzerland.
LeClerc H, Devries LA, Mossel DAA (1996). Taxonomical changes in intestinal (faecal) enterococci and streptococci: consequences on their use as indicators of faecal contamination in drinking water. Journal of Applied Bacteriology, 81:459-466.
Sinton LW, Donnison AM (1994). Characterisation of faecal streptococci from some New Zealand effluents and receiving waters. New Zealand Journal of Marine and Freshwater Research, 28:145-158.
WHO (World Health Organization) (2003). Guidelines for Safe Recreational Water Environments. Volume 1 Coastal and fresh waters. WHO, Geneva, Switzerland.
WHO (World Health Organization) (2004). Guidelines for Drinking-Water Quality. WHO, Geneva, Switzerland.
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