Escherichia coli

(endorsed 2011)

Guideline

Escherichia coli should not be detected in any 100 mL sample of drinking water. If detected in drinking water, immediate action should be taken including investigation of potential sources of faecal contamination.

General description

Coliforms are Gram-negative, non-spore-forming, rod-shaped bacteria that are capable of aerobic and facultative anaerobic growth in the presence of bile salts or other surface active agents with similar growth-inhibiting properties. They are found in large numbers in the faeces of humans and other warm-blooded animals, but many species also occur in the environment.

Thermotolerant coliforms are a sub-group of coliforms that are able to grow at 44.5 ± 0.2°C. E. coli is the most common thermotolerant coliform present in faeces and is regarded as the most specific indicator of recent faecal contamination because generally it is not capable of growth in the environment. In contrast, some other thermotolerant coliforms (including strains of Klebsiella, Citrobacter and Enterobacter) are able to grow in the environment and their presence is not necessarily related to faecal contamination. While tests for thermotolerant coliforms can be simpler than for E. coli, E. coli is considered a superior indicator for detecting faecal contamination.

Thermotolerant coliforms, including E. coli, can ferment lactose (or mannitol) at 44.5 ± 0.2°C with the production of acid within 24 hours. Thermotolerant coliforms that produce indole from tryptophan at 44.5 ± 0.2°C are regarded as being E. coli. E. coli also gives a positive result in the methyl-red test and a negative Voges-Proskauer test, and it cannot use citrate as the sole source of carbon. Most E. coli produce the enzyme ß-glucuronidase.

Source and occurrence

E. coli is a normal inhabitant of the intestine, generally present in high numbers in human and animal faeces, and it generally does not grow in natural waters, although there have been reports that it can multiply in tropical waters (Fujioka et al. 1999) and three atypical strains were reported as being able to grow in two Australian lakes (Power et al. 2005). Two of the three atypical strains were ß-glucuronidase negative. While most E. coli are non-pathogenic, there are some pathogenic subtypes that can cause enteric illness, including enteropathogenic, enteroinvasive, enterotoxigenic and enterohaemorrhagic strains (Bopp 1999). These are described in the fact sheet on Pathogenic Escherichia coli.

Method of identification and detection

The presence of E. coli in water samples can be determined using a number of methods. A common method involves membrane filtration (MF) for concentration of the organisms from water, followed by growth in enrichment/selective media or multiple tube dilution (most probable number – MPN) procedures (AS/NZS 4276.6 2007, AS/NZS 4276.7 2007). Specific secondary tests are used with both MF and MPN procedures to confirm the identification of E. coli.

Alternatively, E. coli can be detected by testing for the production of the enzyme ß-glucuronidase (AS 4276.21 2005). Test media include enzyme substrates such as 4-methylumbelliferyl-ß-D-glucuronide (MUG) which is hydrolysed by ß-glucuronidase to produce the fluorogenic metabolite 4-methylumbelliferyl. Both enumeration and presence/absence tests are available from a number of commercial suppliers.

Indicator value and application in practice

E. coli is used as a specific indicator of recent faecal contamination. It can be used to assess:

• source water quality and potential impacts of human and animal waste;

• inadequate treatment;

• post-treatment ingress of human and animal waste into distribution systems;

• the effectiveness of risk management plans in assuring delivery of safe drinking water at consumers’ taps.

E. coli is not an effective indicator for the presence of enteric protozoa or viruses.

E. coli should not be present in any 100 mL sample of drinking water. Risk management plans should incorporate corrective actions in the event of the detection of E. coli in drinking water. The presence of these organisms can indicate faecal contamination of the water supply, and if they are detected in drinking water, the cause should always be investigated. Possible causes include inadequate treatment or ingress of contamination. Investigation will generally require further testing.

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NOTE: Important general information is contained in PART II, Chapter 5

References

AS/NZS 4276.6 (2007). Water Microbiology. Method 6: Coliforms, Escherichia coli and thermotolerant coliforms – Determination of most probable number (MPN). Standards Australia, Sydney, NSW; Standards New Zealand.

AS 4276.7 (2007). Water Microbiology. Method 7: Escherichia coli and thermotolerant coliforms – Membrane filtration method. Standards Australia, Sydney, NSW; Standards New Zealand.

AS 4276.21 (2005). Water Microbiology. Method 21. Examination for coliforms and Escherichia coli-Determination of most probable number (MPN) using enzyme hydrolysable substrates. Standards Association of Australia, Sydney, NSW.

Bopp CA, Brenner FW, Wells JG, Strockbine NA (1999). Escherichia, Shigella and Salmonella. In: Murray PR, Baron EJ, Pfaller MA, Tenover FC, Volken RH (eds), Manual of Clinical Microbiology, 7th edition. ASM Press, Washington DC, pp 459-474.

Fujioka R, Sian-Denton C, Borja M, Castro J, Morphew K (1999). Soil: the environmental source of Escherichia coli and Enterococci in Guam’s streams. Journal of Applied Microbiology Symposium Supplement, 85:83S-89S.

Power ML, Littlefield-Wyer J, Gordon DM, Veal DA and Slade MB (2005) Phenotypic and genotypic characterization of encapsulated Escherichia coli isolated from blooms in two Australian lakes. Environmental Microbiology 7(5), 631-640.