Thermotolerant coliforms
(endorsed 2011)
Guideline
Thermotolerant coliforms 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.
Thermotolerant coliforms, including E. coli, can ferment lactose (or mannitol) at 44.5 ± 0.2°C with the production of acid within 24 hours.
Source and occurrence
Thermotolerant coliforms are normal inhabitants of the intestine, generally present in high numbers in human and animal faeces;. However, environmental thermotolerant coliforms, can occur in natural waters. These organisms are of lesser significance.
Method of identification and detection
The presence of thermotolerant coliforms 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 thermotolerant coliforms.
Indicator value and application in practice
Thermotolerant coliforms can be used as an indicator of faecal contamination but they are not as specific as E. coli, which is the preferred indicator. The group includes types that can grow in the environment and be present in the absence of faecal contamination.
The presence of thermotolerant coliforms may provide evidence of recent faecal contamination. Thermotolerant coliforms 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.
Thermotolerant coliforms are not an effective indicator for the presence of enteric protozoa or viruses.
Thermotolerant coliforms 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 thermotolerant coliforms in drinking water. The presence of these organisms may 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.
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.
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