Total coliforms
(endorsed 2011)
Guideline
No guideline value has been set for total coliforms in drinking water. If used as an indicator, numbers should be established on a system-specific basis. Increased concentrations should be investigated.
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 able to ferment lactose with the production of acid within 48 hours at 35-37°C. Fermentation by these organisms begins with the cleavage of lactose into galactose and glucose by the enzyme ß-galactosidase. Coliforms are oxidase-negative. These characteristics are not taxonomic criteria, but practical working definitions used for water examination purposes.
Coliforms are a diverse group of bacteria including Escherichia coli and other thermotolerant coliforms (see also Fact Sheets on Escherichia coli and Thermotolerant Coliforms, under Microbial Indicators, and Pathogenic Escherichia coli, under Bacteria). Human and animal faeces contains large numbers of coliform bacteria, but there are many species that occur naturally in the environment. For this reason, coliforms can be present and grow in biofilms in drinking-water distribution systems. Coliforms that have been recovered from distribution systems include the non-thermotolerant genera Serratia, Hafnia and Pantoea as well as thermotolerant genera including Klebsiella and Enterobacter. Their presence in water, in the absence of E. coli, does not necessarily indicate faecal contamination.
Source and occurrence
Total coliform bacteria (excluding E. coli) occur in both sewage and natural waters. Some of these bacteria are excreted in the faeces of humans and animals, but many coliforms are heterotrophic and able to multiply in water and soil environments. Total coliforms can also survive and grow in water distribution systems, particularly in the presence of biofilms.
Method of identification and detection
Total coliforms can be quantified in water by a number of techniques. Membrane filtration (MF) can be used 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 4276.5. 2007, AS/NZS 4276.6. 2007). Specific secondary tests are used with both MF and MPN procedures to confirm the identification of coliform organisms.
Alternatively, the presence of coliform bacteria can be detected by testing for the production of the enzyme ß-galactosidase (AS 4276.21 2005). Enzyme substrate tests incorporate chromogenic substrates such as ortho-nitrophenyl-ß-D-galactopyranoside (ONPG) or chlorophenol red-ß-Dgalactopyranoside (CPRG). When the substrates are hydrolysed, a colour change is produced.
It has been reported that more coliform bacteria may be detected using enzyme substrate-based methodology than with MF-based methodology (Adcock and Saint 1997).
Indicator value and application in practice
Total coliforms (excluding E. coli) are not considered useful as indicators of the presence of faecal contamination and enteric pathogens, as there are many environmental coliforms that are not of faecal origin. The presence of these coliforms may represent release from pipe or sediment biofilms, and may be part of the normal flora of the drinking-water distribution system.
No guideline value has been set for total coliforms in drinking water. If used as an indicator, numbers should be established on a system-specific basis, taking into consideration relevant historical data and an understanding of the characteristics of the system. While coliforms can be used in operational monitoring to indicate inadequate treatment, breakdowns in system integrity, or the presence of biofilms, there are better indicators for these purposes. As a disinfection indicator, the test for total coliforms is far slower and less reliable than direct measurement of disinfectant residual. Heterotrophic plate count tests detect a wider range of microorganisms and are generally considered a better indicator of distribution system integrity and cleanliness.
NOTE: Important general information is contained in PART II, Chapter 5
References
Adcock PW, Saint C (1997). Trials of Colilert System. Water, 24(2):22-25.
AS/NZS 4276.5 (2007). Water Microbiology. Method 5 Coliforms – Membrane filtration method. Standards Australia, Sydney, NSW; Standards New Zealand.
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 4276.21 (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.
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