Coliphages

(endorsed 2011)

Guideline

Coliphages should not be detected in any 100 mL sample of 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

Bacteriophages (also known as phages) are viruses that exclusively use bacteria as their hosts for replication. Coliphages use Escherichia coli and closely related coliform bacteria as hosts and can be released by these bacterial hosts into the faeces of humans and other warm-blooded animals.

Coliphages used in drinking-water quality assessment are divided into two major groups: somatic coliphages and F-RNA coliphages. Differences between the two groups include the route of attachment and infection of bacterial cells.

Somatic coliphages initiate infection by attaching to receptors permanently located on the cell wall of host bacteria. The somatic coliphages replicate more frequently in the gastrointestinal tract of warm-blooded animals and could potentially replicate more readily in water environments than F-RNA coliphages. Somatic coliphages have DNA genomes that can be single or double-stranded. They comprise a wide range of phage families (Myoviridae, Siphoviridae, Podoviridae and Microviridae) with a spectrum of morphological types (Grabow 2001).

F-RNA coliphages initiate infection by attaching to fertility (F-) fimbriae on E. coli hosts. These F-fimbriae are produced only by bacteria carrying the fertility (F-) plasmid. Since F-fimbriae are produced only in the logarithmic growth phase at warmer temperatures, typically above 30°C, F-RNA coliphages are unlikely to replicate in environments other than the gastrointestinal tract of warm-blooded animals. F-RNA coliphages comprise a restricted group of closely related phages, which belong to the family Leviviridae and consist of a single-stranded RNA genome and an icosahedral capsid that is morphologically similar to that of picornaviruses (Grabow 2001).

Source and occurrence

Both somatic coliphages and F-RNA coliphages are routinely found in sewage. Somatic coliphages are found more commonly than F-RNA coliphages in the gastrointestinal tracts of humans and are typically found at higher numbers in sewage. This can be advantage in source water assessment. On the other hand, because F-RNA coliphages are unlikely to grow in the environment, they can be used as specific indicators of faecal contamination.

Method of identification and detection

There are standard methods for detection of somatic and F-RNA coliphages (ISO 1995, 2000) using plaque assays. The assays are based on growing lawns of specific host bacteria on agar-based media. Plaques represent holes within the lawn where coliphage infection has lysed the bacterial cells.

Somatic coliphages are detectable by relatively simple and inexpensive plaque assays, which yield results within 24 hours. Plaque assays for F-RNA coliphages are not quite as simple, as the culture of host bacteria has to be in the logarithmic growth phase at a temperature above 30°C to ensure that F-fimbriae are present.

Indicator value and application

Phages share many properties with human viruses, notably composition, morphology, structure and mode of replication. As a result, coliphages are useful models or surrogates to assess the behaviour of enteric viruses in water environments. In this regard, they are superior to faecal bacteria. However, there is no direct correlation between numbers of coliphages and numbers of enteric viruses. Coliphages are sensitive to disinfectants such as chlorine (Grabow 2001, WHO 2004).

The presence of coliphages is indicative of the likely presence of faecal contamination, and they are more robust than bacterial indicators. Coliphages can be used in source water quality assessment to help detect the presence of possible viral contamination. They are particularly suitable for groundwater assessment, where the larger bacterial indicators might not be found due to natural filtration and adsorption processes. Coliphages are less useful for assessing surface waters, where concentrations tend to be low.

Coliphages are widely used in validation of treatment processes. The physical similarities between coliphages and viruses are particularly useful in validating efficacy of filtration (e.g., USEPA 2005) and they can also be used to validate disinfection processes (e.g., USEPA 2006).

Although testing is more costly than for bacterial indicators, coliphages can be valuable components of verification monitoring. The presence of coliphages in drinking water indicates shortcomings in treatment or in the protection of distribution systems, and detection should always lead to further investigations.

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

References

Grabow WOK (2001). Bacteriophages: Update on application as models for viruses in water. Water SA, 27:251-268.

ISO 10705-1 (1995). Water quality – Detection and enumeration of bacteriophages – Part 1 Enumeration of F-specific RNA bacteriophages. International Organization for Standardization, Geneva, Switzerland.

ISO 10705-2 (2000). Water quality – Detection and enumeration of bacteriophages – Part 2 Enumeration of somatic coliphages. International Organization for Standardization, Geneva, Switzerland.

USEPA (United States Environmental Protection Agency) (2005). Membrane Filtration Guidance Manual. EPA 815-R-06-009, USEPA, Cincinatti, USA.

USEPA (United States Environmental Protection Agency) (2006). Ultraviolet Disinfection Guidance Manual. EPA 815-D-03-007, USEPA, Cincinatti, USA.

WHO (World Health Organization) (2004). Guidelines for Drinking-water Quality, Section 11.6.6, Coliphages. World Health Organization, Geneva, Switzerland.