Just like any other bacteriophages, coliphages have a narrow range of possible hosts. Some coliphages can infect coliform bacteria other than E. coli (for example Shigella or Klebsiella). The specificity is affected by the way of entering the cell. As we explained when defining bacteriophages, we differentiate between somatic (entering the cell through the cell wall) and F-specific bacteriophages (entering the cell using the sexual pilli), and this can be applied as well to coliphages. As for the morphology of coliphages, every group previously described (Myoviridae, Syphoviridae, Podoviridae, Microviridae, Inoviridae and Leviviridae) is represented among them.

Two groups of bacteriophages infecting E. coli, somatic and F-specific coliphages, have been investigated in academia for many years as both fecal and viral indicators.

The high concentrations of coliphages found in raw wastewater and in many other matrixes contaminated with fecal remains; the easy, fast and cost effective detection and enumeration methods; and the persistence in the water environment and the resistance to treatments that resemble those of viruses, make indicator bacteriophages good surrogate indicator candidates for various set-ups.

For practical purposes, coliphages are organized in operational groups rather than in taxons, mostly because of the current methods for their detection and quantification. Somatic, F-specific and the subset F-specific RNA coliphages have been extensively studied for this purpose. Humans and animals excrete both groups.

Coliphages have the properties required for an indicator of fecal and/or viral pollution: they are present in high numbers in sources of fecal pollution; they are more resistant to natural and anthropogenic stressors than bacterial indicators and as resistant as viruses; they are present in water environments whenever fecal pollution exists; they are specific for fecal or sewage pollution; their chances of multiplying in water environments is, if any, extremely low; they are not pathogenic and detectable by simple, rapid and inexpensive methods.

Additionally, they present some attractive features for working with them:

Somatic coliphages are those infecting E. coli through the cell wall. Among them we find bacteriophages from the families Myoviridae, Syphoviridae, Podoviridae, Microviridae. There is consensus about using the host strain (CN13, WG5) to detect phages by standardized methods; other host strains might detect different numbers. Their strength relies both on the concentration found in sources of fecal pollution and method simplicity.

F-specific coliphages, also named sexual coliphages, are those infecting E. coli through the sexual pili and comprise phages of the families Inoviridae and Leviviridae. E. coli (HS) and Salmonella (WG49) strains have been designed to detect mostly F-specific bacteriophages. Their strength is due to their morphology, similar to that of many animal viruses. (Kuprovic et al, 2016)

Historically, the F-specific phages strategy was developed since it was the common belief that somatic coliphages were replicating in the environment, and since pili are not synthesized below 32ºC, F-specific were less prone to replicate outside the gut. However, nowadays there are many arguments to believe that the replication of somatic coliphages or F-specific coliphages outside the gut is very unlikely.

F-specific RNA phages can be further differentiated by counting plaques on detection media with and without RNAse. Since the process of infection of Leviviridae is sensitive to RNAse in the detection medium, the difference between the counts gives the numbers of F-specific RNA bacteriophages (link ISO 10705-1)

The sum of somatic and F-specific coliphages constitutes the group total coliphages. Host strains (for example CB390) counting total coliphages have been developed (Guzman et al., 2008).

3.2 Coliphages in guidelines and regulations

Furthermore, regulatory authorities in different areas of the world are beginning to consider coliphages as indicators for water quality criteria. Examples of ruling guidelines that include bacteriophages are displayed in table 3.1

3.3 Phages infecting Bacteroides as markers of the source of fecal pollution

Bacteroides is one of the most abundant bacterial genera in the human gut. It is a strict anaerobe. This limits even more its possibilities to reproduce outside the gut as it passes to aerobic environments. The most abundant phages infecting bacteroides are tailed (at least the ones described so far), and most of them are classified among the Siphoviridae. They are somatic bacteriophages and their host range is quite narrow. They are pretty abundant in fecally-contaminated water. Different strains of bacteroides detect bacteriophages present in the gut of different animals, which is useful for detecting whether the source of contamination is human or animal. The main drawback of this system is that there are doubts about their geographical continuity, so possible hosts have to be isolated in different geographical areas. Bacteriophages infecting bacteroides are useful for microbial source tracking (Jofre et al., 2014).

References

Ashbolt, N.J., Grabow, O.K. and Snozzi, M. (2001). Indicators of microbial water quality. In Water Quality: Guidelines, Standards and Health, ed. Fewtrell, L. and Bartram, J. pp. 289–315. London, UK: IWA Publishing.

Guzmán, C, Mocé-Llivina, L., Lucena, F. and J. Jofre (2008). Evaluation of the Escherichia coli host strain (CB390) for simultaneous detection of somatic and F-specific coliphages. Applied and Environmental Microbiology 74: 531-534    

IAWPRC, Study Group on Health Related Water Microbiology (1991). Bacteriophages as model viruses in water quality control. Water Res 25, 529–545.

Jofre, J., A.R. Blanch, F. Lucena and M. Muniesa. (2014). Bacteriophages infecting Bacteroides as markers for microbial source tracking. Water Research 55: 1-11.

Jofre, J., F. Lucena, A. R. Blanch and M. Muniesa. (2016). Review. Coliphages as Model Organisms in the Characterization and Management of Water Resources. Water 8, 199; doi:10.3390/w8050199.

Krupovic, M., Dutilh, B.E., Adriaenssens, E.M. et al. Taxonomy of prokaryotic viruses: update from the ICTV bacterial and archaeal viruses subcommittee. Arch Virol (2016) 161: 1095. https://doi.org/10.1007/s00705-015-2728-0.

McMinn, B.R., Ashbolt, N.J. and Korajkic, A. (2017). Bacteriophages as indicators of faecal pollution and enteric virus removal. Lett ApplMicrobiol 65, 11-26.