Bacteriophages: update on application as models for viruses in water

Abstract

Phages are valuable models or surrogates for enteric viruses because they share many fundamental properties and features. Amon g these are structure, composition, morphology, size and site of replication. Even though they use different host cells, colipha ges and Bacteroides fragilis phages predominantly replicate in the gastro-intestinal tract of humans and warm-blooded animals where enteric viruses also replicate. A major advantage of phages is that, compared to viruses, they are detectable by simple, inexp ensive and rapid techniques. In view of these features, phages are particularly useful as models to assess the behaviour and survival of enteric viruses in the environment, and as surrogates to assess the resistance of human viruses to water treatment and disinfec tion processes. Since there is no direct correlation between numbers of phages and viruses, phages cannot to a meaningful extent be used to indicate numbers of viruses in polluted water. The presence of phages typically associated with human and animal excre ta indicates the potential presence of enteric viruses. However, the absence of these phages from water environments is generally a meaningful indication of the absence of enteric viruses. This is because phages such as somatic coliphages, F-RNA coliphages a nd B. fragilis phages generally outnumber enteric viruses in water environments, and they are at least as resistant to unfavourable conditions including those in water treatment and disinfection processes. However, using highly sensitive molecular techniques viruses have been detected in drinking water supplies which yielded negative results in conventional tests for phages. Initial ly, data on phages were rather confusing because a wide variety of techniques was used. However, techniques for the detection of phages are being standardised internationally. This applies in particular to somatic and F-RNA coliphages, and B. fragilis phages, which are most commonly used in water quality assessment. Reliable and practical techniques now available include direct quantitativ e plaque assays on samples of water up to 100 ml, and qualitative tests on 500 m l or more using highly sensitive enrichment procedures.