Rates of inactivation of waterborne coliphages by monochloramine

Implications of free Shiga toxin-converting bacteriophages occurring outside bacteria for the evolution and the detection of Shiga toxin-producing Escherichia coli

In this review we highlight recent work that has increased our understanding of the distribution of Shiga toxin-converting phages that can be detected as free phage particles, independently of Shiga toxin-producing bacteria (STEC). Stx phages are a quite diverse group of temperate phages that can be found in their prophage state inserted within the STEC chromosome, but can also be found as phages released from the cell after activation of their lytic cycle. They have been detected in extraintestinal environments such as water polluted with feces from humans or animals, food samples or even in stool samples of healthy individuals. The high persistence of phages to several inactivation conditions makes them suitable candidates for the successful mobilization of stx genes, possibly resulting in the genes reaching a new bacterial genomic background by means of transduction, where ultimately they may be expressed, leading to Stx production. Besides the obvious fact that Stx phages circulating between bacteria can be, and probably are, involved in the emergence of new STEC strains, we review here other possible ways in which free Stx phages could interfere with the detection of STEC in a given sample by current laboratory methods and how to avoid such interference.

Persistence of infectious Shiga toxin-encoding bacteriophages after disinfection treatments

In Shiga toxin-producing Escherichia coli (STEC), induction of Shiga toxin-encoding bacteriophages (Stx phages) causes the release of free phages that can later be found in the environment. The ability of Stx phages to survive different inactivation conditions determines their prevalence in the environment, the risk of stx transduction, and the generation of new STEC strains. We evaluated the infectivity and genomes of two Stx phages (Φ534 and Φ557) under different conditions. Infectious Stx phages were stable at 4, 22, and 37°C and at pH 7 and 9 after 1 month of storage but were completely inactivated at pH 3. Infective Stx phages decreased moderately when treated with UV (2.2-log10 reduction for an estimated UV dose of 178.2 mJ/cm(2)) or after treatment at 60 and 68°C for 60 min (2.2- and 2.5-log10 reductions, respectively) and were highly inactivated (3 log10) by 10 ppm of chlorine in 1 min. Assays in a mesocosm showed lower inactivation of all microorganisms in winter than in summer. The number of Stx phage genomes did not decrease significantly in most cases, and STEC inactivation was higher than phage inactivation under all conditions. Moreover, Stx phages retained the ability to lysogenize E. coli after some of the treatments.

Stability and infectivity of cytolethal distending toxin type V gene-carrying bacteriophages in a water mesocosm and under different inactivation conditions

Two cytolethal distending toxin (Cdt) type V-encoding bacteriophages (Φ62 and Φ125) were induced spontaneously from their wild-type Escherichia coli strains and from the lysogens generated in Shigella sonnei. The stability of Cdt phages was determined at various temperatures and pH values after 1 month of storage by means of infectivity tests using a plaque blot assay and analysis of phage genomes using real-time quantitative PCR (qPCR): both were highly stable. We assessed the inactivation of Cdt phages by thermal treatment, chlorination, UV radiation, and in a mesocosm in both summer and winter. The results for the two Cdt phages showed similar trends and were also similar to the phage SOM23 used for reference, but they showed a much higher persistence than Cdt-producing E. coli. Cdt phages showed maximal inactivation after 1 h at 70°C, 30 min of UV radiation, and 30 min of contact with a 10-ppm chlorine treatment. Inactivation in a mesocosm was higher in summer than in winter, probably because of solar radiation. The treatments reduced the number of infectious phages but did not have a significant effect on the Cdt phage particles detected by qPCR. Cdt phages were quantified by qPCR in 73% of river samples, and these results suggest that Cdt phages are a genetic vehicle and the natural reservoir for cdt in the environment.

Usefulness of different groups of bacteriophages as model micro-organisms for evaluating chlorination

AIMS

To assess the usefulness of bacterial and viral indicators in chlorination processes and to collect quantitative information necessary for risk assessment analysis in water disinfection processes based on chlorination.

METHODS AND RESULTS

Naturally occurring bacterial indicators, bacteriophages and enteroviruses were determined to evaluate the effect of chlorination in groundwater and secondary sewage effluents. Additionally, the effect of chlorinating on selected bacteriophages, enteroviruses and Escherichia coli was also tested in spiked samples of bottled water and sewage effluents. Results indicate that chlorination inactivates more efficiently bacteria than phages and enteroviruses. Among the human viruses, phages infecting Bacteroides fragilis and selected somatic coliphages belonging to the Siphoviridae family were the most persistent to chlorination.

CONCLUSIONS

The three groups of bacteriophages studied were all more resistant to chlorination than bacteria and some of the phages were more resistant than enteroviruses. Results presented here indicate that it is very risky to generalize from information obtained with inactivation experiments done with single isolates of any phage or virus. If possible, inactivation studies should be done with naturally occurring populations. Phages offer a good opportunity for studying naturally occurring populations. Thus, the bacteriophages offer a range of resistance to chlorination that may represent most of the viruses that can be found in water.

SIGNIFICANCE AND IMPACT OF THE STUDY

Data reported in this study support the inclusion of bacteriophages as additional indicators of the efficiency of water chlorination processes and water quality.

Removal and inactivation of indicator bacteriophages in fresh waters

AIMS

The removal and inactivation of faecal coliform (FC) bacteria, enterococci (ENT), sulphite-reducing clostridia (SRC), somatic coliphages, F-specific RNA bacteriophages and bacteriophages infecting Bacteroides fragilis in fresh waters.

METHODS AND RESULTS

Removal was studied in two areas of a river. The results showed different removal of each group of microbes. Faecal coliform bacteria were removed faster than any other, whereas SRC and bacteriophages infecting Bact. fragilis were the most persistent. Inactivation was measured by 'in situ' experiments, which showed significant differences in survival of the different groups of bacterial and bacteriophage indicators. The SRC and bacteriophages were more resistant than faecal coliforms and enterococci, with the exception of F-specific RNA bacteriophages in the summer. Inactivation experiments with pure cultures of bacteriophages confirmed that phage B40-8 of Bact. fragilis was the most resistant.

CONCLUSIONS

Bacteria and bacteriophages show different resistance to natural inactivation. The use of phages allows information to be obtained in addition to that provided by bacterial indicators. Somatic coliphages and phages infecting Bact. fragilis might supply that indicator function.

SIGNIFICANCE AND IMPACT OF THE STUDY

Confirmation was obtained that bacteriophages provided additional information to that provided by bacterial indicators to monitor the natural inactivation of viruses and/or pathogens.

Study of the potential relationship between the morphology of infectious somatic coliphages and their persistence in the environment

The proportions of different morphological types of infectious somatic coliphages were determined in faecally polluted freshwaters. Myoviridae, followed by Siphoviridae, were the most frequently isolated morphological types in raw sewage, treated sewage and river water collected a few metres downstream from a sewage outfall. However, in river water collected further downstream from the pollution point, in river water after 'in situ' inactivation experiments and in chlorinated raw and treated sewage significant changes in the proportions of the different somatic coliphage morphological types occurred. In all cases, Siphoviridae, especially those with flexible and curled tails, became more abundant to the detriment of Myoviridae.