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Intérêt du génotypage des phages ARN F-spécifiques pour estimer la pollution fécale et virale des eaux

Abstract : F-specific RNA phages, which are non pathogenic viruses with similar size and structure to human enteric viruses, have been proposed like faecal pollution indicators, like models for pathogenic viruses in environment and like tools for microbial source tracking. The key trait on which the F-specific RNA phage approach of source tracking is based is that genogroups I and IV are predominantly isolated from non human faeces, while genogroups II and III are predominantly isolated from human faeces and sewage. Paradoxically, few data are available as for the genogroups distribution in environmental waters. So, the topic of this study was to provide additional information about the relationships between the genogroups of F-specific RNA phages and the level of faecal and viral pollution to environmental waters. First, the methodological work undertaken at the beginning of this project made it possible to develop the first real-time RT-PCR assays able to typing the F-specific RNA phages. The major advantages of this approach are a good sensitivity, a quick detection and a great specificity. Compared with the current tools, this method allows to avoid the phage cultivation and thus to play down the biases associated with the survival characteristics of infectious F-specific RNA phages in the environmental waters. Indeed, survival studies, realized in urban wastewater and also in ground water, have shown that the inactivation rates of infectious particles are always more important that this of viral RNA, whatever the experimental design was. Secondly, the analysis of urban wastewater samples enabled to check that F-specific RNA phages could give interesting information as for the characterization of faecal pollution in spite of the change of reference frame of measurement inherent in our detection method (genome versus infectious particle). In these kinds of samples, the majority of phages isolated belonged to the genogroups II and III, and they exhibited steady concentrations. In several particular samples, the high concentration of genogroup I phages has been associated with rainfall events and with the presence of zoonotic pathogens (Cryptosporidium and Giardia). This observation suggests the presence of an animal pollution after streaming phenomena. All the results obtained with urban wastewaters strengthen the use of F-specific RNA phages like reliable source identification tool. On the other hand, the comparison between the pathogenic virus concentrations and the results of genotyping has shown that phage genogroups are not relevant indicators for the presence of enteric viruses. For instance, norovirus and enterovirus concentrations in wastewater displayed a seasonal distribution while human genogroups exhibited steady concentrations over the time. Thirdly, two particular case studies devoted to natural waters constitute the major aspect of our work. In river water principally influenced by human wastes, genotyping results show that genogroup II is very largely isolated. For the first time, positive correlations between the concentrations of genogroup II phages, bacterial indicators (E. coli, enterococci) and human adenoviruses was observed, which attests the human faecal origin of this genogroup. Genogroup I was also often isolated but it appeared irregularly distributed. The correlation analysis has shown that genogroup I was linked neither with the concentration of genogroup II nor with that of bacterial or viral faecal indicators. The absence of a link between the concentrations of these two genogroups supports the assumption of another faecal origin. Conversely, a relationship was shown between genogroup I and the water turbidity observed at the sampling. This suggests that the origin of this genogroup could be related to streaming phenomena following precipitations. Thus, in river water the genogroup I and II would be the two most interesting genogroups in order to characterize faecal pollution. As a consequence, genogroup II/genogroup I ratio may be an interesting tool for faecal source tracking. Indeed, depending on the sign of the ratio, it seems possible to determine the main source of pollution at a given point. For example, for an E. coli concentration of 3.6log10 MPN/100mL, log-ratio values could as well be 3.8 as -1.7. With the water turbidity, this log-ratio was the only parameter enabled to highlight a change of faecal pollution nature. In ground waters protected from faecal pollution, genome of F-specific RNA phages was not observed while genome of adenoviruses was isolated in 7 samples on the 60 analyzed. This observation suggests that more persistent markers than RNA of phages could be detected in ground waters. More over, in persistence study, no degradation of adenoviral DNA was observed during all the time (200 days) of the experiment. Finally, the typing method newly developed during this study led to a better knowledge of the distribution of different the genogroups within environmental waters. F-specific RNA phage typing provides original information compared to the bacterial indicators, but does not constitute alone the universal indicator of faecal or viral pollution of waters.
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Leslie Ogorzaly. Intérêt du génotypage des phages ARN F-spécifiques pour estimer la pollution fécale et virale des eaux. Microbiologie et Parasitologie. Université Henri Poincaré - Nancy 1, 2009. Français. ⟨NNT : 2009NAN10025⟩. ⟨tel-01748485⟩

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