Monitoring the emergence and re-emergence of viral diseases with the goal of containing the spread of viral agents requires both adequate preparedness and quick response. Identifying the causative agent of a new epidemic is one of the most important steps for effective response to disease outbreaks. Traditionally, virus discovery required propagation of the virus in cell culture, a proven technique responsible for the identification of the vast majority of viruses known to date. However, many viruses cannot be easily propagated in cell culture, thus limiting our knowledge of viruses. Viral metagenomic analyses of environmental samples suggest that the field of virology has explored less than 1% of the extant viral diversity. In the last decade, the culture-independent and sequence-independent metagenomic approach has permitted the discovery of many viruses in a wide range of samples. Phylogenetically, some of these viruses are distantly related to previously discovered viruses. In addition, 60-99% of the sequences generated in different viral metagenomic studies are not homologous to known viruses. In this review, we discuss the advances in the area of viral metagenomics during the last decade and their relevance to virus discovery, clinical microbiology and public health. We discuss the potential of metagenomics for characterization of the normal viral population in a healthy community and identification of viruses that could pose a threat to humans through zoonosis. In addition, we propose a new model of the Koch's postulates named the 'Metagenomic Koch's Postulates'. Unlike the original Koch's postulates and the Molecular Koch's postulates as formulated by Falkow, the metagenomic Koch's postulates focus on the identification of metagenomic traits in disease cases. The metagenomic traits that can be traced after healthy individuals have been exposed to the source of the suspected pathogen.
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