The isolation of viruses infecting Archaea

A mere 50 viruses of Archaea have been reported to date; these have been investigated mostly by adaptingmethods used to isolate bacteriophages to the unique growth conditions of their archaeal hosts. The most numerousare viruses of thermophilic Archaea. These viruses have been discovered by screening enrichment culturesand novel isolates from environmental samples for their ability to form halos of growth inhibition, or by usingelectron microscopy to screen enrichment cultures for virus-like particles. Direct isolation without enrichmenthas not yet been successful for viruses of extreme thermophiles. On the other hand, most viruses of extremehalophiles, the second most numerous archaeal viruses, have been isolated directly from hypersaline environments.Detailed methods for the isolation of viruses of extremely thermoacidophilic Archaea and extremelyhalophilic Archaea are presented in this manuscript. These methods have been extremely effective in isolatingnovel viruses. However, Archaea comprise much more than extreme thermoacidophiles and extreme halophiles.Therefore a vast pool of archaeal viruses remain to be discovered, isolated, and characterized, particularly amongthe methanogens and marine Archaea. Some suggestions for expansion of the described methods are discussed.We hope these suggestions will provide an impetus for future work on these and other Archaeal viruses.