</>X174 and its close relative S13 are members of the family Microviridae, genus Microvirus. They were found to be among the smallest DNA bacteriophages in the early days of phage research (1920-1950) but they were not particularly popular as subjects of study because they were less easy to work with than the T phages and not as intriguing as temperate phages such as lambda. However, the physicochemical studies of Robert Sinsheimer and the genetic studies of Irwin Tessman convinced many members of a generation of phage workers to concentrate their efforts on these organisms (see Denhardt et al in the Further Reading list for a review of this period). A major defining characteristic of these phages is their possession of a single molecule of single-stranded circular DNA as their sole genetic material. Other members of this group, less closely related to 4>X, include a3, St-1 and G4. They all grow on various strains/species of Enterobacteriaceae, typically Escherichia coli, Salmonella and Shigella species, and are found wherever these bacteria are found. E. coli C is the best host for many isolates, though phage able to grow on E. coli B (e.g. a3) and E. coli K12 (e.g. St-1) are also known.
The reviews by Hayashi et al and Baas and Jansz in the Further Reading List contain details and references that of necessity are omitted here. Most of the specific information presented below concerns (f>X 174 ('phi-ex'), and is assumed to apply to all members of this group unless indicated otherwise.
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