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Life is a battlefield for your cell, and other cells are not the only enemy. Attack can come from another front: viruses called phage (short for bacteriophage, or “bacterium eaters” because they were first identified infecting and killing bacteria). Like all viruses, phage use cells as vehicles for their replication. And, just like eukaryotic viruses, there is an army of different phage that prey on bacteria and archaea. No species is known to be immune.
The basic strategy of a phage is to inject its genome into a host cell, use the cell’s machinery to replicate the genome and manufacture the proteins it encodes, then use these materials to build the next generation of phage. Since the phage is dependent on the cell’s machinery for replication, it does not fulfill the criteria for a living organism. (Despite this distinction, we still talk about the viral “lifecycle.”) By cleverly outsourcing, phage can be highly streamlined structurally, little more than a genome in an envelope en route to its next destination. All phage have a genome (either double or single-stranded DNA or RNA), tightly packed inside a protective envelope of either protein (forming a “capsid”) or lipid, or both. The genome-containing shell is called the head of the phage. Many phage also have a tail, like this myophage attacking Shewanella oneidensis. Tail fibers at the tip recognize features on their target host and allow the phage to dock.