bacteriophage

Not every virus attacks animals or plants. The most abundant viruses on Earth attack bacteria, and they are called bacteriophages — literally 'bacteria-eaters.' They are everywhere bacteria are: a single drop of seawater can hold millions of them. If you could count all the bacteriophages on the planet, the number would dwarf the count of stars in the visible universe.

Many bacteriophages have a strange, almost mechanical appearance: an angular head holding the genetic material, a tube-like tail, and spidery leg fibers that grip the surface of a target bacterium. The phage lands on the bacterium, anchors itself, and injects its DNA through the cell wall like a syringe, leaving the empty shell outside. Once inside, the phage's genes take over, either bursting the cell open with new phages (the lytic cycle) or hiding quietly in the bacterium's chromosome (the lysogenic cycle). Phages are extremely picky, each kind usually infecting only one or a few species of bacteria.

Bacteriophages have been central to biology twice over. In the 1950s, experiments with phages helped prove that DNA — not protein — is the molecule of heredity, because researchers could watch which part of the phage actually entered the cell to direct the making of new phages. Today, as antibiotic resistance grows, doctors are revisiting 'phage therapy,' using these natural bacteria-killers as precision weapons against infections that drugs can no longer cure. Their pickiness is both a strength (they spare helpful bacteria) and a limitation (you must match the right phage to the right bug).