Bacteriophage

A bacteriophage, or phage, is a virus that infects bacteria. Phages are probably the most abundant biological entities on Earth, with an estimated 10³¹ particles, and they shape bacterial populations in the oceans, soils and the human gut.

A typical tailed phage such as T4 has a protein head holding its genome, a tail and tail fibers that recognize receptors on the surface of a specific bacterium. The phage attaches, injects its nucleic acid through the cell wall and takes over the cell's machinery. The genome can be DNA or RNA, and each phage usually infects only a narrow range of hosts.

In the lytic cycle the phage makes many copies and bursts the cell open, releasing the new particles. In the lysogenic cycle, as in phage lambda, the genome joins the host's chromosome as a prophage and is copied with it until a stress signal triggers release. Phages can also move bacterial genes between cells, a process called transduction that contributes to horizontal gene transfer.

Experiments on phages, such as the 1952 Hershey–Chase experiment, showed that DNA is the genetic material, and phage work helped establish molecular biology. Bacteria's defenses against phages gave rise to CRISPR-Cas tools. Phage therapy, which uses phages to treat infections, is being revisited as antimicrobial resistance grows.

• Virus — the broader group
• CRISPR-Cas Systems — a bacterial defense against phages
• Antimicrobial Resistance — the problem phage therapy may help address