Bacteriophage

Bacteriophages were discovered independently by Frederick Twort in 1915 and Félix d'Herelle in 1917. d'Herelle coined the term "bacteriophage" (from Greek phagein, "to eat") and proposed their therapeutic use against bacterial infections. Early phage therapy was widely used in the 1920s–1930s, but declined in the West after the advent of antibiotics, while continuing in the Soviet Union and Eastern Europe.

Phages are classified by morphology and nucleic acid type. Most known phages have a head (icosahedral capsid) and a tail, but others are filamentous or pleomorphic. The International Committee on Taxonomy of Viruses (ICTV) places phages in the order Caudovirales (tailed phages) and several other families.² They can be double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), or RNA viruses.

Lytic phages (e.g., T4) immediately hijack the host machinery to produce progeny and lyse the cell. Temperate phages (e.g., lambda) can integrate their genome into the host chromosome as a prophage, replicating passively with the host (lysogeny).³ Environmental triggers can induce the lytic cycle. This dual lifestyle influences bacterial evolution through horizontal gene transfer.

Phages are major drivers of bacterial mortality and nutrient cycling in oceans and soils. They mediate horizontal gene transfer via transduction, contributing to the spread of antibiotic resistance genes and virulence factors. Phage predation selects for bacterial resistance mechanisms, fueling an evolutionary arms race.

Phage therapy is being revisited as an alternative to antibiotics, especially for multidrug-resistant infections. Phages are also used in diagnostics (phage typing), as biocontrol agents in food safety, and as tools in genetic engineering (e.g., phage display).⁴ The CRISPR-Cas system, derived from bacterial phage defense, has revolutionized genome editing.

Beyond the well-known T4 and lambda, phages include the giant phages (e.g., Bacillus phage G) with genomes over 500 kb. Some phages produce enzymes called endolysins that degrade bacterial cell walls, now explored as "enzybiotics."⁵ Phages can also infect archaea, and some have unusual morphologies like bottle-shaped or star-shaped. The first phage genome sequenced was φX174 in 1977, a milestone in genomics.

Lytic cycle

A phage replication cycle that ends with host cell lysis and release of progeny phages.

Lysogenic cycle

A phage replication cycle where the viral genome integrates into the host chromosome and replicates passively.

Prophage

The integrated phage genome in a lysogenic host.

Transduction

The transfer of bacterial DNA from one cell to another via a phage.

Phage therapy

The use of bacteriophages to treat bacterial infections.