Bacteriophages

Bacteriophages are viruses that infect bacteria, the most abundant biological entities on Earth, and they have served as foundational tools for molecular biology while shaping microbial communities.

Definition

Bacteriophages, or phages, are viruses that specifically infect and replicate within bacteria, exhibiting lytic and, in some cases, lysogenic life cycles.

Scope

This topic covers phage structure and diversity; the lytic cycle, in which the phage replicates and bursts the host cell; the lysogenic cycle, in which the phage genome is maintained as a prophage; the regulatory switch between these states; transduction as a route of gene transfer; and the ecological and applied roles of phages, including phage therapy. It treats phages both as model systems and as ecological agents.

Core questions

• How do phages infect and replicate within bacteria?
• What distinguishes the lytic from the lysogenic cycle?
• How does a temperate phage decide between lysis and lysogeny?
• What roles do phages play in microbial ecology and biotechnology?

Key concepts

• Phage structure and diversity
• The lytic cycle
• The lysogenic cycle and prophage
• The lysis-lysogeny decision
• Transduction and phage ecology

Mechanisms

A phage attaches to specific surface receptors and injects its genome into the host. In the lytic cycle the phage redirects the cell to make new phage particles, which are released by lysis. Temperate phages may instead enter the lysogenic cycle, integrating their genome as a prophage that is replicated with the host until conditions trigger a switch to the lytic pathway. During packaging, phages can occasionally carry host DNA, mediating transduction.

Clinical relevance

Phages were central to early molecular biology, including the demonstration that DNA carries genetic information, and they remain important as agents of gene transfer, as shapers of microbial communities and global nutrient cycles, and as potential therapeutic tools against bacterial infections.

History

Bacteriophages were discovered independently by Frederick Twort and Félix d'Hérelle in the 1910s. From the 1940s the phage group led by Max Delbrück and Salvador Luria used these viruses as model systems, producing key insights into mutation, genetics, and the molecular basis of heredity.

Key figures

• Félix d'Hérelle
• Frederick Twort
• Max Delbrück
• Salvador Luria

Seminal works

• madigan2018
• willey2020

Frequently asked questions

What is the difference between a lytic and a lysogenic phage?

A lytic phage replicates immediately and destroys the host cell to release new particles. A temperate phage can instead enter a lysogenic state, integrating its genome into the host and replicating quietly until conditions trigger a switch to the lytic cycle.