Bacteriophages

Bacteriophages, or phages, are viruses that infect bacteria. They are among the most abundant biological entities on Earth and inhabit soil, water, and the human microbiome. Phages shape bacterial communities, drive evolution, and can transfer genes among bacteria.

Structure and diversity: Most characterized phages have a protein capsid enclosing a genome of DNA or RNA. Many possess a tail that recognizes bacterial receptors and injects the genome. The best-studied forms are tailed phages with icosahedral heads.

Life cycles: Phages replicate by two general strategies. In the lytic cycle, infection leads to rapid replication, assembly, and host cell lysis. In the lysogenic cycle, the phage genome integrates into the host as a prophage and can remain dormant until induction.

Host range and defenses: Phages are usually highly specific for particular bacterial strains. Bacteria defend against infection with CRISPR-Cas systems, restriction-modification, and abortive infection. Phages can mediate horizontal gene transfer through transduction.

Applications: Phages are used to control bacterial populations in medicine, agriculture, and food safety. Phage therapy seeks alternatives to antibiotics; phage display enables the presentation of peptides or proteins on phage surfaces for research and development.

History and status: Phages were discovered in the 1910s by Frederick Twort and Félix d'Herelle. Western interest declined after the rise of antibiotics but has revived with increasing resistance, fueling ongoing research, regulatory discussions, and clinical trials.