A phage is a virus that infects bacteria or archaea. Short for bacteriophage, these viruses attach to a specific host cell, inject their genetic material and hijack the host’s biosynthetic machinery to produce new viral particles. Phages occur in diverse forms and are ubiquitous in every environment where microbes are found.
Biology and Classification
Most phages consist of a nucleic acid genome encapsulated in a proteinaceous head and, in many cases, a tail structure used for attachment and genome delivery. Their genomes can be single-stranded or double-stranded DNA or RNA and range in size from a few thousand to hundreds of thousands of bases. Tailed phages in the order Caudovirales are traditionally grouped into families such as Myoviridae with contractile tails, Siphoviridae with long non‑contractile tails and Podoviridae with short tails. Phages initiate infection by recognising receptors on the host cell surface and injecting their genome. Lytic phages replicate rapidly, lyse the host and release progeny, whereas temperate phages can integrate into the host genome as prophages and remain quiescent during a lysogenic cycle. Prophages influence host physiology and contribute to horizontal gene transfer, virulence and antibiotic resistance. The ecological abundance of phages in soil, seawater, the gut and other habitats makes them major drivers of microbial evolution and nutrient cycling.
Roles and Applications
Well‑studied examples include enterobacteria phage T4, which infects Escherichia coli and follows a strictly lytic life cycle, and lambda phage, a temperate phage of E. coli that integrates into the bacterial chromosome. Filamentous phages such as M13 replicate without killing their host and are widely used as cloning vectors and for phage display to study protein interactions and to develop antibodies. In medicine, phage therapy exploits naturally occurring viruses to treat bacterial infections, offering an alternative to antibiotics against drug‑resistant pathogens. Food industries employ phages to reduce contamination by Listeria or Salmonella on meat and produce. Conversely, phage contamination can disrupt dairy fermentations by infecting starter cultures. Environmental studies use phage diversity as indicators of microbial community structure and as tools for tracking fecal pollution.
Phages represent a vast and dynamic component of the microbial world. By infecting and modulating bacterial and archaeal populations, they shape ecosystems and influence the evolution of their hosts. Their specificity and genetic diversity make them valuable tools in biotechnology, medicine and ecological research.
Related Terms: Bacteriophage, Virus, Lytic cycle, Lysogenic cycle, Phage therapy