A bacteriophage, commonly shortened to phage, is a virus that infects bacteria and reproduces using resources within a bacterial cell. Phages vary greatly in structure, genetic composition, and infection strategy. Some destroy their hosts when releasing newly formed particles, whereas others maintain long-term associations with them. They influence bacterial communities and serve as experimental systems for studying heredity, viral reproduction, and interactions between hosts and pathogens. (ncbi.nlm.nih.gov)
Discovery and scientific importance
Frederick Twort described a transmissible phenomenon affecting bacterial cultures in 1915. In 1917, Félix d’Hérelle independently reported an agent that produced clear patches in bacterial cultures and interpreted it as a virus parasitizing bacteria. He introduced the name bacteriophage, combining “bacteria” with a Greek word meaning “to eat.” The name describes the disappearance of bacteria during infection rather than literal ingestion. (journals.asm.org)
Phages became important tools in genetics and molecular biology. In the 1952 Hershey–Chase experiment, researchers separately labelled the DNA and protein of phage T2 with radioactive isotopes. During infection, DNA entered bacterial cells much more readily than the protein coat, supporting the identification of DNA as the phage’s hereditary material. Phage systems subsequently helped researchers investigate replication, recombination, and the regulation of viral development. (ncbi.nlm.nih.gov)
Structure and genetic diversity
A phage particle contains a genome enclosed by a protective protein shell, or capsid. Many familiar phages have a head containing double-stranded DNA and a tail involved in recognizing the host and delivering the genome. Tail architecture varies; not every tailed phage possesses a contractile sheath. Other phages are filamentous or lack tails altogether. (ncbi.nlm.nih.gov)
Phage genomes are not exclusively double-stranded DNA: bacterial viruses also include single-stranded DNA and RNA forms. For example, the International Committee on Taxonomy of Viruses records tailed double-stranded DNA viruses, small icosahedral single-stranded DNA viruses, and spherical single-stranded RNA viruses among bacterial hosts. “Bacteriophage” therefore describes a host relationship, not a single taxonomic family or uniform body plan. (elliot1.ictv.global)
Phages depend on host machinery, especially ribosomes, to manufacture viral proteins. Their genomes may nevertheless encode enzymes and regulatory factors that redirect cellular processes toward phage reproduction. They do not multiply through independent cell division; new particles are assembled from components produced during infection. (ncbi.nlm.nih.gov)
Infection and reproductive strategies
Infection commonly begins with attachment to specific structures on the bacterial surface. These interactions help determine a phage’s host range, which may encompass only particular strains within a bacterial species. Attachment alone does not guarantee productive infection, because intracellular bacterial defences can prevent subsequent development. (ncbi.nlm.nih.gov)
In a lytic cycle, the phage genome directs the production of viral components, which assemble into progeny particles. Release typically involves disruption of the cell membrane and cell wall. Phage-encoded enzymes called endolysins break down peptidoglycan, while other proteins help coordinate the rupture of the bacterial envelope. The infected bacterium is killed during this process. (ncbi.nlm.nih.gov)
Temperate phages can establish lysogeny, in which the viral genome persists as a prophage and is inherited by bacterial descendants. A prophage may integrate into a bacterial chromosome, as phage lambda does, or remain as an extrachromosomal element, as phage P1 does. Under suitable conditions, including certain forms of DNA damage, a prophage can be induced to resume productive development. Some filamentous phages instead release particles continuously without immediately lysing the host, so the lytic–lysogenic distinction does not encompass every infection strategy. (ncbi.nlm.nih.gov)
Genetic exchange and bacterial defences
Phages participate in horizontal gene transfer through transduction. In generalized transduction, bacterial DNA can be accidentally packaged into phage particles and delivered to another bacterium. Specialized transduction occurs when imprecise excision of an integrated prophage carries adjacent bacterial genes with it. These processes contribute to genetic exchange without reproduction between the donor and recipient cells. (ncbi.nlm.nih.gov)
Prophage genes can also change bacterial properties, a phenomenon called lysogenic conversion. Certain bacterial toxins, including diphtheria toxin and some Shiga toxins, are encoded by phage-associated genes. Thus, phages can affect bacterial disease-causing capacity as well as bacterial survival. (ncbi.nlm.nih.gov)
Bacteria resist phages through mechanisms that include altered surface receptors and destruction of incoming genetic material. CRISPR–Cas systems provide sequence-specific defence using stored fragments of previously encountered foreign DNA. A 2007 experiment demonstrated that acquisition of phage-derived CRISPR spacers could confer resistance to matching phages. Host resistance and phage countermeasures contribute to reciprocal evolutionary change. (nature.com)
Ecological roles
Phages shape bacterial abundance, community composition, and evolution in natural environments. Their effects depend on host susceptibility, environmental conditions, and infection strategy; they are not simply agents that uniformly suppress all bacteria. (nature.com)
In marine systems, cell lysis releases organic material that other microorganisms can reuse. This redistribution, known as the viral shunt, influences microbial food webs and the carbon cycle. Phage infection can therefore alter both the identities of organisms present and the movement of nutrients through an ecosystem. (nature.com)
Research and applications
Phage display presents peptides or proteins on a phage surface while retaining their encoding genes inside the particle. This physical connection allows researchers to select binding properties and recover the corresponding genetic information. The method is used in protein engineering and antibody development; its development was recognized by part of the 2018 Nobel Prize in Chemistry. (nobelprize.org)
Phage therapy uses bacteriophages against bacterial infections. Research examines their use alongside or as alternatives to antibiotics, including against bacteria exhibiting antibiotic resistance. Clinical outcomes cannot be reliably inferred from laboratory killing alone: host matching, bacterial adaptation, immune interactions, and preparation quality affect therapeutic performance. Production, scalability, and regulation remain active areas of development. (nature.com)