Bacteriophage
A bacteriophage is a virus that infects bacteria. Phages shape microbial ecosystems, move genes between bacterial cells, and are studied as tools for genetics, biotechnology, and possible treatment of some bacterial infections.
What a bacteriophage is
A bacteriophage, often called a phage, is a virus whose host is a bacterium. Like other viruses, it carries genetic material inside a protein shell and depends on host-cell machinery to make more virus particles. A phage is not a tiny bacterium; it is a virus specialized for bacterial hosts.
How phages find hosts
Phages usually infect only particular bacterial species or strains. They attach to specific structures on the bacterial surface, such as receptors on the cell wall, membrane, capsule, or appendages. That host range is one reason phages can be highly selective, but it also means a phage that works against one bacterium may not work against another.
The lytic cycle
In a lytic cycle, a phage injects its genome, directs the host cell to make viral parts, assembles new phage particles, and then breaks open the cell to release them. This cycle can rapidly reduce a susceptible bacterial population, though bacteria can evolve defenses and resistant variants.
The lysogenic cycle
Temperate phages can enter a lysogenic state, in which phage DNA is maintained inside the bacterium, often integrated into the bacterial chromosome as a prophage. The prophage may remain quiet for many cell divisions before switching into a productive cycle under certain conditions.
Gene movement and transduction
Phages can contribute to horizontal gene transfer through transduction. In some cases, bacterial DNA is packaged or carried during the phage life cycle and then delivered to another cell. This can spread genes through bacterial populations, including genes that affect metabolism, virulence, or resistance.
Bacterial defenses
Bacteria are not passive targets. They can block phage attachment, cut foreign DNA with restriction enzymes, use CRISPR-based immune systems, or trigger other defenses. Phages also evolve countermeasures, so phage-bacteria relationships are often a continuing evolutionary contest.
Phage therapy
Phage therapy uses bacteriophages to attack bacterial infections. It is being studied as one possible response to antibiotic-resistant bacteria, but it is not a universal replacement for antibiotics. Clinical use depends on matching phages to the pathogen, checking safety, and understanding how bacteria and immune systems respond.
Why it matters
Bacteriophages are abundant drivers of microbial ecology and useful tools in molecular biology. They help explain how bacteria evolve, how genes move through microbial communities, and why targeted biological approaches can be both powerful and complicated.