A virus that specifically infects bacteria is known as a bacteriophage, often shortened to “phage.” These microscopic entities are among the most abundant biological agents on Earth, found wherever bacteria exist. They play a fundamental role in microbial ecosystems by influencing bacterial populations and facilitating genetic exchange.
Understanding Bacteriophages
Bacteriophages are a diverse group of viruses that specifically target and replicate within bacterial cells. Their existence was independently discovered by Frederick Twort in Great Britain in 1915 and Félix d’Hérelle in France in 1917. D’Hérelle coined the term “bacteriophage,” which means “bacteria eater,” reflecting their ability to destroy bacterial hosts.
These viruses are remarkably ubiquitous, outnumbering all other organisms combined. They are found in vast numbers in various environments, including soil, water, and the human gut. Despite their abundance, bacteriophages are highly specific, typically infecting only a single bacterial species or particular strains within a species. This specificity is a defining characteristic that differentiates them from broader-spectrum antimicrobial agents.
Mechanisms of Infection
Bacteriophages initiate infection by interacting with a bacterial cell. Their basic structure includes a protein head (capsid) encasing their genetic material, and often a tail structure. This genetic material can be DNA or RNA, single or double-stranded.
Infection begins with attachment, where proteins on the phage’s tail fibers bind to receptor sites on the bacterial cell surface. These receptors can include components like lipopolysaccharides, teichoic acids, or proteins on the bacterial cell wall. Following attachment, the phage penetrates the bacterial cell wall. Many tailed phages inject their genetic material directly into the bacterial cytoplasm, leaving the protein capsid outside.
Lytic and Lysogenic Cycles
Once a bacteriophage has injected its genetic material into a bacterial cell, it typically follows one of two primary life cycles: the lytic cycle or the lysogenic cycle. The lytic cycle, characteristic of virulent phages, results in the rapid replication of the phage and the destruction of the host cell. In this cycle, the phage takes over the bacterial machinery to produce numerous new phage particles. The host cell’s DNA is degraded, and its resources are redirected to synthesize viral components. Ultimately, enzymes produced by the phage cause the bacterial cell to burst, or lyse, releasing hundreds of new phages to infect other bacteria.
In contrast, the lysogenic cycle is a more temperate pathway, primarily used by temperate phages. Here, the phage’s DNA integrates into the bacterial chromosome, forming what is known as a prophage. In this dormant state, the prophage replicates along with the bacterial DNA during cell division, without immediately harming the host. The bacterial cell continues to function normally, carrying the viral genes. However, certain environmental stressors, such as UV light or specific chemicals, can induce the prophage to excise from the bacterial chromosome and enter the lytic cycle, leading to the destruction of the host cell and release of new phages.
Therapeutic and Research Applications
Bacteriophages have practical applications beyond their natural role in bacterial population control. One application is phage therapy, which uses phages to treat bacterial infections. This approach has gained renewed interest, especially with increasing antibiotic resistance, as phages can specifically target and destroy resistant bacterial strains while leaving beneficial bacteria unharmed. Phage therapy has been explored and used in various contexts, including treating skin infections and septicemia.
Beyond therapy, bacteriophages serve as valuable tools in molecular biology and genetic engineering. Their ability to transfer genetic material between bacteria, known as transduction, is harnessed for genetic manipulation. Researchers also utilize phages in technologies like CRISPR-Cas systems, which originated as a bacterial defense against phage infection, and in phage display for protein engineering and drug discovery. Phages are also investigated for food safety, acting as biocontrol agents to reduce harmful bacteria in food products and processing environments without affecting food quality.