Bacteriophage

Bacteriophage
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Name	Bacteriophage
Caption	Transmission electron micrograph of multiple bacteriophages attached to a bacterial cell wall
Taxon	Various families across Caudoviricetes, Leviviricetes, and others

Bacteriophage. A bacteriophage, often simply called a phage, is a virus that infects and replicates within bacteria and archaea. These entities are among the most abundant biological agents on Earth, found in every environment where their host organisms exist, from soil and seawater to the human gut microbiome. The study of phages, known as phage biology, has been fundamental to the development of molecular biology and continues to offer promising avenues in therapeutic medicine and biotechnology.

Structure and classification

Bacteriophages exhibit a remarkable diversity of morphology and genetic material. The most commonly studied group are the tailed phages of the class Caudoviricetes, which possess an icosahedral capsid attached to a helical tail structure used for host recognition and genome injection. Other major morphological families include the filamentous phages of the family Inoviridae and the lipid-containing phages of the family Corticoviridae. Classification is primarily based on virion morphology, nucleic acid type (double-stranded DNA, single-stranded DNA, or RNA), and genome organization, as formalized by the International Committee on Taxonomy of Viruses. Landmark studies by researchers like Michael Rossmann and Robert W. Horne have elucidated many intricate structural details using techniques such as X-ray crystallography and cryo-electron microscopy.

Life cycle

The phage life cycle typically begins with the specific adsorption of the virion to receptor molecules on the bacterial surface, such as lipopolysaccharide or porin proteins. Following attachment, the viral genome is translocated into the host cytoplasm, often through the phage tail. Phages then generally follow one of two pathways: the lytic cycle or the lysogenic cycle. In the lytic cycle, exemplified by the classic T4 phage, the host's transcription and translation machinery are hijacked to produce new viral components, leading to cell lysis and release of progeny. In the lysogenic cycle, typified by lambda phage, the viral DNA integrates into the host chromosome as a prophage, replicating passively with the host until induced to enter the lytic cycle by environmental stressors like UV radiation or mitomycin C.

Ecology and evolution

Bacteriophages are key drivers of microbial ecology and evolutionary biology, profoundly influencing bacterial population dynamics and biogeochemical cycles in environments like the Sargasso Sea and the human gut. Through processes like predation and horizontal gene transfer (via transduction), they exert immense selective pressure, shaping bacterial virulence, antibiotic resistance, and metabolic diversity. The constant evolutionary arms race between phages and their hosts, described by models like the Red Queen hypothesis, leads to rapid co-evolution of defense systems such as CRISPR-Cas in bacteria and corresponding anti-CRISPR proteins in phages. Research by Forest Rohwer and Matthew B. Sullivan has highlighted their global ecological impact.

Applications in medicine and biotechnology

The therapeutic use of phages, known as phage therapy, was pioneered by Félix d'Herelle and has seen renewed interest as an alternative to antibiotics for treating multidrug-resistant infections caused by pathogens like Pseudomonas aeruginosa and Staphylococcus aureus. Companies like AmpliPhi Biosciences and the Eliava Institute are active in this field. In biotechnology, phages are indispensable tools; M13 phage is used in phage display technology for antibody development, a contribution recognized by the Nobel Prize in Chemistry awarded to George P. Smith and Gregory Winter. Furthermore, phage-derived enzymes, or endolysins, are being developed as novel antimicrobial agents.

History of discovery and research

Bacteriophages were independently discovered by Frederick Twort in 1915 and Félix d'Herelle in 1917. D'Herelle coined the term and pioneered early therapeutic applications. The Phage Group, centered around Max Delbrück, Alfred Hershey, and Salvador Luria in the mid-20th century, used phages as model systems to establish the foundations of molecular genetics; their work on the Hershey–Chase experiment and bacterial conjugation earned them the Nobel Prize in Physiology or Medicine in 1969. Subsequent research by Sydney Brenner, François Jacob, and Jacques Monod on lambda phage was instrumental in elucidating the operon model of gene regulation. Modern research continues at institutions like the Howard Hughes Medical Institute and the J. Craig Venter Institute.

Category:Viruses Category:Microbiology