Bacteriophages “bacteria-eater” are infectious agents that replicate as obligate intracellular parasites in bacteria with high selectivity. They are the powerful regulators of bacterial populations in natural ecosystems and are found in the soils, plants, rivers, and also as the human microbiome.
Phages colonize all body niches, including the skin, oral cavity lungs, gut, and urinary tract. Although considered sterile, metagenomic analysis has shown that blood contains bacterial virus of family Myoviridae, Podoviridae, Siphoviridae, Microviridae, and Inoviridae families.
Phages were first observed in 1915 by Frederic Twort in England and in 1917 by Felix d’Herelle in France. Felix d’Herelle named them Bacteriophages. In the 1920-1930s, phages were used to treat bacterial infections in Europe and Soviet Unions but after the discovery of penicillin use of bacteriophage as antimicrobial agents decreased. Due to the rise in antimicrobial resistance, there is renewed interest in phage therapy.
Use of phages to treat bacterial or animal infections is called phage therapy.
Structure of Bacteriophage
Bacteriophage structures are diverse, but most of them share some common characteristics. For example, bacteriophage T4 of Escherichia coli has an icosahedral head structure made of repeat protein sub-units known as the capsid. This head structure contains a linear double-stranded viral genome.
Phage genome varies in size from approximately 2 to 200 kilobases per strand of nucleic acid. Considerable variability is found in the nucleic acid of phages, and it may be ds DNA, ds RNA, ss DNA, ss RNA. Most known bacteriophages contain dsDNA genomes.
The head of bacteriophage T4 is attached to a helical tail through a collar (neck). Tails contain a series of tail fibers and tail pins at the end. These specialized syringe-like structures bind to receptors on the cell surface. All bacteriophages do not contain a ‘tail’ structure.
Phages are classified into two major groups on the basis of their mode of replication.
- Virulent (lytic phage): Virulent phage replicate in the susceptible bacteria producing many copies of themselves and destroys the host cells in the process by lysis. e.g. T-even; T2 and T4 phages of E. coli.
- Temperate phages: Infection by temperate phages can have either of two outcomes.
- Lytic growth or
- Lysogeny (non-lytic prophage state)
Replication of Lytic Phages
Bacteriophage adsorption is the first step to start the infection process. The binding proteins of the bacteriophage mostly located on the tail fibers interact and recognize specific receptors present on the bacterial cell wall.
The attachment of phage to its host cell results in morphological changes to both phage and bacterium facilitating the penetration of the phage into a host bacterium.
Lysozyme like enzyme found in the phage tails weakens the bacterial cell wall. Tail sheath contracts, hollow tube (core) penetrate the weakened cell wall and come in contact with the cell membrane.
Viral DNA moves from the head via the tube to the bacterial cytoplasm while the phage capsid remains outside.
Phage genes take control of the host cell’s metabolic machinery and directs host cells to produce only viral products. Bacterial DNA is disrupted and their nucleotides are used as building blocks for new phage.
Phage DNA is transcribed to mRNA using the host cell’s machinery. Translation of mRNA and capsid proteins and viral enzymes are produced.
Head of T4 Phages is assembled in the host cells cytoplasm from newly synthesized capsid proteins. A viral dsDNA molecule is packed into each head.
Phage tails are assembled from newly formed base plates, sheaths, and collars.
When the head is properly packed with DNA, each head is attached to a tail.
Tail fibers are added and fully formed mature, infective phages develop.
Lysis and Release
Lysozyme breaks down bacterial cell wall and bacterial host cells is lysed. Released phage infects other susceptible bacteria and the new infection process is started.
Burst time: Time from adsorption to cell lysis, generally 20-40 minutes.
Burst size (viral yield): Number of new virions released from each bacterial host cells. In T4 phage burst size is 50-200.
Replication of Temperate Phages
Temperate phages mostly exhibit lysogeny but can replicate by lytic pathways after induction. The best characterized temperate phage is the E. coli phage λ.
Lysogeny is a special type of latent viral infection. It’s a stable long term relationship between the phages and its host in which the phage nucleic acid becomes incorporated into the host chromosome. The phage genome replicates as a prophage in the bacterial cell.
In most lysogenic bacteria the genes required for lytic phage development are repressed and the production of infectious phage does not occur.
Process of Lysogeny
Lambda phages attach to bacterial cells and insert their linear DNA into the bacterial cytoplasm.
Phage DNA circularize and then integrates into the circular bacterial chromosome at a specific location. Insertion of a lambda phage into a bacterium alters the genetic characteristics of the bacterium.
Once established as a prophage, the virus can remain dormant for a long time. Each time a bacterium divides; the prophage is copied as a part of the bacterial chromosome and retains in the progeny bacteria. The period of bacterial growth with a prophage represents a lysogenic cycle.
However, either spontaneously or in response to some outside stimulation, the prophage can become active and initiate a typical lytic cycle. This process is called induction.
Lysogenic or Lytic cycle; what determines the fate?
The choice depends on the balance between two proteins, the
- repressor produced by the c-I gene and
- antagonizer of the repressor produced by the cro gene.
If the repressor predominates, transcription of other early genes is shut off and lysogeny ensues. Transcription is inhibited by binding of the repressor to the two operator sites that control early protein synthesis.
If the cro gene product prevents the synthesis of sufficient repressor, replication, and lysis of the cell result.
Expression of the integrated phage (prophage) genes confers new properties to the host bacteria. This phenomenon is known as lysogenic conversion. For example; synthesis of several exotoxins in bacteria, such as diphtheria, botulinum, cholera, and erythrogenic toxins. Without prophage, these bacteria are non-pathogenic.
Virulence factors encoded by bacteriophages
|Streptococcus pyogenes||Erythrogenic toxin|
|Escherichia coli||Shiga-like toxin|
|Staphylococcus aureus||Enterotoxins A, D, E, Staphylokinase, toxic shock syndrome toxin-1 (TSST-1)|
|Clostridium botulinum||Neurotoxins C, D, E|
|Corynebacterium diphtheriae||Diphtheria toxin|
Lysogenic conversion is mediated by the transduction of bacterial genes from the donor bacterium to the recipient bacterium by bacteriophages. During integration into host genes, the phage loses genes required for replication, this prevents the induction of the lytic cycle and killing of host bacteria.
Transduction is the process by which the DNA is mobilized between cell by a virus.
Prophage also provides “immunity” to infection by other phages of the same type. The gene however does not protect the lysogen against infection by a different type of temperate phage or by a virulent phage.
Usefulness of Phages
Phages have been studied as model organisms to gain insights into basic genetic concepts, such as viral gene expression. Researches on bacteriophage gave us much of our understanding of viruses and many fundamental concepts of molecular biology.
Lytic phages can be used as a replacement for antibiotic therapy. Using phages to treat bacterial infections was developed back in the 1920s and 1930s in Eastern Europe and the Soviet Union with varying success. Due to the spread of multi-drug resistant bacteria, phage therapy is re-emerging after a century. In 2019, the FDA approved the first US clinical trial of intravenously administered bacteriophage therapy.
Applications in Biotechnology and Research
Phages are excellent vehicles for horizontal gene transfer by transduction. So, they find wide use in recombinant-DNA technology to construct mutants and to transfer genes of interest from one bacterium to another. Phages can also be used as biocontrol agents in agriculture and petroleum industry.
References and further readings
- Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.
- Manohar, P., Tamhankar, A. J., Leptihn, S., & Ramesh, N. (2019). Pharmacological and Immunological Aspects of Phage Therapy. Infectious Microbes & Diseases, 1(2), 34–42.
- Navarro, F., & Muniesa, M. (2017). Phages in the Human Body. Frontiers in Microbiology, 8.
- Van Belleghem, J. D., Dąbrowska, K., Vaneechoutte, M., Barr, J. J., & Bollyky, P. L. (2018). Interactions between Bacteriophage, Bacteria, and the Mammalian Immune System. Viruses, 11(1).