Bacteriophage (phage) are obligate intracellular viruses that especially infect bacteria. They were discovered independently by two researchers, Frederick wilhelm Twort1 in ~ the college of London in 1915, and Félix d’Herelle2 who shown the finding and coined the term bacter in 1917 and have been much studied since.

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Bacteriophage Structure

Phage have a very simple structure (Figure 1). Their genetic material is had in a prism shame head, surrounding by a protein capsid. This is linked to the elongated sheath (sometimes referred to as the tail) through a neck or collar region.

The sheath creates a hole tube with which the viral DNA/RNA is injected into the host cell and is surrounded by safety sheath proteins. At the bottom that the sheath is the basic plate to which the tail fibers (normally six) that facilitate attachments to the host cell room attached.


Figure 1.Example structure of a bacteriophage.In order come reproduce, phage must an initial enter the host cell. They bind to particular receptors on the bacterial cell surface with your tail yarn (adsorption) and create a hole, a procedure which, together with attachment, is combination by the basic plate3. A rigid pipe is pushed out that the sheath, puncturing a feet in the bacterial cabinet membrane with which they inject their hereditary material (DNA or RNA, double or single stranded). They can then hijack the organize cell’s cellular machinery for their very own replication if surrounding conditions are unfavorable in a process called the revolt cycle. Alternatively, lock may get in a dormant state, known as the lysogenic cycle, within the organize cell if problems are favorable.

Lytic cycle

In the riot cycle (Figure 2), occasionally referred to as virulent infection, the infecting phage eventually kill the hold cell to produce plenty of of their own progeny. Instantly following injection into the hold cell, the phage genome synthesizes early proteins that failure the host DNA, permitting the phage to take control of the to move machinery. The phage then offers the host cell to synthesize the staying proteins forced to build brand-new phage particles. The heads and sheaths are assembled separately, the brand-new genetic material packed right into the head and brand-new daughter phage particles constructed. During this process, the host cells gradually come to be weakened through phage enzymes and eventually burst, releasing on mean 100-200 brand-new phage progeny right into the surrounding environment.


Figure 2.
depiction of the stages of the bacteriophage lytic cycle.Watch the revolt cycle in action here.

Lysogenic cycle

The lysogenic bicycle (Figure 3), periodically referred come as warm or non-virulent infection, does no kill the hold cell, instead using it as a refuge wherein it exists in a dormant state. Adhering to the injection the the phage DNA into the organize cell, it integrates itself right into the host genome, v the assist of phage-encoded integrases, wherein it is then termed a prophage. The prophage genome is then replicated passively together with the organize genome together the hold cell divides because that as long as it continues to be there and does not type the proteins forced to create progeny. As the phage genome is normally comparatively small, the bacterial hosts space normally reasonably unharmed by this process.


Figure 3.
relenten of the stages of the bacter lysogenic cycle.

Transition native lysogenic come lytic

If a bacter containing prophage is exposed come stressors, such as UV light, low nutrient conditions, or chemicals like mitomycin C, prophage may spontaneously extract themselves from the host genome and enter the revolt cycle in a procedure called induction.

This process, however, is no perfect and also prophage may sometimes leave portions of their DNA behind or take sections of host DNA v them once they re-circularize. If they climate infect a new host cell, they may transport bacterial gene from one strain to one more in a process called transduction. This is one an approach by i m sorry antibiotic resistance genes, toxin and superantigen-encoding genes and other virulence traits may spread with a bacter population.

Recent work-related has shown that shift between lytic and lysogenic epidemic is likewise dependent ~ above the abundance of phage in one area as they are able to produce and also sense tiny peptides in a procedure akin to quorum sensing4.

Bacterial immune to phage infection

Not all bacteria space helpless versus phage attack, own an “immune system” that permits them to fight back. CRISPR-Cas, i m sorry is currently synonymous with genetic modification, was first proposed as a bacterial “adaptive immune system” by Francisco Mojica5 and independently by a group from Université Paris-Sud6 in 2005. The CRISPR locus is variety of quick repeated sequences separated by spacers with distinctive sequences. These spacer sequences were uncovered to have homology come viral and also plasmid DNA, including phage. When assaulted by a previously unencountered phage, new spacers are included at one next of the CRISPR, making the CRISPR a chronological document of the phage the cell and also its ancestors have actually encountered. In solution to phage invasion, the CRISPR sequences space transcribed and, in partnership v Cas proteins, target and also destroy the phage assignment that room homologous to the spacers sequences.

Phage as genetic and molecular biologic tools

The Lambda phage, originally isolated from Escherichia coli, is among the best studied phage and formed the communication of plenty of genetic tools. That has also been claimed that the use of phage as tools at some point led to the advance of molecule biology as a discipline7. In the 1950s, the phage’s capability to recombine with host DNA was first exploited to manipulate the genomes that Salmonella types and so the process of transduction was born8. Due to the fact that then, it has actually been supplied as a automobile to move genetic material in between many organisms, consisting of fungal gene manipulations9 and even human being genes. The was thanks to the humble phage that human being insulin was very first safely and cheaply produced. It has additionally opened increase applications in high throughput screening that clones, nanomaterial development10, antibacterial therapy for food items, as a diagnostic tool and drug discovery and delivery systems11.

The phage ϕX174 ended up being an unwitting pioneer in 1977 once it to be the an initial organism to have actually its entire nucleotide sequence identified thanks to Fred Sanger and colleagues12.

Phage therapy

Prior to the exploration of antibiotics through Alexander Fleming in 1928, phage to be being explored together a method for treating bacterial infections. In the post-antibiotic era, the convenient broad-spectrum activity of antibiotic treatment expected that in many organization’s research right into phage therapy was abandoned. However, in numerous of the previous Soviet countries where there was a lack of west antibiotics, research into phage therapies continued through necessity. V the increasing an international problems that antibiotic resistance, there has actually been a revival in the phage therapy ar in current years. Whilst phage space able to infect and also destroy bacteria and have been properly used come treat life-threatening infection13, their species and also strain specificity and also potential for pre-existing immunity of some bacteria typical targeting a phage treatment is currently not a trivial process and need to be tailored come the separation, personal, instance infection. This renders it costly and lengthy. Consequently, that is currently a critical resort and also there is still lot work forced in this field.

The phage family members tree

With the increasing ease of access and affordability that nucleotide sequencing, there has actually been an explosion in the numbers of phage genomes submitted to databases end the past two decades14.

Phage are classified by the global Committee on Taxonomy the Viruses (ICTV), as of your 2017 update, there room 19 families of phage that infect bacteria and also archaea (Table 1) yet as more samples from more remote areas are sequenced this is only likely to grow in the future.

For mobile users, role left and right to view the table data below.

OrderFamilyMorphologyNucleic acidExamplesSubfamiliesGenera
MyoviridaeNonenveloped, contractile tailLinear dsDNAT4 phage,Mu, PBSX, P1Puna-like, P2, I3, Bcep 1, Bcep 43, Bcep 78641
SiphoviridaeNonenveloped, noncontractile tail (long)Linear dsDNAλ phage,T5 phage, phi, C2, L5,HK97, N1511100
PodoviridaeNonenveloped, noncontractile tail (short)Linear dsDNAT7 phage,T3 phage,Φ29, P22, P37323
LigamenviralesLipothrixviridaeEnveloped, rod-shapedLinear dsDNAAcidianus filamentous virus 13
RudiviridaeNonenveloped, rod-shapedLinear dsDNASulfolobus islandicus rod-shaped virus 11
UnassignedAmpullaviridaeEnveloped, bottle-shapedLinear dsDNA1
BicaudaviridaeNonenveloped, lemon-shapedCircular dsDNA1
ClavaviridaeNonenveloped, rod-shapedCircular dsDNA1
CorticoviridaeNonenveloped, isometricCircular dsDNA1
CystoviridaeEnveloped, sphericalSegmented dsRNA1
FuselloviridaeNonenveloped, lemon-shapedCircular dsDNA2
GlobuloviridaeEnveloped, isometricLinear dsDNA1
GuttaviridaeNonenveloped, ovoidCircular dsDNA2
InoviridaeNonenveloped, filamentousCircular ssDNAM137
LeviviridaeNonenveloped, isometricLinear ssRNAMS2,Qβ2
MicroviridaeNonenveloped, isometricCircular ssDNAΦX17426
PlasmaviridaeEnveloped, pleomorphicCircular dsDNA1
TectiviridaeNonenveloped, isometricLinear dsDNA2

Table 1.

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ICTV taxonomic category of bacteriophage infecting bacteria and also archaea.


1.F. W. Twort, Lancet, 2, 1241-1243. (1915)2.F. D"Herelle, C. R. Acad. Sci. Paris, 165, 373-375. (1917) 3.M. I. Nicholas et al., Nature, 533, 346-352. (2016)4.Z. Erez, Nature, 541, 488-493. (2017)5.F. J. Mojica, J Mol Evol., 60(2), 174-82. (2005)6.C. Pourcel, Microbiology, 151(3), 653-63. (2005)7.W. C. Summers, new Haven: Yale university Press. (2000)8.N. D. Zinder, J Bacteriol., 64(5), 679-99. (1952)9.M. K. Chaveroche, main point Acids Res., 28(22), E97. (2000)10.W.-J. Chung, considerable Biomaterials, 95-111. (2011)11.L. O"Sullivan. F1000Res., 5, 2782. (2016)12.F. Sanger, Nature, 265(5596), 687-95. (1977)13.R. T. Schooley, Antimicrob Agents Chemother., 61(10), e00954-17. (2017)14.E. Adriaenssens, Viruses, 9(4), 70. (2017)