Bacteriophage
- This article is about a biological infectious particle; for other uses, see phage (disambiguation).
A bacteriophage (from 'bacteria' and Greek phagein, 'to eat') is a virus that infects bacteria. The term is commonly used in its shortened form, phage.
Like viruses that infect eukaryotes (plants, animals and fungi), phages vary greatly in structure and function. Typically, they consist of an outer protein hull, or capsid, enclosing genetic material. The genetic material can be either RNA or DNA, but is usually double-stranded DNA. The phage genome varies in size from between 2 to 200 kilo base pairs per strand of nucleic acid (Baron TK).
Phages are ubiquitous and can be found in many reservoirs populated by bacteria, such as soil or the intestine of animals. One of the densest natural sources for phages and other viruses is sea water, where up to 109 virions, or complete virus particles, per milliliter have been found at the surface. Up to 70 percent of marine bacteria may be infected by phages (Prescott 1993).
Structure
The classic structure of a bacteriophage is shown on the diagram to the right, which features complex symmetry. The head of the structure, which contains the genetic material, features icosahedral symmetry, whereas the tail features helical symmetry. A hexagonal base plate has multiple tail fibers projecting off of it. In reality, this structure is present mainly in the T-even and coliphages. Other complex bacteriophages may lack tail fibers or may even possess contractile tails.
Various other phage morphologies have been observed, such as the long, filamentous Inoviridae family, rod-like structures, or the spherical Cystoviridae family.
Replication
Bacteriophages may have a lytic cycle or a lysogenic cycle. Few viruses are capable of carrying out both cycles. In the lytic cycle, characteristic of virulent phages such as the T4 phage, in the last stage of infection the host cells break open, or lyse, and die after the virion replicates. Once the cell is destroyed the viruses have to find new host.
In contrast, the lysogenic cycle does not kill the host cell. The lysogenic cycle allows the host cell to survive and reproduce, and the virus is reproduced in all of the cell’s offspring.
Phages able to undergo a lytic cycle and a lysogenitic cycle are known as temperate phages. Their viral genome will integrate with host DNA and replicate along with it fairly harmlessly, or may even become established as a plasmid. The virus remains dormant until host conditions deteriorate, perhaps due to depletion of nutrients, then the endogenous phages (known as prophages) become active. They initiate the reproductive cycle resulting in the lysis of the host cell.
Sometimes prophages may provide benefits to the host bacterium while they are dormant by adding new functions to the bacterial genome in a phenomenon called lysogenic conversion. A famous example is the conversion of a harmless strain of Vibrio cholerae by a phage into a highly virulent one, which causes cholera.
Attachment and penetration
To enter a host cell, bacteriophages attach to specific receptors on the surface of bacteria, including lipopolysaccharides, teichoic acids, proteins or even flagella. This specificity means that a bacteriophage can only infect certain bacteria bearing receptors that they can bind to. As phage virions do not move, they must rely on random encounters with the right receptors when in solution.
Complex bacteriophages, such as the T-even phages, are thought to use a syringe-like motion to inject their genetic material into the cell. After making contact with the appropriate receptor, the tail fibres bring the base plate closer to the surface of the cell. Once attached completely, conformational changes cause the tail to contract, possibly with the help of ATP present in the tail (Prescott, 1993). While the genetic material may be pushed through the cell membrane, it may also be deposited on the surface. Other bacteriophages may use different methods to insert their genetic material.
Synthesis of proteins and nucleic acid
Within a short amount of time, sometimes just minutes, bacterial polymerase starts translating viral mRNA into protein. For RNA-based phages, RNA replicase is synthesised early in the process. Early proteins and a few proteins that were present in the virion may modify the bacterial RNA polymerase so that it preferentially transcribes viral mRNA. The host’s normal synthesis of proteins and nucleic acids is disrupted, and it is forced to manufacture viral products. These products go on to become part of new virions within the cell, helper proteins which help assemble the new virions, or proteins involved in cell lysis.
Virion assembly
In the case of the T4 phage, the construction of new virus particles is a complex process which requires special helper molecules to assist it. The base plate is assembled first, with the tail being built upon it afterwards. The head capsid, constructed separately, will spontaneously assemble with the tail. The DNA is packed efficiently within the head in a manner which is not yet known. The whole process takes about 15 minutes.
Release of virions
Phages may be released via cell lysis or by host cell secretion. In the case of the T4 phage, in just over twenty minutes after injection upwards of three hundred phages will be released via lysis. This is achieved by an enzyme called lysozyme which attacks and breaks down the cell membrane. Some phages however may develop a symbiotic relationship where the host cell continually secretes new virus particles. The new virions bud off the plasma membrane, taking a portion of it with them to become enveloped viruses possessing a viral envelope. All released virions are capable of infecting a new bacterium.
Phage Therapy
Phages were tried as anti-bacterial agents after their discovery. However Antibiotics, upon their discovery, proved to be more practical. Research on phage therapy was largely discontinued in the West, but phage therapy has been used since the 1940s in the former Soviet Union as an alternative to antibiotics for treating bacterial infections.
The evolution of bacterial strains through natural selection that are resistant to multiple drugs has led some medical researchers to re-evaluate phages as alternatives to the use of antibiotics. Unlike antibiotics, phages adapt along with the bacteria, as they have done for millions of years, so a sustained resistance is unlikely. Additionally, when an effective phage has been found it will seek out the bacteria and continue to kill bacteria of that type until they are all gone.
A specific type of phage often infects only one specific type of bacterium (ranging from several species, to only certain subtypes within a species), so one has to make sure to identify the correct type of bacteria, which takes about 24 hours. Sometimes mixes of several strains of phage are used to create a broader spectrum cure. An added advantage is that no other bacteria are attacked, making it work similarly to a narrow spectrum antibiotic. However this is a disadvantage in infections with several different types of bacteria, which is often the case. Another problem with bacteriophages is that they are attacked by the body's immune system.
Phages work best when in direct contact with the infection, so they are best applied directly to an open wound. This is rarely applicable in the current clinical setting where infections occur systemically. Despite individual success in the former USSR where other therapies had failed, many researchers studying infectious diseases question whether phage therapy will achieve any medical relevance. There have been no large clinical trials to test the efficacy of phage therapy yet, but research continues because of the rise of multiple antibiotic resistance.
Model bacteriophages
Following is a list of bacteriophages that are extensively studied:
- λ phage - Lysogen
- T4 phage (169 to 170 kbp, 200 nm long)
- T7 phage
- R17 phage
- M13 phage
- MS2 phage (23-25 nm in size)
- G4 phage
- P1 phage
- P2 phage
- N4 phage
- ф6 phage
- ф29 phage
See also
- RNA viruses
- DNA viruses
ReferencesISBN links support NWE through referral fees
- Häusler, T. (2006) "Viruses vs. Superbugs", Macmillan
- Evergreen State College material on bacteriophages
- Phage.org general information on bacteriophages
- Pittsburgh Bacteriophage Institute
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