Bacteriophage

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This article is about a biological infectious particle; for other uses, see phage (disambiguation).

A bacteriophage (from 'bacteria' and Greek φαγειν, '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), a large diversity of phage structures and functions exist. Typically, they consist of an outer protein hull enclosing genetic material. The genetic material can be ssRNA, dsRNA, ssDNA, or dsDNA between 5 and 500 kilo base pairs long with either circular or linear arrangement. Bacteriophages are usually between 20 and 200 nm in size.

Phages are ubiquitous and can be found in all reservoirs populated by bacterial hosts, 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 per millilitre have been found at the surface, and up to 70% of marine bacteria may be infected by phages.[1]

Contents

[edit] History

In 1915, British bacteriologist Frederick Twort discovered a small agent that infects and kills bacteria, but did not pursue the issue further. Independently, French-Canadian microbiologist Félix d'Hérelle announced on September 3, 1917 that he discovered "an invisible, antagonistic microbe of the dysentery bacillus" which he named bacteriophage.

[edit] Replication

Bacteriophages may have a lytic cycle or a lysogenic cycle, however a few viruses are capable of carrying out both. In the lytic cycle, characteristic of virulent phages such as the T4 phage, host cells will be broken open (lysed) and suffer death after immediate replication of the virion. As soon as the cell is destroyed the viruses will have to find new hosts.

In contrast, the lysogenic cycle does not result in immediate lysing of the host cell, those phages able to undergo lysogeny 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. Interestingly, as the lysogenic cycle allows the host cell to continue to survive and reproduce the virus is reproduced in all of the cell’s offspring.

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.

[edit] Attachment and penetration

A coloured electron micrograph of multiple bacteriophages
A coloured electron micrograph of multiple bacteriophages

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, which i.e. determines the phages hostrange. As phage virions do not move, they must rely on random encounters with the right receptors when in solution (blood and lymphatic circulation).

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 membrane, it can also be deposited on the cell surface. Other bacteriophages may use different methods to insert their genetic material.

[edit] Synthesis of proteins and nucleic acid

Within a short amount of time, sometimes just minutes, bacterial ribosomes start 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. Walter Fiers (University of Ghent, Belgium) was the first to establish the complete nucleotide sequence of a gene (1972) and of the viral genome of Bacteriophage MS2 (1976).[2]

[edit] Virion assembly

In the case of the T4 phage, the construction of new virus particles is a complex process which requires the assistance of special helper molecules. 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.

[edit] 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 endolysin which attacks and breaks down the peptidoglycan. "Lysogenic" phages, do not kill the host but rather become long-term parasites and make the host cell continually secrete 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.

[edit] Phage Therapy

Main article: Phage therapy
A 3D render of a T4 type bacteriophage landing on a bacterium to inject genetic material
A 3D render of a T4 type bacteriophage landing on a bacterium to inject genetic material

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. 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. Sometimes mixes of several strains of phage are used to create a broader spectrum cure. 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.

Georgia, a former Soviet state, is currently using phage treatments on most patients with bacterial infections. Many patients from Western countries that have been told amputation was their only option, have been cured of their infections in Georgia.[citation needed]

[edit] Other areas of use

In August, 2006 the United States Food and Drug Administration (FDA) approved using bacteriophages on certain meats to kill the Listeria monocytogenes bacteria. [1] Another large use of bacteriophages is by the company Cambrios Technologies. Its founder, Dr. Angela Belcher, pioneered the use of the M13 bacteriophage to create nanowires and electrodes. She started her research by studying how abalone snails create their shells from things that naturally occur in their environment. Specifically, she discovered the snails take abalone and make them transform into two distinct crystalline structures. One of the structures was hard, the other was fast-growing. She took this concept and applied it to bacteriophages. One of her ventures consisted of implanting gold and cobalt oxide in a bacteriophage to create a paper-thin electrode. The gold was for conductivity. The cobalt oxide was for the actual use of the battery.

[edit] Model bacteriophages

Following is a list of bacteriophages that are extensively studied:

[edit] See also

[edit] References

  1. ^ Prescott, L. (1993). Microbiology, Wm. C. Brown Publishers, ISBN 0-697-01372-3
  2. ^ Fiers W et al., Complete nucleotide-sequence of bacteriophage MS2-RNA - primary and secondary structure of replicase gene, Nature, 260, 500-507, 1976

[edit] External links

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