Bacteriophage

A bacteriophage (or phage ) is a Virus infecting only Bactérie Greek S. In , phageton means food/consumption. They are also called bacterial viruses . They are fundamental tools of research and study in molecular Génétique. The bacteriophages are useful inter alia, of vector S of Clonage of Gène S.

The bacteriophages are present in the whole of the Biosphère. Indeed, they are present everywhere, but in larger quantity in the excrements, the ground and sewages. They were discovered in 1915 by a British researcher, Frederick W. Twort, but in fact really work of Felix d' Hérelle, a Franco-Canadian scientist opened the field in 1917.

The support genomic of the bacteriophages can be a DNA or a ARN.

Characteristics

Constitution

As the Virus which infect the Eucaryote S, the phages consist of an external proteinic envelope (called Capside) protecting the genetic material (DNA or ARN). For more than 95% of the known phages, this material is a molecule of DNA double-bit of a size from 5 to 650 kpb and their size varies from 24 to 200 Nm.

The organization responsible for the nomenclature and the taxonomy of the viruses is called off International Commitee one Taxonomy Viruses (ICTV). One counts 21 different morphologies at the bacterial viruses currently recognized by the ICTV. In 2000, more than 5000 different bacteriophages had been observed and described. More than 95% of them had a tail implied in the entry of the DNA of the phage in the bacterial cell.

In the years 1940, the work carried out on the bacteriophages made it possible to discover that the nucleic acid were the principal components of the genetic material. It is with this discovery that the vast domain of molecular biology occurred.

Reproduction

The phages infect only one specific bacterium. Some phages are virulent, i.e. at once that they infect a cell, it is immediately put to reproduce and, in a short amount of time, the phage explodes the cell what releases many others phages.

The famous microbiologist Mark Muller said one day: “The bacteria do not die, they explode in multiples phages. ” Certain bacteriophages (called average phages ) can enter a rather inoffensive state while entering their genetic material the DNA of the host (the bacterium) like a Rétrovirus or like Plasmide endogenous S. These phages, referred like prophages , is copied with each cellular division with the whole with the DNA of the bacterium. The DNA of this one is not destroyed, on the contrary! Its genes can be transferred via the prophage. When the cell shows some signs of stress (that can mean that it soon will die) the endogenous phage still starts to be active and begins its reproductive cycle. What results from it, it is the multiplication of the phage inside the cell. An example is the lambda phage of Escherichia coli . Sometimes, the prophages bring something to the relation bacterium-phage when the cell is in dormancy, by adding new functions to the Génome of the bacterium, a phenomenon called Conversion lysogene. A known example is the inoffensive bacterium Vibrio which, when it is transformed by a phage, causes the Choléra!

Cycles lytic and lysogenic

The bacteriophages, ubiquitaires of nature, persist in the bacterial world under two distinct states: as a phage virulent (which is retorted in a receptive bacterial cell) or in form lysogene (inserted in the genome in the form of a prophage, he becomes integral part of the genome of the host). All the viruses bacteriophages have a lytic cycle (infectious) during which the virus, incompetent to reproduce by his own means, injects its genetic material in the bacterium. Thanks to the Enzyme S and the Ribosomes of the host, the virus can be retorted with more than one hundred specimens before the host does not burst. But sometimes, certain bacteriophages behave differently. Their genetic material is integrated into the chromosome of the bacterium which transmits it to its descendants (Lysogénie). In a case percent thousand, the viral DNA is activated and starts a lytic cycle.

The bacteriophages take part in the evolution of the bacteria

As the phages can carry in their genome of additional genes to their life cycle, they take part in the horizontal transfers of genes between bacterial populations. It is the transduction. When these additional genes code factors of virulence, the infected bacterium sees its increased pathogenic capacity - it is the phenomenon of “lysogenic conversion”.

A well-known example is that of genes of the Toxine S Stx of the enterohemorragic Escherichia coli (EHEC). These genes stx are localized in sequences of bacteriophages lambdoïdes integrated in the chromosome. The EHEC would thus have emerged by lysogenic conversion. One knows many other examples of this type, as choleraic toxin of Vibrio cholerae which is carried by phage CTX.

The bacteriophages lysogenes are often integrated in the chromosome at the level of loci coding ARN of transfer (ARNt). For example, the phage ¨PhiR73 of Escherichia coli is inserted into the level of the locus selC . The acquisition of foreign genes per horizontal transfer, thanks to bacteriophages being integrated into the level of such “hot spots” is plausible, since the sequences coding ARNt are highly preserved between the various bacterial species. Lastly, the persistence of genes of virulence in the genomes phagic suggests that they confer a selective advantage, perhaps due to the greatest multiplication and diffusion of the bacterium host.

Bacteriophages like fundamental tool of research

The phages allowed the rise of the Molecular biology

In the years 1960, latest researches undertaken on the mechanisms host/phage by American physiologists, max Delbrück, Alfred Hershey and El Salvador Luria, were worth with these researchers the Nobel Prize of medicine-physiology in 1969.

See also: Experiment of Delbrück and Luria

The phages allowed various discoveries:

  • the Experiment of Harshey and Chase which made it possible to confirm the function of DNA as a genetic data carrier.
  • In 1980, the British biochemist Frederick Sanger accepted the Nobel Prize to have made a success of with séquencer DNA by using a phage.

The study of the phages has important implications in Médecine and Génétique, in particular for the comprehension of the viral infections, the genetic anomalies, the human embryology, the causes of the Cancer and the resistance of the bacteria to antibiotics.

Use in genetic Engineering

The phages are used multiple manners in molecular biology. They are used as vectors of cloning to insert DNA in the bacteria. The method of the phage display is a method which allows the selection of a peptide thanks to its presentation on the surface of phages. August 1st

Use in the sequencing of whole genomes

The Séquençage of a genome is not made an only one blow, but gradually on fragments of genomes. For that these fragments of DNA must be stored and multiplied in organizations being used as bank of DNA. The phages as vectors of cloning allow it.

See also: Project human genome

Use of the phages as an antimicrobic agent

In 2006, with the the United States, a preparation containing six viruses bacteriophages was authorized like food Conservateur, in particular, to fight against the Listériose.

Use in Medicine

In the ex-USSR and more particularly in Georgia and Poland, the phages are used to treat the bacterial infections with or without antibiotic addition of treatment. In many cases, the phages are as effective as antibiotics and often even higher, in particular in the chronic cases of infections.

List principal bacteriophages

  • phage λ - Lysogène
  • Phage T4 (169 to 170 pairs of bases, 200 Nm length)
  • Phage T7
  • Phage R17
  • Phage M13 - Phagemid

See too

  • Felix d' Herelle

Simple: Bacteriophage

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