Bacteriophage
As bacteriophages or simply phages in the short form (singular: phage, from Ancient Greek βακτήριον baktērion " sticks " and ancient Greek φαγεῖν phagein "eat " ) refers to a group of viruses that are specialized in bacteria and archaea as host cells. This host specificity is considered in the taxonomic classification of the phages to rate. Thus a distinction is made, for example, E. coli, staphylococci, diphtheria or Salmonella bacteriophage. With an estimated number of 1030 virions in the entire seawater phages are the most common creatures on earth (if viruses are considered living things, although virologists are largely agree that viruses are not living ) and form the so-called Virioplankton.
History
Phages were described in 1917 by Canadian Félix Hubert d' Hérelle first time. Although the Englishman Frederick Twort had in 1915 to staphylococcal cultures observed decomposition processes, which are due to the action of bacteriophage, but its publication has been virtually ignored. D' Hérelle thus applies in addition to Frederick Twort as one of the discoverers of bacteriophages, the so-called " bacteria eaters ". However, your name and their discovery they owe d' Hérelle. Parallel to d' Hérelle postulated the German microbiologist Philalethes Kuhn from observations of the changes in bacterial cultures under certain conditions, the existence of bacteria parasites. He described them as Pettenkoferien and saw the described by d' Hérelle "invisible, the Ruhrbazillus counteracting microbe " as a special case of this parasite on. As it turned out later, his observations, however, were not based on the existence of a bacterial parasite, but only changes in the shape of the examined bacteria from him.
D' Hérelle turned the bacteriophage as an " ultra- visibles, corpuscular beings ", which exists in a basic form and adaptive to different hosts, ie bacteria. Indeed, bacteriophages are according to current knowledge highly specialized parasites that are bound to a specific host. The first phage were examined seven phage bacteria were Escherichia coli. They were in the order of their discovery as type 1 (T1 ), type 2 ( T2) and so named on.
Construction
The shape of the bacteriophage was elucidated mainly on the phage of the T- series (T- series) of Escherichia coli. ( The coliphage T2 consists of a polyhedral head of 100 nm in length, on which a long tail about the same sitting. ) Bacteriophages are taxonomically classified according to their morphology, their genome and their host. Thus one differentiates DNA phages with single-stranded DNA, so-called ss - DNA phages (of English. Single-stranded ), and those with double-stranded DNA, called ds- DNA phage (of English. Double-stranded ). The example here treated E. coli phages of the T series are counted among the latter group.
The so-called T- phages ( eg, T4 phage ) are distinguished from other bacteriophages by a relatively complex structure of. Basically, they consist of a base plate ( 9), an injection apparatus ( injection apparatus, 2) and a head ( 1), consisting of the so-called capsid ( 4) and the nucleic acid contained therein ( 3) together. The modules head and injection apparatus are connected by a neck (collar or 5). The base plate ( as constructed capsid and injection apparatus of proteins ) is busy with tail fibers (7) and spikes ( 8), which serve to adsorption to the host cell wall. The injection apparatus consists of a thin tube, and the tail pipe (6) mentioned, by the phage DNA (3) is injected into the host cell. The tube is surrounded by a contractile tail sheath, which contracts during the injection. The capsid is built with icosahedral symmetry of 152 capsomeres and contains the DNA of the phage. Because of this structure include the phage T4 -like viruses of the genus ( family Myoviridae ) to the structurally most complex viruses.
Phages with single-stranded DNA, however, are usually small, spherical and tailless or filamentous. The exist also occurring RNA phages usually (as far as described up to this point ) of a protein shell that encloses a single-stranded RNA molecule. The diameter of this phage is about 25 nm, so they are among the smallest phage.
Lysehof of gamma phage for Bacillus anthracis right uninfested single colony
Phage S- PM2 from seawater
Multiplication of bacteriophages
Viruses require the absence of its own metabolism to reproduce a host, in the case of bacteriophages a living (appropriate ) bacterial cell. The reproduction can be divided into five phases:
Adsorption to specific cell wall receptors
During adsorption, the ends of the tail fibers accumulate on the surface structure.
Injection of phage DNA into the host cell
Injection, at which the own DNA or RNA phage enters the bacterium then follows. The empty sleeves of the phage remain as non-functional proteins on the surface of the bacterium.
Latency phase
During this phase can be in the bacterium itself does not prove phage. Now, the transcription of the viral genome, the translation of viral mRNA and the replication of the viral nucleic acid begins. This process takes a maximum of a few hours.
Production phase
After the phage genes have become active in a fixed order, all viral components, such as envelope proteins, phage DNA and tail filaments are formed.
Mature phase
In this phase of the assembly to mature phage particles morphogenesis or the so-called assembly takes place. First, a head part or capsid is formed. The proteins inside serve as placeholders and are later replaced by the phage DNA enters the capsid. The threads of DNA play a one ball of wool -like space-saving form.
Release
The release of the final virus particles is carried out by the lysis of the host cell, which in turn is caused by the enzyme lysozyme, which is formed by the bacterium itself reprogrammed. It dissolves the bacterial Mureinzellwand. The cell bursts and about 200 infectious phages are free.
The propagation does not always runs off at some phage types according to the scheme described above, lytic. In temperate phages, one distinguishes between lysogenic and lytic replication cycles or cycles of infection. In a cycle, the lysogenic phage DNA is incorporated into the chromosome of the bacterium, whereby a prophage obtained. With each subsequent cell division genes of the phage and the bacteria are doubled and passed together. This cycle can later lead to the lytic cycle.
Areas of application
Phages have been found in medicine, biology, agricultural sciences, especially in the field of genetic engineering a wide range of applications. So one uses phages in medicine because of their host specificity for the determination of bacterial pathogens. This process is called Type differentiation. Due to the increasingly frequent multiple antibiotic resistance is currently being intensively on the application of bacteriophages as antibiotics substitute in human medicine (see: phage therapy ) research. Problems arise in this case due to the low stability of phages in the body, as they are disposed of in a very short time by phagocytes as a foreign body. This application of phages as antibiotics substitute was discovered in 1916 by Felix d' Hérelle, but was considered to be impractical with the introduction of chemotherapy by antibiotics and fell into oblivion. D' Hérelle founded in 1934 together with the Georgian microbiologist Georgi Eliava in Georgia the Eliava Institute for phage research, which still exists today.
In genetic engineering, temperate phages as vectors are used (eg the λ phage ). This phage are prepared so that its genome, the genes which induce the virulence, removed and replaced by genes that are of interest for genetic engineering considerations, such as genes that are required for the production of insulin. This modified phage are then reacted with suitable bacteria, for example E. coli, in contact. Upon review, if the desired gene was integrated into the genetic material of the bacterial genome ( given to using genexprimierter antibiotic resistances that are connected to the to be cloned desired genes), the modified bacteria cells are further cultured and produced in this case, insulin can be isolated. Similarly, phages are used in agricultural technology for transduction of specific genes in crop plants. An important application of biotechnology is the phage display for isolation of new active compounds. Simpler than the use of phage, however, the transformation of free DNA, which is nowadays used predominantly for the transfer into the bacterial cell.
Damage to humans by phage
Bacteriophage can cause damage anywhere where bacterial processes serve the people. Infection of lactic acid bacteria ( LAB) by phages from raw milk is the most common cause of decreased or absent enzyme activity in starter cultures for cheese or thick milk production.
System
Bacteriophages can be found in the Nomenclature of Virus Taxonomy in the following taxonomic groups:
- DsDNA bacteriophages:
- Family: Myoviridae
- Family: Siphoviridae
- Family: Podoviridae
- Family: Tectiviridae
- Family: Corticoviridae
- Family: Plasmaviridae
- Family: Podoviridae
- Family: Lipothrixviridae
- Family: Rudiviridae
- Family: Fuselloviridae
- Genus: Salterprovirus
- Family: Guttaviridae
- SsDNA bacteriophages:
- Family: Inoviridae
- Family: Microviridae
- DsRNA bacteriophages:
- Family: Cystoviridae
- SsRNA bacteriophages:
- Family: Leviviridae