Phage
From Freepedia
A phage (short for bacteriophage, from 'bacteria' and Greek phagein, meaning 'to eat') is a small virus that infects only bacteria. Like viruses that infect eukaryotes (plants, animals and fungi), phages consist of an outer protein hull and the enclosed genetic material (which consists of double-stranded DNA in 95% of the phages known) of 5 to 650 kbp (kilo base pairs) with a length of 24 to 200 nm. The vast majority of phages (95%) have a tail to let them inject their genetic material into the host. Phages are omnipresent, with millions in just one drop of ordinary water.
Image:Tevenphage.png Image:Bacteriophage.jpg
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Discovery and research
Phages were discovered independently in 1915 by Frederick Twort and in 1917 by Félix d'Herelle, who came up with the name bacteriophage. Twort did not pursue the matter further, but d'Herelle (being a communist) went to Tbilisi in the Soviet Union, to continue the research. When antibiotics were discovered, the West forgot about phages, but in Tbilisi the research continued, as it does to this day. It is still the place to go for those infected with a disease that is resistant to antibiotics, to get a phage treatment. Unlike antibiotics, phages adapt along with the bacteria, as they have done for millions of years, so resistance is unlikely.
Life cycle
Phages infect only specific bacteria. There is probably a phage for every species of bacterium. Some phages are virulent, meaning that upon infecting a cell they immediately begin reproducing, and within a short time lyse (destroy) the cell, releasing new phages. (A famous quote from the microbiologist Mark Müller says: Bacteria don't die, they just phage away.) Some phages (so-called temperate phages) can instead enter a relatively harmless state, either integrating their genetic material into the chromosomal DNA of the host bacterium (much like endogenous retroviruses in animals) or establishing themselves as plasmids. These endogenous phages, referred to as prophages, are then copied with every cell division together with the DNA of the host cell. They do not kill the cell, but monitor (via some proteins they code for) the status of their host. When the host cell shows signs of stress (meaning it might be about to die soon), the endogenous phages become active again and start their reproductive cycle, resulting in the lysis of the host cell. An example is phage λ of E. coli. Sometimes, prophages even provide benefit to the host bacterium while they are dormant, by adding new functions to the bacterial genome, a phenomenon called lysogenic conversion. A famous example is the harmless Vibrio bacteria strain, which is turned into Vibrio cholerae by a phage, causing cholera.
Therapy
A specific type of phage infects only one specific type of bacterium, so one has to make sure to use exactly the right phage (an exception are the family of Enterobacteriaceae, probably because they are so much alike). But if so, the phage will continue to kill bacteria of that type until they are all gone. An added advantage is that no other (possibly benevolent) bacteria are attacked. Phages play an important role in molecular biology as cloning vectors to insert DNA into bacteria. Phage therapy has been used since the 1940s in the former Soviet Union as an alternative to antibiotics for treating bacterial infections, because killing bacteria is what phages do best. In contrast with antibiotics, phages work best when in direct contact with the infection, so they are best applied directly to an open wound. There is an extensive library of research into specific phages and their therapeutic uses in the Tbilisi Institute in Georgia. The development of bacterial strains that are resistant to multiple drugs has led Western medical researchers to re-evaluate phages as alternatives to the use of antibiotics.
Phage display is a test to screen for protein interactions by integrating multiple genes from a gene bank into phages.
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
See also
Internal links
Resources
http://www.uq.edu.au/nanoworld/nanohome.html
