Like viruses that infect eukaryotes (plants, animals and fungi), a large diversity of phage structure and function exists. Typically, they consist of an outer protein hull enclosing genetic material. The genetic material can be either RNA or DNA, but is usually double-stranded DNA between 5 and 500 kilo base pairs long. Bacteriophages are usually between 20 and 200 nm in size.
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 per millilitre have been found at the surface, and up to 70% of marine bacteria may be infected by phages.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.information from: http://en.wikipedia.org/wiki/Bacteriophage