Bacteriophage is a virus that infects bacteria, which was originally discovered in 1913 but was not named until 1917 by a French-Canadian scientist Félix D'Hérelle. He discovered that there was an invisible "thing" that was killing bacteria and changing it, and he eventually named it bacteriophage.(Purves 215) Bacteriophages are vast in numbers. They strike with unrelenting force or lay dormant for just minutes or even years. They cause many harmless cells to become a virulent bacterium.
A bacteriophage looks similar to the Lunar Lander used by the US when man first landed on the moon. Not all bacteriophages look the same but they all have the same basic makeup. T2 and T4 Bacteriophages are similar. They both have a head also called the capsid. This part of the virus is made up of many duplicates of one or more proteins. These proteins protect the virus' DNA. The next part is the tail. In the more complex bacteriophages such as the T4 this is called the sheath, and there are more tail structures at the base of the sheath. This is a hollow tube made up of proteins used for the passing of the virus' DNA during the infection process. The T4 also has a base plate. The base plate is used to cement the bacteriophage to the cell wall of the bacteria host.  (Purves 215)
Bacteriophages have no form of movement. They rely on chance encounters with bacterial cells. They also can only attach themselves to specific points on the cell. Certain phages can connect to certain points such as the flagellum. This is the reason not all phages can strike at any bacteria.  The phage attaches to its host with its tail end not its head. Once it makes contact it starts the process of injecting its DNA into its host. Bacteriophages that have a sheath will use that part of its body to penetrate by blunt force through the cellular envelope of the bacteria cell. Phages that do not have a sheath enter into the bacteria by other means. One way is secreting an enzyme that eats open the wall. After this has happened the DNA enters the cell and takes over. 
Bacteriophages can be divided into three major groups: some that perform just a lytic cycle, others that perform only the lysogenic cycle, and finally there are some that can perform both. Bacteriophages can appear to be positive while they lay dormant, but become quite potent once they are done with the host.
A virulent bacteriophage takes over the host cell in stages. First its DNA is injected and it beings to create specialized mRNA. This RNA will in turn create proteins and other mRNAs. The proteins created are used to stop the bacteria's major functions, before they turn the bacteria in to a virulent bacteriophage factory.  While it creates the new bacteriophages it breaks down the chromosomes of the bacteria and acquires other nutrients. Once the bacteria cell becomes too full with bacteriophages, it explodes sometimes releasing up to 1,000 new bacteriophage viruses. 
During the lytic cycle the virulent bacteriophage injects its own DNA into the bacteria cell. The process of transduction is a flaw in the incorporation of the phage's DNA in the cell. When the Phage DNA binds with the host DNA to begin production of capsids (A protein coat) to store the viral DNA recombination can occur. When recombination occurs some of the new capsids for the bacteriophages can been filled with the host's DNA rather than the viral DNA making it ineffective viral infection.(Purves p. 266)
A temperate phage is created during the lysogenic stage. The predator bacteriophage attaches itself to a host cell releasing its DNA into the cell (Generally bacteria cells). The viral DNA will then implement itself into the DNA of the bacteria when it undergoes DNA replication. It will do this harmlessly allowing the bacterium to perform as usual as the viral DNA lays dormant. In many cases the viral DNA can become a [plasmid].  (Purves 216) The viral DNA also creates a protein called repressor. This protein attaches itself to the viral DNA causing it to lie dormant. It will stay in this state for however long it feels it is needed. Until all the nutrients are gone or until its host organism recognizes it. By this time all the daughter cells from the first infected cell are infected because of the replicated viral DNA. When the phage comes out of dormancy it quickly takes control of the cell causing it to create more bacteriophages. In the end it destroys its host cell and leaves with many bacteriophages that move on to infect the next passing bacteria cell. The process after dormancy till the lysis occurs can take as little as 20 minutes. 
A bacteria cell is usually infected by only one bacteriophage virus. It is an extremely rare occurrence for more than one phage to strike the same bacteria cell. Once the infection process has begun proteins created by the infecting phage prevents any continued infections. If another bacteriophage infects the bacteria cell before the proteins can be synthesized a unique opportunity is available. The host cell now has two sets of the viral genomic code. This allows for recombination of the two genomes while the merge with the host cells DNA. When this happens the bacteriophage that will be created in this bacteria cell will be a genetically unique virus that has a new gene strains. (Purves 260)
Experiment with Bacteriophages
When bacteriophages were first discovered they did not know if the hereditary material was from the DNA or from the protein. Scientist at the time did not know the significance of DNA thinking that proteins were more important the DNA. For this reason the guessed that the protein was the hereditary material. Two scientists, Alfred Hershey and Martha Chase developed an experiment in 1952 to find out which was the hereditary material. They took a bath of bacteriophages infecting half of them with 32P containing DNA and the other half with 35P proteins (32P and 35P are radioactive isotopes). They then put each batch separately into a solution with E. coli bacteria. They mixed it up and the put it into a centrifuge. This separated the infected bacteria from the viral bacteriophages. From doing this experiment they found out that the Bacteriophages that had marked DNA lost all of its DNA to the Bacteria whereas the marked protein stayed with the bacteriophage. This concluded that DNA was the heritable material.(Purves 215-216)
Phage therapy was developed by a French-Canadian Scientist microbiologist Felix D'Herelle in 1917. He thought of the idea by realizing that Bacteriophages attack and destroy bacteria cells that cause diseases. He tried his theory on chickens. He split the chickens into two separate groups. The first group was the control group and was infected with Salmonella gallinarum. The other group was infected with Salmonella but it was also given phage. The group with just the Salmonella died whereas the other group that had been inoculated with the phage never became infected by the infectious bacteria Salmonella. He then went on to treat a bacteria-plague that was infecting men and women in Egypt, and it worked perfectly. (Purves 261)
Phage therapy is much like the use of antibiotics. Bacteriophages work by infecting the bacteria cell and killing it through a process of either lytic or lysogenic. The bacteriophages destroy the bacteria never allowing it to infect the organisms cells. Antibiotics work by creating a repressor enzyme that binds to a specific gene that codes for certain proteins that are vital to the bacterias life cycle. The problem with these antibiotics are that the bacterias have begun to create immunities to these antibiotics. In recent years the phage therapy process has started to make a come back because at the moment there are no immunities. (Purves 261)
- Bacteriophage Wikipedia
- Bacteriophages University of S. Carolina
- Life: The Science of Biology, Gordensville, VA. 2004, by: Purves, William K. et al.