- A bacteriophage is a virus that infects bacteria. Bacteriophages, first discovered around 1915, have played a unique role in viral biology. They are perhaps the best-understood viruses, yet at the same time, their structure can be extraordinarily complex.
- A bacteriophage is essentially a virus consisting of DNA or RNA that is enclosed within a protein shell.
- The protein shell or capsid protects the viral genome. Some bacteriophages, like the T4 bacteriophage that infects E.coli, also have a protein tail composed of fibers that help attach the virus to its host.
- The use of bacteriophages played a prominent role in elucidating that viruses have two primary life cycles: the lytic cycle and the lysogenic cycle.
Virulent Bacteriophages and the Lytic Cycle
Bacteriophages are viruses that infect bacteria. T- phages consist of an icosahedral (20-sided) head, which contains the genetic material (either DNA or RNA), and a thick tail with several bent tail fibers. The tail is used to inject the genetic material into the host cell to infect it. The phage then uses the bacterium’s genetic machinery to replicate itself. When a sufficient number have been produced the phages exit the cell by lysis, a process that kills the cell. KARSTEN SCHNEIDER/SCIENCE PHOTO LIBRARY/Getty Images
Viruses that kill their infected host cell are said to be virulent. The DNA in these type of viruses is reproduced through the lytic cycle. In this cycle, the bacteriophage attaches to the bacterial cell wall and injects its DNA into the host. The viral DNA replicates and directs the construction and assembly of more viral DNA and other viral parts. Once assembled, the newly produced viruses continue to increase in numbers and break open or lyse their host cell. Lysis results in the destruction of the host. The whole cycle can be completed in 20 – 30 minutes depending on a variety of factors such as temperature. Phage reproduction is much faster than typical bacterial reproduction, so entire colonies of bacteria can be destroyed very quickly. The lytic cycle is also common in animal viruses.
Temperate Viruses and the Lysogenic Cycle
Temperate viruses are those that reproduce without killing their host cell. Temperate viruses reproduce through the lysogenic cycle and enter a dormant state. In the lysogenic cycle, the viral DNA is inserted into the bacterial chromosome through genetic recombination. Once inserted, the viral genome is known as a prophage. When the host bacterium reproduces, the prophage genome is replicated and passed on to each bacterial daughter cells. A host cell that carries a prophage has the potential to lyse, thus it is called a lysogenic cell. Under stressful conditions or other triggers, the prophage may switch from the lysogenic cycle to the lytic cycle for the rapid reproduction of virus particles. This results in lysis of the bacterial cell. Viruses that infect animals may also reproduce through the lysogenic cycle. The herpes virus, for example, initially enters the lytic cycle after infection and then switches to the lysogenic cycle. The virus enters a latent period and can reside in nervous system tissue for months or years without becoming virulent. Once triggered, the virus enters the lytic cycle and produces new viruses.
Bacteriophages may also exhibit a life cycle that is a little different from both the lytic and lysogenic cycles. In the pseudolysogenic cycle, the viral DNA does not get replicated (as in the lytic cycle) or inserted into the bacterial genome (as in the lysogenic cycle). This cycle typically occurs when there are not enough nutrients available to support bacterial growth. The viral genome becomes known as a preprophage that does not get replicated within the bacterial cell. Once nutrient levels return to a sufficient state, the preprophage may either enter the lytic or lysogenic cycle.