Bacteriophages are a type of virus that prey upon bacterial cells. The reproduction of bacteriophages always involves death of the infected bacterium, when the viruses burst out or lyse their prokaryotic host cell. Bacteriophages reproduce by commandeering a bacterium and ultimately killing it.
But sometimes, through lysogeny, the phage doesn’t immediately take over its host. The viral genetic material may remain hidden and inactive in the bacterial DNA, allowing the host to reproduce. Each time the infected bacterium duplicates its DNA (nucleic acid) and divides, it also creates another copy of the viral nucleic acid; a very sneaky and effective tactic for creating a huge number viruses from one initially infected bacterial cell.
* The Basics of Viral Reproduction *
All viruses, both bacteriophages and animal virions, reproduce via three basic steps:
1. Viruses deliver their genomes into a host cell.
2. Viruses commandeer the host cell transcription and translation machineries and utilize host cell building blocks to copy viral genomes and synthesize viral proteins.
3. Viral genomes and proteins are self-assembled and exit host cells as new infectious particles.
These steps are just a general outline of the events of viral replication. The details of each of these steps vary among different types of viruses.
* Viral Lytic Replication *
A bacteriophage, also known as simply a phage, reproduces specifically via lytic replication, a type of viral replication that ultimately kills the infected bacterial cell. The five stages of the lytic cycle are as follows:
1. Attachment: The phage encounters and connects to a bacterial cell.
2. Entry: The phage injects its nucleic acid (genetic material) into the bacterium and destroys the bacterial DNA.
3. Synthesis: No longer having its own DNA to work with, the bacterial cell begins replicating, transcribing and translating the viral nucleic acid.
4. Assembly: The viral components made by the bacterial cell self-assemble into new viruses.
5. Release: The bacterial cell is lysed (broken open), killing the cell and releasing the new viruses.
* Lysogenic Cycle of Bacteriophages *
Sometimes, after a phage enters its host cell it does not destroy the host’s DNA and the phage’s genome does not immediately take control of the cell. Instead the viral DNA is inserted into the bacterial chromosome, but virus remains inactive. In this state, the virus is called a prophage and the mode of reproduction is called lysogeny.
The prophage remains inactive by coding for a protein that suppresses the viral genes. A side effect of this suppression is that it renders the bacterium resistant to additional infection by other viruses of the same type. In effect, the prophage is claiming its territory.
At some later time, the viral DNA is reactivated, by being excised (cut) from the host’s genome. External factors that cause induction are typically the same physical and chemical agents that damage DNA molecules, such as UV light, X-rays and carcinogenic chemicals.
* Sources *
Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.