Bacteriophages are a particular type of virus that infects only bacterial cells. Ultimately the virus kills its host, but sometimes it first becomes a hidden prophage.

How a Virus Makes More Viruses

Although there are variations depending on the type of virus, all viruses reproduce using at least these three basic steps:

• A virus delivers its genome into a host cell.
• Induce the host to begin making copies of the viral genome and viral proteins.
• New viruses self-assemble and exit the host cell to infect new cells.

Lytic Replication and Lysogeny

Bacteriophages, also known as phages, ultimately reproduces through lytic replication. This form of viral reproduction ultimately kills the infected bacterial cell. Sometimes, rather than immediately taking over the cells transcription and translation machineries, the viral DNA inserts itself into the host bacteria’s genome. In this form, the virus is known as a prophage and it relies on the division of bacterial call (binary fission) to create more viral DNA.

Lytic Replication

The stages of the viral lytic cycle are attachment, entry, synthesis, assembly and release.

Attachment

Viruses are not motile. They can’t move about on their own. Contact with a potential bacterial host may occur when a virus randomly collides with the appropriate type of bacterium.

Attachment of the virus to the bacterium is dependent on the chemical attraction and precise fit between attachment proteins on the phage tail fibers and receptor proteins on surface of the host cell. This specificity is what makes one type of bacteriophage the specific pathogen to one type of bacteria.

Entry

Bacteriophages release a lysozyme (digestive enzyme) that weakens the main structural material (peptidoglycan) of the bacterial cell wall. The phage’s tail sheath then contracts, forcing an internal hollow tube within the tail through the bacteria’s cell wall and membrane. The viral genetic material is then injected through this tube.

Synthesis

Once the virus is inside the host, viral enzymes destroy the bacterial DNA, and the bacterium instead begins synthesizing new viruses under the direction of the viral genome.

Assembly

Proteins that make up the structure of the viral covering, called a capsid, accumulate within the cell and spontaneously attach to each other. The viral genetic material ultimately ends up within the capsid.

Release

Newly assembled viruses are released from the host bacterial cell as the lysozyme completes its degradation of the cell wall and the bacterium disintegrates.

Lysogenic Cycle of Bacteriophages

In lysogeny the bacteriophage does not immediately destroy the hosts DNA and take control of the cell. Instead inactive viral DNA is inserted into the bacterial chromosome. In this state, the virus is called a prophage.

Bacteria reproduce by dividing. Each time the bacterium divides, the cell’s genetic material must be copied so that each new cell has a genome. In a bacterium infected with a prophage, each division not only copies the bacterial DNA, but also the prophage. In this way, millions of bacteria can become infected through the action of only one bacteriophage.

Eventually the viral DNA is excised (cut) from the host’s genome; a process called induction that reactivates the virus. The phage then proceeds through lytic replication, killing its host.

More Information on Microbes

See the science education websites Science Prof Online and MicrobeWiki for more microbiology information.

Sources

Bauman, R. (2005) Microbiology.

Park Talaro, K. (2008) Foundations in Microbiology.