Bacteriophages are a special type of virus that exclusively infects bacterial cells. Here’s how they recognize, take over and ultimately kill their host bacteria.
* Viral Host Recognition *
Most viruses infect only a certain type of host. This specificity is due to the affinity of viral surface proteins or glycoproteins to corresponding glycoproteins on the surface of the host cell. Bacterophages, which look much like War of the Worlds spaceships, have proteins in their tail fibers that attach to proteins on the surface of the prokaryotic bacterial cell.
* How Do Viruses Reproduce? *
All viruses reproduce in three basic steps:
1. Virus delivers its genome (genetic material) into a host cell.
2. The virion (intracellular viral nucleic acid) commandeers the host cell transcription and translation machinery and utilizes host cell building blocks to copy viral genomes and synthesize viral proteins.
3. Viral genomes and proteins self-assemble and exit the host cell as new infectious particles.
The details of each of these steps vary among different types of viruses.
* Stages of Lytic Replication *
A bacteriophage, also known simply as a phage, reproduces via lytic replication, a type of viral reproduction that ultimately kills the infected bacterial cell. The five stages of the lytic cycle are attachment, entry, synthesis, assembly and release.
* Attachment: Viruses don’t move around on their own. Contact with a host bacterium occurs by random collision, and the structures responsible for attachment are the tail fibers. Attachment 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 between proteins is what makes one type of bacteriophage the specific pathogen to one type of bacteria. For example, T4 bacteriophages are specific to E. coli.
* Entry: How does the bacteriophage get its genetic material past the bacteria’s thick cell wall and plasma membrane? T4 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; much like a hypodermmic needle penetrating skin.
* Synthesis: After entry, viral enzymes (either carried in the capsid or coded by viral genes) degrade the bacterial DNA. As a result, the bacterium stops synthesizing its own molecules and begins synthesizing new viruses under the direction of the viral genome (a molecule of nucleic acid that is either DNA or RNA).
* Assembly: Scientists don’t entirely understand how phages are assembled inside their host bacterial cell. It appears that proteins that make up the structure of the viral protective covering, called a capsid, accumulate within the cell and spontaneously attach to each other.
Sometimes a viral capsids assembles around leftover pieces of host DNA. When this virus emerges and infects a new host cell, it transfers DNA from the former host into the new host. This process is known as transduction and it is one way that bacteria, organisms that essentially reproduce by cloning themselves, can increase their genetic diversity.
* Release: The newly assembled viruses are released from the host bacterial cell as the lysozyme completes its degradation of the cell wall and the bacteria disintegrates.
* Sources *
Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.