Friday, December 04, 2009

Bacteriophage (Part 1)

What are bacteriophage? Bacteriophages (or phages) are viruses that infect bacteria rather than eukaryotic cells. The word bacteriophage comes from "bacteria" and "to eat" because many phages literally destroy the bacteria they infect.

Bacteriophage T4 is a classic example of phage virus. It infects the E. coli bacterium. Below is a diagram of a T4 phage and an actual electron micrograph of the phage.
The T4 phage looks alot like the lunar lander doesn't it? The "legs" of the T4 phage allow for attachment to the E. coli host bacterium and the phage "tail" is hollow to allow for the injection of the phage DNA into the bacterium. You can see an animation of the T4 phage infecting a bacterium here.

Viruses have two different types of life cycles called lytic and lysogenic. The lytic life cycle is the more common type of life cycle used by viruses and it results in the destruction of the bacterium or cell. The below excellent diagram, from the U. of Delaware, shows the lytic life cycle of a T4 phage.
Basically, the bacterium is infected, the phage takes over the metabolic machinery of the bacterium, reproduces itself and destroys the bacterial host cell thus releasing more virus available to infect new host cells.

The lysogenic cycle is different and is pictured below.
In this phage "life style", the phage DNA is actually incorporated into the bacterial genome. This newly integrated phage DNA is referred to as a prophage. The advantage of this life cycle is that every time the bacterium divides, the DNA of the prophage is replicated along with the bacterial genome. At some time in the future, the prophage can be induced to enter the lytic phase where new phages are reproduced and the bacterial cell is destroyed, releasing the phage virions.

So phages are good because they infect and destroy bacteria. Well not so fast. Phages are also known to increase the disease-causing capability (virulence) of some bacteria. For instance, the toxin produce by Vibrio cholerae bacterium diarrhea and rapid dehydration of the infected person, is due to infection of the bacterium with CTX phage. The toxin which causes cholera is actually encoded by the phage genome and not the bacterial genome. It is possible that phage infection of bacteria could be responsible for the evolution of more virulent bacteria. Other toxins reported to be encoded by phages include shigalike toxin in some pathogenic E. coli strains, botulinum toxin, diptheria toxin and erythrogenic toxin. This is serious business because it may be possible for phage to transfer virulence genes from disease-causing bacteria to unrelated bacteria which normally do not cause disease. You can read more about this phenomenon here.

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