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Bacteriophage: T4 Phage

Bacteriophage: T4 Phage Infecting an E. coli Bacterium

Image courtesy of Dr. K. Sathasivan. Used with permission.


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. The use of bacteriophages played a prominent role in elucidating that DNA in viruses can reproduce through two mechanisms: the lytic cycle and the lysogenic cycle.

Virulent Bacteriophages and the Lytic Cycle

Viruses that kill their infected host cell are called virulent. The DNA in these type of viruses reproduces through the lytic cycle. When these viruses reproduce, they break open, or lyse, their host cells, resulting in the destruction of the host. The whole cycle can be complete 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 can be destroyed very quickly.

Temperate Viruses and the Lysogenic Cycle

Temperate viruses are those that reproduce without killing their host cell. Typically they reproduce in two ways: through the lytic cycle and the lysogenic cycle. In the lysogenic cycle, the phage's DNA recombines with the bacterial chromosome. Once it has inserted itself, it is known as a prophage. A host cell that carries a prophage has the potential to lyse, thus it is called a lysogenic cell. The image above illustrates both the lytic and lysogenic cycles of a bacteriophage.

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