The third mode of bacterial gene transfer is transduction, which is mediated by viruses. It is a frequent mode of horizontal gene transfer in nature. Indeed evidence suggests that the number of genes moved by marine viruses from one host cell to another is huge (perhaps 1024 per year). Furthermore, viruses in marine environments and hot springs move genes between organisms in all three domains of life.

Virus particles are structurally simple, often composed of just a nucleic acid genome protected by a protein coat called the capsid. Viruses are unable to multiply autonomously. Instead, they infect and take control of a host cell, forcing the host to make many copies of the virus. Viruses that infect bacteria are called bacteriophages, or phages for short. Virulent bacteriophages multiply in their bacterial host immediately after entry. After the progeny phage particles reach a certain number, they cause the host to lyse, so they can be released and infect new host cells. Thus this process is called
the lytic cycle.
Temperate bacteriophages, on the other hand, do not immediately kill their host. Many temperate phages enter the host bacterium and insert their genomes into the bacterial chromosome. The inserted viral genome is called a prophage. The host bacterium is unharmed by this, and the phage genome is passively replicated as the host cell's genome is replicated. The relationship between these viruses and their host is called lysogeny, and bacteria that have been lysogenized are called lysogens. Temperate phages can remain inactive in their hosts for many generations. However, they can be induced to switch to a lytic cycle under certain conditions, including UV irradiation. When this occurs, the prophage is excised from the bacterial genome and the lytic cycle proceeds.

Transduction is the transfer of bacterial or archaeal genes by virus particles. It is important to understand that host genes are packaged in the virus particle because of errors made during the virus's life cycle. The virion containing these genes then transfers them to a recipient cell. Two kinds of bacterial transduction have been described: generalized and specialized.

No comments