In bacteria, transduction is the process of DNA transfer from one bacterium to another, mediated by a virus. The technique of transduction is often employed by molecular biologists to introduce foreign DNA into a bacterium using a viral vector. Transduction is one of a few processes, including conjugation and transformation, by which foreign DNA may enter a bacterial cell. The recipient cell of a transduction process is called a transductant.

Transduction occurs because an invading bacteriophage hijacks the bacterial chromosome, either to remain dormant during the lysogenic cycle or to create progeny during the lytic cycle. Recombination between the bacterial and viral genomes sometimes causes the virus to carry with it the DNA of previous hosts, which can then be introduced into new bacterial hosts later down the line.

There are different types of transduction: notably generalised and specialised transduction.

Generalised transduction may occur in one of two ways:
  • Chromosomal DNA from the bacterium is accidentally encapsulated in the viral capsid as the virus creates its new progeny.
  • In headful packaging, spare capacity in the viral genome is made up using bacterial DNA, a process mediated by the viral packaging mechanism.

In both cases, the recombined bacterial DNA may enter a new bacterial host when the virus moves on to infect another bacterium. In essence, transduction is thus a sexual process between bacteria that is mediated by viruses.

When the bacterial DNA is inserted into a new bacterium by viral infection, it may fall into one of three (random) fates:

1. The DNA may be absored, degraded and recycled for spare parts (i.e. nucleotides, etc)
2. If the DNA was borne of a plasmid, then it will re-circularise a form a plasmid again in its new host
3. If the DNA has a homologous region in the chromosome of its new host, then recombination between the foreign DNA and the chromosome will occur

Lysogenic generalised transduction could also occur in two possible scenarios: either where the viral genome has a random site of integration, and an erroneous excision process leads it to carry some bacterial DNA with it, or where the viral genome is activated into the lytic cycle and random incorporation of bacterial DNA subsequently occurs.

Specialised transduction refers specifically to the transfer of DNA that is adjacent to the site where a prophage inserts itself into the bacterial genome. This is caused by incorrect removal of the lysogenic phage when it is converted to the lytic cycle, such that it carries some bacterial DNA with it. The fate of such DNA may be absorption and degradation, or homologous recombination, with subsequent hosts as described above, or alternatively the bacterial DNA may behave as if it is part of the virus, causing it to be replicated multiple times with the phage genome.