Transduction in Bacteria

Transduction definition in a simple manner is a method of transfer of foreign DNA into a bacterial cell. If you are asked to define transduction then you can elaborate the transduction definition by saying that transduction is the process of transferring foreign DNA into a bacterial cell using a virus (more specifically viral vector). 

Transduction, as hinted from the definition, is one of the most commonly applied techniques in scientific research for understanding genetic transfers and gene editing because of its property of transferability. Other such methods are transformation and conjugation. An important point for you to note is that the viruses are required to infect the bacteria and the viruses that infect the bacteria are known as bacteriophages. 

Transduction by Bacteriophages

The process of transduction occurs through two kinds of life-cycles of the bacteriophages.

The life-cycle of the bacteriophage sets the stage for the transduction definition to become a reality. These life-cycles are namely, the lytic cycle and the lysogenic cycle. Even though the production of viral particles happens by using bacterial machinery there is a significant difference between the two. The two processes are explained below:

Lytic cycle: In this cycle, the bacteriophage (virus) infects the bacteria and exploits the bacterial machinery for replication i.e. making more copies of the viral genetic material, transcription i.e. production of RNA from DNA, and translation for production of viral proteins. In this cycle, the virus exists as a separate body in the bacterial cytoplasm and makes use of the required machinery for its own purposes separately from the processes that the bacterial genetic material undergoes. Viruses following this type of life-cycle are called virulent phages. 

Lysogenic cycle: Unlike the lytic cycle, the main characteristic of the lysogenic cycle is that it does not exist as a separate body in the bacteria, rather the viral genetic material fuses with the bacterial genetic material i.e. the viral DNA integrates into the bacterial DNA. This integrated viral genome into the bacterial genome is termed a prophage. Since the viral genome is integrated into the bacterial chromosome it is damage proof from DNAse, an enzyme that cuts the foreign genetic substance to protect the bacteria. Also, because of this, the viral set of genes remain intact over various generations of the bacteria and remain dormant. Only when the prophage is induced i.e. prompted to activate by external factors usually the UV light, the prophage is cut from the bacterial genome and the prophage starts the lytic cycle of viral reproduction. 

In the end, in both cycles, the cells are lysed by the rupturing of the membrane and new viral particles are released. These cycles, also termed reproductive cycles, lead to different types of transduction. In transduction, these new virions are transferred to another bacterial cell where they might be inserted into the new bacterial genome or remain as an extrachromosomal body in the new host bacterium. This fate is unpredictable but it underlines one method which leads to the failure of antibiotic drugs because of the transfer of antibiotic-resistance genes in between different bacteria.

Types of Transduction

There are commonly known two types of transduction that involve the transfer of a foreign genome into another bacterial cell. They are listed below.

(I) Generalised Transduction - The phage carries any random part of bacterial DNA.

(II) Specialised Transduction - The phage carries a specific part of the bacterial DNA.

These two kinds of transduction are explained as follows.

Generalised Transduction

In this type of transduction, a random segment of the bacterial genetic material is packed along with the viral genetic material. This bacterial DNA can be packed into the virus capsule either as a part of the viral genome or in the head of the capsule. When a virus replicates using head packaging then the bacterial genome is fit into the head of the virus capsule. Or else when there is a spare capacity then the bacterial genome is incorporated into the new virion. Later on, the new virion that is produced by either the lytic cycle or the lysogenic cycle is used for infecting another bacterial cell and thus there is gene transfer as per transduction definition.

Specialised Transduction

In this type of transduction, a specific segment of the bacterial genetic material is packed along with the viral genome. This kind of transduction occurs only through the lysogenic life-cycle. The prophage can be used for being integrated into the bacterial chromosome in a targeted manner. This prophage when excised brings along with it the extending bacterial genomic region as well into the new virion. Since, the prophage was attached at a specific part of the genome it can be employed to use a particular part of the bacterial genome for the transduction. Thus, in the end, the gene transfer is carried out by inserting the new virion into another bacterial cell.

There also exists another type of transduction called the lateral transduction but so far you can only find it working in Staphylococcus aureus and hence is not yet common. In this kind, very long fragments of DNA are inserted into the bacterium. Lateral transduction is a type of transduction in which the prophage starts to replicate before being cut or excised from the bacterial genome. This ensures a higher efficiency and packaging of several bacterial genes extending to kilobases into new virus particles and can be further inserted into a different bacterium.

Thus different types of transduction in bacteria provide not only a natural method for the virus to reproduce but also for utilisation in scientific studies and in medical applications such as gene therapy. The entire process of transduction is shown below in the given diagram.

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