Transduction in Bacteria

Bacteria need to be able to adapt to the changing and challenging environments in which they live to survive and multiply. One of the genetic processes by which they do this is transduction.

In this article, we will discuss the mechanisms of bacterial transduction and how scientists can utilize it to manipulate bacterial genomes for a wide variety of purposes.

What is bacterial transduction?

Transduction is one of three forms of horizontal gene transfer (HGT) in bacteria, along with transformation and conjugation. In transduction, small sections of exogenous DNA are introduced into a bacterial cell via a bacterial virus, more commonly known as a bacteriophage. The process was first identified in Salmonella by Norton Zinder and Joshua Lederberg in the early 1950s and has been studied extensively since then.¹

When viruses replicate within a cell they can accidentally incorporate host DNA into their genomes, which can then be transferred to a new host. The new host cell consequently acquires novel DNA, which can include virulence factors, antibiotic resistance genes etc. This process plays a vital role in the evolution of bacteria and adaption to changing environments and external threats.

There are two main processes by which transduction occurs: generalized and specialized (explained below in more detail). Recently, Chen et al discovered a third mechanism in Staphylococcus aureus,² in which large sections of host DNA (several hundred kilobases) are packaged into phage heads, which can then be transferred to a new host.

Although far more commonly associated with prokaryotes, transduction does occur in eukaryotic systems as well, often associated with retroviruses. Here, host DNA fragments are incorporated into the viral genome and can then be subsequently passed onto a new host. Researchers utilize this process in generating genetic therapies,³ generating transgenic organisms and general research.

Transduction biology – Understanding the process in bacteria, including bacterial transduction diagrams

There are two main types of bacterial transduction: generalized and specialized.⁴

Generalized transduction

Generalized transduction occurs when random sections of host DNA are incorporated into bacteriophage during the lytic cycle in the process of making more bacteriophage particles, as shown in Figure 1.

Initially, a bacteriophage binds to the bacterial cell surface via specific receptor molecules (e.g. lipopolysaccharides, teichoic acids and proteins) and subsequently injects its DNA into the bacterial cell’s cytoplasm (Figure 1A). The bacteriophage then takes over the host’s cellular machinery to replicate its genetic material and the protein components required to make new bacteriophage particles. During this process, the bacterial host’s DNA may become fragmented (Figure 1B). New bacteriophage particles are assembled, some of which can contain host DNA, and are subsequently released when the bacterial cell lyses (Figure 1C). A bacteriophage carrying host DNA then binds to a new host cell and injects its DNA package, including the DNA from the original host (Figure 1D). If the original host DNA matches a region of the new host, homologous recombination may occur and the new host can acquire novel genetic material (Figure 1E). If the original host DNA was in a plasmid format, then these generally re-circularize and remain in the cytoplasm.

Figure 1: Diagram showing the process of generalized bacterial transduction. A) Bacteriophage binding and infection. B) Bacteriophage replication. C) Cell lysis and bacteriophage release. D) Bacteriophage carrying host DNA infects a new host cell. E) Recombination and insertion of original host DNA into the new host’s chromosome. Credit: Technology Networks.

Specialized transduction

In specialized transduction, a restricted group of host genes are transferred to a new host by temperate (lysogenic) bacteriophage. A bacteriophage binds (Figure 2A) and infects (Figure 2B) a host bacterium. Instead of initiating the lytic cycle, the bacteriophage DNA integrates into the host genome in what is known as the lysogenic cycle (Figure 2C), becoming a prophage. The prophage may remain integrated for prolonged periods before becoming induced (usually by some sort of stress factor, e.g. oxidative, temperature nutritional and UV light) to excise from the chromosome. Sometimes this process does not happen accurately and host genes adjacent to the bacteriophage are excised along with bacteriophage DNA (Figure 2D). Bacteriophage replication is initiated (Figure 2E) and bacteriophage are packaged and released from the bacterial cell by lysis (Figure 2F). These released bacteriophage are then capable of infecting a new host (Figure 2G) and transferring the original host DNA to the new host (Figure 2H).

Figure 2: A diagram showing the process of specialized bacterial transduction. A) Bacteriophage binding. B) Bacteriophage infection. C) Bacteriophage DNA integration. D) Bacteriophage de-integration. E) Bacteriophage replication. F) Bacteriophage packaging and release. G) Bacteriophage infects a new host. H) Stable transfer of DNA into a new host. Credit: Technology Networks.

Generalized transduction vs specialized transduction

Specialized and generalized transduction are similar in that they are both mechanisms of HGT that generate genetic diversity and are mediated by bacteriophages. However, there are also some key differences, which are summarized in Table 1.

Table 1: Differences between generalized and specialized transduction.

Transformation vs transduction

While both transformation and transduction are examples of HGT processes, the key difference is that transformation involves the uptake of free DNA from the environment. In contrast, transduction consists of the transfer of DNA mediated by bacteriophage.

How is viral transduction in bacteria harnessed in science?

Scientists have used bacteriophage transduction for many years to study and manipulate bacteria. For example, it has played a role in bacteriophage therapy to combat infectious disease,^5,6 many aspects of basic research, industrial, biotechnological and pharmaceutical applications,^7,8,9 cancer gene therapy¹⁰ and phage display techniques.¹¹

In conclusion, bacterial transduction is one of the processes that contribute to genetic diversity and the spread of traits, such as antibiotic resistance and virulence factors among bacterial populations. It has been utilized by scientists in a variety of basic research applications and to develop therapeutic, biotechnological and pharmacological agents.