Horizontal gene transfer (HGT) describes the movement of genetic material between organisms through means other than parent-to-offspring inheritance. This process is prevalent in the microbial world, allowing bacteria to acquire new traits from unrelated organisms. Transduction represents a distinct mechanism of HGT, specifically involving viruses that infect bacteria, known as bacteriophages, as carriers of genetic information. This viral mediation shapes bacterial evolution and adaptation.
Bacteriophages as Genetic Messengers
Bacteriophages are viruses that exclusively target and infect bacterial cells. These viruses consist of a protein coat, called a capsid, enclosing their genetic material, which can be DNA or RNA. To initiate infection, a phage attaches to specific receptors on the bacterial cell surface and injects its genetic material into the host.
Once inside the bacterium, bacteriophages follow one of two life cycles: lytic or lysogenic. In the lytic cycle, the phage takes over the host cell’s machinery to produce many new phage particles, ultimately leading to the lysis of the bacterial cell and the release of new viruses. This rapid replication and destruction of the host characterize virulent phages.
The lysogenic cycle involves the integration of the phage’s genetic material, now called a prophage, into the bacterial host’s chromosome. In this dormant state, the prophage is replicated along with the bacterial DNA each time the cell divides, without immediately harming the host. Environmental stressors, such as certain chemicals or UV radiation, can induce the prophage to excise from the bacterial chromosome and enter the lytic cycle, leading to the production of new phages and cell lysis.
The Mechanics of Transduction
Transduction, the transfer of bacterial DNA by bacteriophages, occurs through two primary mechanisms: generalized transduction and specialized transduction. Both processes involve a phage picking up bacterial genetic material and delivering it to a new bacterial host.
Generalized transduction happens during the lytic cycle of a bacteriophage. When a virulent phage infects a bacterium, it replicates its own DNA and produces new viral proteins, often fragmenting the host bacterium’s DNA in the process. During the assembly of new phage particles, some phages may mistakenly package random fragments of the bacterial DNA into their capsids instead of or in addition to their own viral genome.
These phage particles, now containing bacterial DNA, are called transducing particles. When such a particle infects a new bacterium, it injects the carried bacterial DNA into the recipient cell. This transferred DNA can then integrate into the recipient bacterium’s chromosome through recombination.
Specialized transduction, in contrast, involves lysogenic phages and occurs when a prophage excises imperfectly from the bacterial chromosome. During the lysogenic cycle, the phage DNA integrates into a specific site on the host’s genome. If this integrated prophage is later induced to enter the lytic cycle, it normally excises precisely from the chromosome.
However, occasionally, the excision is imprecise, causing the prophage to take a small, specific piece of the adjacent bacterial DNA with it. This combined phage-bacterial DNA is then packaged into new phage particles. When these phages infect a new bacterium, they introduce both their viral DNA and the specific bacterial genes from the previous host, which can then integrate into the new host’s genome.
Broader Implications of Transduction
Transduction impacts the microbial world, influencing bacterial evolution, the spread of new traits, and human health. It allows bacteria to rapidly acquire new genetic information, fostering diverse and adaptable populations.
Its contribution to the spread of antibiotic resistance genes among bacteria is a significant implication of transduction. Phages can pick up genes that confer resistance to antibiotics from one bacterium and transfer them to another, even across different bacterial species. For example, methicillin-resistant Staphylococcus aureus (MRSA) can acquire resistance through phage-mediated transduction of the mecA gene.
Beyond antibiotic resistance, transduction also plays a role in bacterial evolution and adaptation by facilitating the transfer of virulence factors. These are genes that enable bacteria to cause disease or evade the host’s immune system, such as those coding for toxins or specific adhesion proteins. The acquisition of such genes through transduction can transform a harmless bacterium into a pathogenic one or enhance the disease-causing ability of an existing pathogen.
In the field of biotechnology and genetic engineering, the principles of transduction are harnessed as a tool for gene delivery. Scientists can engineer bacteriophages or other viral vectors to carry specific genes into bacterial or even mammalian cells for research purposes or therapeutic applications. This controlled gene transfer is used for gene editing, genetic mapping, and studying gene function, with potential for gene therapy to treat genetic disorders.