Transduction is a biological process where genetic material is transferred from one bacterium to another through a virus, specifically a bacteriophage. This method represents a form of horizontal gene transfer, allowing bacteria to acquire new genetic traits. Unlike other gene transfer methods, transduction does not require direct cell-to-cell contact, relying instead on these viral intermediaries. The genetic information transferred can include genes that provide beneficial characteristics to the recipient bacterium.
Bacteriophages The Viral Agents
Bacteriophages, or phages, are viruses that specifically infect bacteria. These microscopic entities consist of a protein coat, known as a capsid, which encloses their genetic material, typically DNA or RNA. Phages have a distinct structure with a polyhedral head, a short collar, and a helical tail. The tail often includes tail fibers and a basal plate, which help the phage attach to specific receptors on the surface of a bacterial cell during infection.
Once attached, the phage injects its genetic material into the bacterial cytoplasm. Following injection, bacteriophages can undergo one of two primary life cycles: the lytic cycle or the lysogenic cycle. In the lytic cycle, the phage takes over the host cell’s machinery to replicate its genetic material and produce new phages. This process culminates in the lysis of the bacterial cell, releasing hundreds of new phages to infect other bacteria.
In contrast, the lysogenic cycle involves the integration of the phage DNA, now called a prophage, into the bacterial chromosome. In this state, the prophage remains dormant and replicates along with the host bacterium’s DNA during cell division, without immediately harming the host. The bacterial cell containing the prophage is then termed a lysogen. Under environmental stresses, the prophage can excise from the bacterial chromosome and enter the lytic cycle, leading to the production of new phages and eventual cell lysis.
Mechanisms of Transduction
Transduction occurs through two main mechanisms: generalized transduction and specialized transduction. Generalized transduction can transfer any segment of bacterial DNA and occurs during the lytic cycle. During a lytic infection, the bacteriophage causes the fragmentation of the bacterial DNA. As new phage particles are assembled, fragments of the host bacterial DNA are sometimes accidentally packaged into the phage capsid instead of the viral genome.
These newly formed phage particles, now carrying bacterial DNA, are called transducing phages. When these transducing phages infect a new recipient bacterium, they inject the bacterial DNA into the new cell. The transferred bacterial DNA can then undergo recombination with the recipient bacterium’s chromosome, potentially integrating new genes into its genome. This process is considered a relatively rare event, occurring in approximately 1 out of 11,000 phages.
Specialized transduction involves the transfer of only specific bacterial genes and is associated with the lysogenic cycle. In this process, the phage’s DNA integrates into the bacterial chromosome at a particular site, becoming a prophage. When the prophage excises from the bacterial chromosome to re-enter the lytic cycle, an error can occur. This imprecise excision means that the prophage takes with it a small, specific piece of the adjacent bacterial DNA.
The excised DNA is then packaged into new phage particles. When these specialized transducing phages infect a new bacterial host, they introduce this specific bacterial genetic material into the recipient cell. The transferred genes are those located immediately next to the prophage integration site. This mechanism is highly efficient for transferring specific genes and is frequently studied with phages like lambda phage in Escherichia coli.
Impact of Transduction
Transduction influences bacterial evolution and genetic diversity by enabling the transfer of genetic material. This horizontal gene transfer allows bacteria to acquire new traits, rather than relying solely on mutations or vertical inheritance from parent to offspring. The acquisition of novel genes can provide bacteria with adaptive advantages in various environments.
Transduction plays a role in the spread of antibiotic resistance genes among bacterial populations. Bacteriophages can pick up genes that confer resistance to antibiotics from one bacterium and transfer them to another, even across different bacterial species. This mechanism contributes to the growing challenge of multidrug-resistant bacteria, as resistance genes can disseminate widely. For instance, methicillin-resistant Staphylococcus aureus (MRSA) has been shown to acquire resistance genes through phage-mediated transduction.