Transduction in microbiology describes a process where genetic material is transferred from one bacterium to another by a virus. This mechanism allows bacteria to acquire new genes, which can lead to altered traits and adaptations. It represents a form of horizontal gene transfer, meaning the transfer of genetic material between organisms without involving reproduction. This process is a fundamental aspect of bacterial genetics, influencing their evolution and diversity within microbial communities.
The Viral Messenger: Bacteriophages
The agents responsible for transduction are bacteriophages, often simply called phages, which are viruses that specifically infect bacteria. These viruses are composed of a protein coat, known as a capsid, that encloses their genetic material, which can be either DNA or RNA. Many phages, such as the T-even phages, have a distinct structure with a polyhedral head, a short collar, and a helical tail that aids in attachment to bacterial cells.
Bacteriophages exhibit two primary life cycles: lytic and lysogenic. In the lytic cycle, the phage injects its genetic material into the host bacterium, takes over the cell’s machinery to produce new phage particles, and then causes the host cell to burst, releasing the progeny phages. Conversely, in the lysogenic cycle, the phage DNA integrates into the bacterial chromosome, becoming a prophage, and replicates along with the host DNA without immediately destroying the cell. The prophage can remain dormant for many generations until environmental cues, such as UV light, trigger it to excise and enter the lytic cycle.
Mechanisms of Transduction
Transduction occurs through two main mechanisms: generalized transduction and specialized transduction. Generalized transduction involves the accidental packaging of any random fragment of bacterial DNA into a phage capsid during the phage assembly process. During the lytic cycle, the bacterial chromosome is broken down into smaller pieces. Some of these bacterial DNA fragments are mistakenly enclosed within newly formed phage particles.
These “transducing phages” are defective because they lack the complete viral genome. When such a transducing phage infects a new recipient bacterium, it injects the bacterial DNA it carries into the new host cell. The transferred bacterial DNA may then integrate into the recipient bacterium’s chromosome through homologous recombination, or it can remain as a separate plasmid, potentially giving the new host novel genetic traits. This process allows for the transfer of virtually any gene, though it occurs at a relatively low frequency.
Specialized transduction, in contrast, involves the transfer of only specific bacterial genes that are located adjacent to the integrated prophage in the bacterial chromosome. This mechanism exclusively occurs with temperate phages. After the phage DNA integrates into the bacterial chromosome at a specific attachment site, imprecise excision of the prophage can occur. During this rare event, a portion of the bacterial chromosome flanking the prophage is accidentally excised along with the phage DNA.
The resulting phage particle, containing both phage and specific bacterial genes, is then packaged and released. When this specialized transducing phage infects a new bacterium, it injects the combined DNA. The donor bacterial DNA, along with the phage DNA, can then integrate into the recipient’s chromosome, often at the same specific site.
The Far-Reaching Impact of Transduction
Transduction plays a significant role in horizontal gene transfer among bacteria, contributing substantially to their genetic diversity and evolution. Transduction allows bacteria to acquire new traits, including those that enhance their survival in various environments. For instance, transduction can facilitate the spread of virulence factors, genes that enable bacteria to cause disease. It can transform harmless bacterial strains into more pathogenic ones, as seen with Corynebacterium diphtheriae and Vibrio cholerae.
A particularly concerning implication of transduction is its contribution to the dissemination of antibiotic resistance genes among bacterial populations. Phages can pick up and transfer genes that confer antibiotic resistance from one bacterium to another, even across different bacterial species. This process reduces the effectiveness of existing antibiotics, making infections harder to treat and leading to increased illness and mortality. The Centers for Disease Control and Prevention (CDC) estimates that antibiotic-resistant infections cause over 23,000 deaths annually in the United States alone.
Beyond its natural impact, transduction has found applications in biotechnology and genetic engineering. Researchers use transduction as a tool to introduce foreign DNA into host cells, both bacterial and mammalian. This capability is harnessed for various purposes, such as producing therapeutic proteins like insulin or human growth hormone in bacteria, developing vaccine strains, and exploring gene therapy to correct genetic defects. The precision of specialized transduction makes it particularly useful for targeted gene delivery in laboratory settings.