Lysogenic conversion is a biological process where a virus, known as a bacteriophage, integrates its genetic material into the DNA of a bacterium. This integration establishes a stable, long-term relationship between the virus and its bacterial host. Félix d’Hérelle first recognized this phenomenon in the early 20th century. This process is a powerful mechanism for genetic exchange within the microbial world, influencing bacterial characteristics.
The Process of Lysogenic Conversion
A bacteriophage is a virus that infects bacteria, utilizing the bacterial cell’s machinery to reproduce. Some bacteriophages are “temperate phages” because they can either replicate immediately and destroy the host cell (lytic cycle) or integrate their genetic material into the host genome, entering a dormant state called lysogeny. Lysogenic conversion begins when a temperate phage attaches to a susceptible bacterial cell and injects its genetic material, typically DNA, into the bacterial cytoplasm.
Following injection, the phage DNA does not immediately initiate a destructive replication cycle. Instead, it undergoes a precise integration event, inserting itself into a specific site on the bacterial chromosome. Once integrated, the phage DNA is called a “prophage,” becoming a silent part of the bacterial genome. This prophage remains dormant within the bacterium, replicating along with the host’s DNA each time the bacterium divides.
As the bacterium multiplies, the prophage is copied and passed down to all daughter cells, ensuring its persistence within the bacterial lineage. The maintenance of this dormant state is regulated by specific phage-encoded proteins, such as repressor proteins. These repressors bind to particular DNA sequences on the prophage, preventing the expression of most phage genes that would otherwise trigger the lytic cycle and destroy the host cell. This regulatory control allows the phage to coexist peacefully with its bacterial host.
New Traits for Bacteria
The presence of an integrated prophage can profoundly alter the characteristics of the bacterial host through the expression of phage genes. This allows bacteria to acquire new traits that can significantly impact their survival and interactions within their environment. A recognized consequence is an increase in bacterial pathogenicity or virulence, which is the ability to cause disease.
Many pathogenic bacteria gain their disease-causing capabilities from prophage-encoded genes that produce toxins. For instance, the bacterium Vibrio cholerae, responsible for cholera, acquires the gene for cholera toxin from a temperate bacteriophage known as CTXφ. Similarly, Corynebacterium diphtheriae, the causative agent of diphtheria, produces diphtheria toxin only when it is lysogenized by a specific phage carrying the toxin gene. Escherichia coli strains linked to severe food poisoning often produce Shiga toxins, which are also encoded by genes introduced by prophages.
Beyond toxin production, prophages can also alter the surface properties of bacteria. Genes carried by phages can modify components of the bacterial cell wall or outer membrane, such as lipopolysaccharides or capsular polysaccharides. These changes can affect how the bacterium interacts with its environment, including its ability to adhere to host tissues or evade the host immune system. Such modifications can influence bacterial colonization and persistence within a host.
Some prophages can also introduce genes that lead to subtle metabolic alterations within the bacterial host. While less commonly highlighted than virulence factors, these changes might affect the bacterium’s ability to utilize certain nutrients or its overall growth characteristics. These new traits allow the bacterium to express proteins and functions it did not possess before.
Broader Implications
Lysogenic conversion holds broader significance, extending beyond the immediate interaction between a single phage and bacterium. It serves as a powerful mechanism for horizontal gene transfer, allowing genetic material to move between different bacterial cells, even across species boundaries. This genetic exchange drives bacterial evolution and diversification, enabling bacterial populations to adapt to new environmental pressures, including antibiotics or host immune responses.
The process plays a considerable role in human health and disease. Understanding lysogenic conversion is key to comprehending bacterial pathogenesis, as many severe bacterial infections are caused by strains that have acquired virulence factors through this mechanism. While not directly conferring antibiotic resistance, the ability of phages to transfer genes contributes to bacterial genetic adaptability, which can indirectly facilitate the spread or emergence of resistance genes. The emergence of new bacterial diseases or more virulent strains often involves lysogenic conversion events.
Insights gained from studying lysogenic conversion have practical applications in biotechnology and research. The precise integration and regulation mechanisms of temperate phages are being explored for their potential in developing new antimicrobial strategies, such as phage therapy, which uses phages to target and kill specific pathogenic bacteria. The ability to integrate foreign DNA into a host genome also makes temperate phages valuable tools in genetic engineering, allowing researchers to introduce desired genes into bacterial cells for various scientific and industrial purposes.