Phage conversion describes a biological process where bacteria acquire new characteristics after infection by a specific virus called a bacteriophage. This leads to modifications in the bacterial cell, causing it to express traits it did not possess before. The newly gained features can influence the bacterium’s biology, altering its behavior and capabilities. This genetic exchange contributes to bacterial diversity.
Understanding Bacteriophages and Their Interaction with Bacteria
Bacteriophages, often simply called phages, are viruses that specifically infect bacteria. Like all viruses, they cannot reproduce on their own and must use a host cell’s machinery to create new viral particles. When a bacteriophage encounters a bacterium, it injects its genetic material, which can be DNA or RNA, into the bacterial cell.
Phages exhibit two main life cycles within their bacterial hosts: the lytic cycle and the lysogenic cycle. In the lytic cycle, the phage takes over the host cell’s machinery to rapidly produce new viruses, causing the bacterium to burst open and release the progeny phages.
In contrast, the lysogenic cycle allows the phage to coexist with the bacterium without immediately destroying it. Here, the phage’s genetic material integrates into the bacterium’s DNA, becoming a prophage. This integrated prophage DNA is copied along with the bacterial chromosome during division, ensuring all subsequent bacterial generations carry the phage’s genetic information. This dormant state can persist for many generations, but under certain conditions, such as environmental stress, the prophage can excise itself and initiate the lytic cycle.
How Phage Conversion Alters Bacterial Traits
Phage conversion occurs when a bacterium is in the lysogenic state, carrying a prophage integrated into its genome. The integrated prophage DNA carries its own genes. When these phage genes are expressed by the bacterial host, they confer new characteristics or traits to the bacterium.
These newly acquired traits can be diverse, ranging from toxin production to modifications in the bacterium’s surface structures. For example, a prophage might carry genes that enable the bacterium to synthesize specific enzymes or proteins. These new proteins can then alter how the bacterium interacts with its environment or other organisms.
The integration of phage DNA can also serve as an anchor point for bacterial genome rearrangements, contributing to genetic diversity. In some instances, the prophage’s presence can disrupt existing bacterial genes, leading to the inactivation of certain functions and the emergence of new characteristics. This gene transfer through phages is a form of horizontal gene transfer, allowing bacteria to acquire new genetic information.
Impact on Bacterial Virulence and Human Disease
The acquisition of new traits through phage conversion significantly impacts bacterial virulence and human disease. Many bacterial pathogens become more dangerous due to genes acquired from prophages. These genes often encode toxins or other molecules that enhance the bacterium’s ability to cause illness.
A well-known example is Corynebacterium diphtheriae, the bacterium responsible for diphtheria. This bacterium only causes the severe symptoms of diphtheria if it carries a specific prophage that encodes the diphtheria toxin. Without this prophage, the bacterium is non-toxic. Another instance is Vibrio cholerae, the causative agent of cholera, which acquires its cholera toxin genes from a filamentous phage called CTX𝜑. The expression of these toxins by the infected bacteria leads to the characteristic severe diarrheal disease.
Phage conversion can also influence other aspects of bacterial pathogenicity, such as resistance to host immune defenses, the ability to form biofilms, or antibiotic resistance. For example, some prophages carry genes that confer antibiotic resistance to their bacterial hosts, allowing them to survive in the presence of certain medications. The transfer of such virulence factors through prophages impacts the evolution of bacterial strains and public health.