Lysogenic conversion is a fascinating biological process where a bacterium gains new characteristics after its genetic material integrates with the DNA of a bacteriophage. Bacteriophages, often called phages, are viruses that specifically infect bacteria. This unique interaction can profoundly impact bacterial behavior, enabling bacteria to acquire traits that enhance their survival or virulence.
How Lysogenic Conversion Works
Lysogenic conversion begins when a temperate bacteriophage infects a bacterium. Unlike a lytic phage that immediately destroys its host, a temperate phage integrates its viral DNA, known as a prophage, into the bacterial chromosome. This integrated prophage generally remains dormant, replicating along with the bacterial DNA each time the bacterium divides. The bacterium, now called a lysogen, continues to live and reproduce normally, carrying the phage’s genetic information without immediate harm. This process differentiates from the lytic cycle, where the phage immediately takes over the bacterial machinery to produce new viruses, ultimately leading to the host cell’s destruction.
Key Triggers for Lysogenic Conversion
Lysogenic conversion is observed through the expression of new traits from integrated prophage genes. While the prophage is dormant, environmental stressors can induce these genes, leading to observable changes in the bacterium. A common trigger is DNA damage within the bacterial cell, which activates the bacterial SOS response, leading to prophage DNA excision and gene expression. Environmental factors such as UV light, certain chemicals, or even antibiotics can induce this SOS response.
Other triggers include nutrient availability, acidity, osmolarity, and temperature fluctuations, which can influence prophage stability and lead to gene expression. Quorum sensing, a communication system where bacteria coordinate behavior based on population density, can also play a role in inducing prophage gene expression. For instance, some phages can sense bacterial population density through signaling molecules and switch from a dormant state to actively expressing their genes. The presence of specific host factors or even other phages can also influence whether a prophage’s genes are expressed, leading to a phenotypic change in the bacterium.
Why Lysogenic Conversion Matters
Lysogenic conversion has significant practical implications, particularly concerning bacterial virulence and evolution. Many disease-causing bacteria acquire genes for toxins or other virulence factors directly from prophages. For example, the bacteria responsible for diphtheria, cholera, and Shiga toxin-producing E. coli become harmful only after being infected by specific phages that carry the genes for these toxins. This process allows non-virulent bacteria to transform into potent pathogens, influencing the spread and severity of infectious diseases.
Beyond virulence, lysogenic conversion contributes to bacterial survival and adaptation. Prophage genes can provide bacteria with new adaptive advantages, such as altering their surface structures to evade the host’s immune system or resist other phages. While phages are not the primary mechanism for antibiotic resistance gene transfer, lysogenic phages can carry genes that contribute to or modify bacterial resistance mechanisms, indirectly impacting antibiotic effectiveness. This ability to acquire new traits through prophages is a significant driver of bacterial diversity and evolution.