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Lysogeny – Definition, Mechanism and Medical Significance
Lysogeny is a mechanism by which a virus integrates its genetic material into the genome of a host cell without immediately destroying it.
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Lysogeny is a mechanism by which a virus integrates its genetic material into the genome of a host cell without immediately destroying it.
What is Lysogeny?
Lysogeny describes an infection cycle used by certain viruses – most notably bacteriophages (viruses that infect bacteria) – in which the viral genetic material is integrated into the chromosome of the host cell rather than immediately destroying it. The integrated viral genome is referred to as a prophage. The infected host cell, known as a lysogen, continues to divide normally, passing the prophage on to all daughter cells during replication.
Basic Principle and Mechanism
The lysogenic cycle stands in contrast to the lytic cycle, in which a virus actively hijacks the host cell, replicates massively, and ultimately destroys the cell through lysis (cell rupture). In lysogeny, the viral genome remains dormant (latent) within the genome of the host cell.
Steps of the Lysogenic Cycle
- Infection: A bacteriophage injects its DNA into a bacterial cell.
- Integration: The viral DNA is inserted into the bacterial chromosome with the help of viral enzymes (e.g., integrase).
- Latency phase: The prophage is replicated along with the bacterial DNA and passed on to daughter cells, without producing new viral particles.
- Induction: Under certain stress conditions (e.g., UV radiation, chemical mutagens, DNA damage), the prophage can excise itself from the chromosome and switch to the lytic cycle.
- Lytic outbreak: After induction, new viral particles are produced, the host cell is destroyed, and viruses are released.
Medical and Microbiological Significance
Lysogeny is not merely an academic concept – it has considerable medical relevance:
- Virulence transfer: Many pathogens acquire their disease-causing properties through lysogenic phages. For example, Corynebacterium diphtheriae only produces the diphtheria toxin when infected by a prophage. The same applies to Vibrio cholerae (cholera toxin) and certain strains of Escherichia coli.
- Horizontal gene transfer: Lysogeny facilitates the transfer of genes between bacteria, which can contribute to the spread of antibiotic resistance genes.
- Biotechnology: Principles of lysogeny are applied in genetic engineering, such as in the development of viral vectors for gene transfer.
Lysogeny in Humans – Retroviruses
An analogous principle exists in retroviruses that infect human cells. The most prominent example is HIV (Human Immunodeficiency Virus): after infection, HIV converts its RNA into DNA via the enzyme reverse transcriptase and integrates it as a provirus into the human genome. This state is functionally equivalent to lysogeny in bacteriophages. The provirus can remain latent and is passed on during cell division – a central challenge in HIV therapy, since latently infected cells form a viral reservoir that cannot be fully eliminated even by modern antiretroviral therapy.
Diagnosis and Detection
Lysogenic infections are often difficult to detect because no active viral replication is occurring. Detection methods include:
- PCR (Polymerase Chain Reaction): Detection of integrated viral DNA within the host genome.
- Genome sequencing: Comprehensive analysis of the host genome for embedded prophages.
- Induction experiments: Deliberate activation of the prophage under laboratory conditions to detect viral particles.
Clinical Relevance
Understanding lysogeny is fundamental to infectious medicine, as lysogenic phages can profoundly influence the properties of pathogens. Disruptions to the lysogenic balance – for example, by antibiotics that can act as inducing stressors – may enhance the virulence of bacterial infections. This has direct implications for the treatment of certain bacterial diseases.
References
- Flint S.J. et al. – Principles of Virology. 4th edition, ASM Press, 2015.
- Lwoff A. – Lysogeny. Bacteriological Reviews, 17(4):269–337, 1953. PubMed PMID: 13105613.
- World Health Organization (WHO) – Antimicrobial Resistance: Global Report on Surveillance, 2014. Available at: https://www.who.int
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