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Prophage – Definition, Function and Significance

A prophage is the genetic material of a bacteriophage integrated into the chromosome of a host bacterium. It remains dormant and can be reactivated under certain conditions.

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Things worth knowing about "Prophage"

A prophage is the genetic material of a bacteriophage integrated into the chromosome of a host bacterium. It remains dormant and can be reactivated under certain conditions.

What Is a Prophage?

A prophage is the integrated form of a bacteriophage – a virus that infects bacteria – within the chromosome of its host bacterium. In this dormant state, the viral DNA is silently embedded in the bacterial genome and is passively duplicated and passed on to daughter cells with every bacterial cell division. This phenomenon is known as lysogeny.

Formation and Integration

The formation of a prophage begins when a bacteriophage injects its genetic material into a bacterial cell. Rather than immediately destroying the host cell (lytic cycle), the viral genome uses specific enzymes called integrases to insert itself precisely into the bacterial chromosome. This process is called integration, and the resulting state is referred to as the lysogenic cycle.

The viral genetic material becomes part of the bacterial chromosome.
Most viral genes are inactive (repressed) during lysogeny.
The bacterium shows no outward signs of infection.

Reactivation: The Lytic Cycle

Under certain stress conditions – such as UV radiation, DNA damage, or nutrient deprivation – a prophage can be induced. This means it is excised from the bacterial chromosome (excision), and viral replication begins anew. New phages are produced, eventually lysing (rupturing) the host cell and being released to infect new bacteria.

Biological and Medical Significance

Prophages play an important role in microbiology and medicine:

Horizontal gene transfer: Prophages can transfer bacterial genes between different bacterial strains through a process called transduction. This is one mechanism by which antibiotic resistance genes and virulence factors are spread.
Pathogenicity: Many disease-causing bacteria carry prophages that encode toxin genes. A well-known example is Escherichia coli producing Shiga toxin (STEC), in which the toxin gene is encoded on a prophage.
Lysogenic conversion: Integration of a prophage can confer new properties on the bacterium – a process called lysogenic conversion. This can affect toxin production, surface proteins, and other traits.
Biotechnology: Prophages are used in research and biotechnology, for example as tools for genetic engineering or in the development of new therapeutic approaches (phage therapy).

Prophages in the Human Microbiome

The human microbiome – the totality of all microorganisms in and on the human body – contains a vast number of bacteria that in turn carry prophages. These phages within the microbiome significantly influence the composition and stability of bacterial communities. Research suggests that an imbalance in the phageome (the complete set of phages) may be associated with various diseases.

Distinguishing a Prophage from an Active Phage

It is important to distinguish between a prophage and a free, active bacteriophage:

Prophage: Integrated into the host genome, inactive, passively replicated.
Active phage: Free-floating, actively replicating, capable of infecting and lysing host cells.

References

Madigan, M. T. et al. - Brock Biology of Microorganisms (15th edition). Pearson Education, 2019.
Canchaya, C. et al. - Phage as agents of lateral gene transfer. Current Opinion in Microbiology, 6(4):417-424, 2003. PubMed PMID: 12941415.
Nanda, A. M. et al. - Impact of Spontaneous Prophage Induction on the Fitness of Bacterial Populations and Host-Microbe Interactions. Journal of Bacteriology, 197(3):410-419, 2015. PubMed PMID: 25404693.

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