WHAT IS THE LYSOGENIC CYCLE? - BIOLOGY - 2025

The lysogenic cycle, also called lysogeny, is a stage in the reproduction process of some viruses, mainly those that infect bacteria. In this cycle, the virus inserts its nucleic acid into the genome of the host bacteria.

This cycle forms, together with the lytic cycle, the two main replication mechanisms of viruses. When the bacteriophage, during the lysogenic cycle, inserts its DNA into the bacterial genome, it becomes a prophage.

Zlir'a. Spanish version by Alejandro Porto, via Wikimedia Commons

Bacteria infected with this prophage continue to live and reproduce. When bacterial reproduction occurs, a replica of the prophage is also obtained. This results in each daughter bacterial cell being also infected by the prophage.

The reproduction of the infected bacteria, and therefore of its host prophage, can continue for several generations without any manifestation of the virus occurring.

Sometimes, spontaneously, or under conditions of environmental stress, the DNA of the virus separates from the bacterial one. When the separation of the bacterial genome occurs, the virus initiates the lytic cycle.

This reproductive stage of the virus will cause the rupture of the bacterial cell (lysis) allowing the release of new copies of the virus. Eukaryotic cells are also susceptible to being attacked by lysogenic viruses. However, it is not yet known how the insertion of the viral DNA into the genome of the eukaryotic cell occurs.

Bacteriophage virus injecting its genome into bacteria. Taken and edited from: Thomas Splettstoesser. (Translated into Spanish by Alejandro Porto), via Wikimedia Commons.

The bacteriophage

Viruses that infect only bacteria are called bacteriophages. They are also known as phages. The size of this type of virus is quite variable, with a size range that can be between 20 and 200 nm approximately.

Bacteriophages are ubiquitous, and can grow in practically any environment where bacteria are found. It has been estimated, for example, that slightly less than three-quarters of the bacteria that live in the sea are infected by phages.

Cycle of viral infection

Viral infection begins with phage adsorption. Phage adsorption occurs in two stages. In the first one, known as reversible, the interaction between the virus and its potential host is weak.

Any change in environmental conditions can cause the cessation of this interaction. In the irreversible interaction, on the other hand, specific receptors are involved that prevent the interruption of the interaction.

The DNA of the virus can enter the interior of the bacterium only when the irreversible interaction occurs. Subsequently, and depending on the type of phage, these can carry out various reproductive cycles.

In addition to the lytic and lysogenic cycles, already described, there are two other reproductive cycles, the continuous development cycle and the pseudolysogenic cycle.

Lytic cycle

During this stage, replication of the virus within the bacteria occurs rapidly. In the end, the bacteria will undergo a lysis of its cell wall and the new viruses will be released into the environment.

Each of these newly released phages can attack a new bacterium. Successive repetition of this process allows the infection to grow exponentially. Bacteriophages that participate in the lytic cycle are called virulent phages.

Lysogenic cycle

In this cycle, lysis of the host cell does not occur, as it does in the lytic cycle. After the adsorption and penetration stages, the stage of integration of the phage DNA to that of the bacterial cell continues, to become a prophage.

Phage replication will occur simultaneously with bacterial reproduction. The prophage integrated into the bacterial genome will be inherited by the daughter bacteria. The virus can continue without manifesting itself for several bacterial generations.

This process is common when the number of bacteriophages is high compared to the number of bacteria. Viruses that carry out the lysogenic cycle are not virulent and are called temperate.

Eventually, the prophages can be separated from the bacterial genome and transformed into lytic phages. The latter enter the lithogenic cycle that leads to bacterial lysis and infection of new bacteria.

Lytic and Lysogenic Cycle. Taken and edited from: Suly12, from Wikimedia Commons

Continuous development cycle

Some bacteriophages carry out numerous replications inside bacteria. In this case, contrary to what occurs during the lysogenic cycle, it does not cause bacterial lysis.

Newly replicated viruses are released from bacteria by specific places on the cell membrane, without causing their rupture. This cycle is called continuous development.

Pseudolysogenic cycle

Sometimes the availability of nutrients in the medium is poor for the bacteria to grow and reproduce normally. In these cases, it is believed that the available cellular energy is not sufficient for the phages to produce lysogenesis or lysis.

Because of this, the viruses then enter a pseudolysogenic cycle. However, this cycle is still little known.

Lysogenic conversion

Eventually, as a product of the interaction between the prophage and the bacterium, the former can induce the appearance of changes in the phenotype of the bacterium.

This occurs mainly when the host bacteria are not part of the normal cycle of the virus. This phenomenon is called lysogenic conversion.

The changes induced in the bacterium by the DNA of the prophage increase the biological success of the host. By increasing the biological capacity and survival success of the bacteria, the virus also benefits.

This type of beneficial relationship for both participants could be classified as a type of symbiosis. However, it must be remembered that viruses are not considered living beings.

The main benefit obtained by lysogenically transformed bacteria is its protection against attack by other bacteriophages. Lysogenic conversion can also increase the pathogenicity of the bacteria in their hosts.

Even a non-pathogenic bacterium can become pathogenic by lysogenic conversion. This change in the genome is permanent and heritable.

Phagotherapy

Phage therapy is a therapy that consists of the application of phages as a control mechanism to prevent the spread of pathogenic bacteria. This bacterial control methodology was used for the first time in 1919.

On that occasion it was used to treat a patient suffering from dysentery, obtaining a completely favorable result. Phage therapy was used successfully during the beginning of the last century.

With the discovery of penicillin, as well as other antibiotic substances, phage therapy was practically abandoned in Western Europe and on the American continent.

The indiscriminate use of antibiotics, allowed the appearance of bacterial strains multiresistant to antibiotics. These bacteria are becoming more frequent and more resistant.

Because of this, there is a new interest in the western world in the development of phage therapy for the control of contamination and bacterial infections.

Advantages of phage therapy

1) The growth of phages occurs exponentially, increasing their action over time, antibiotics, on the contrary, lose their effect over time due to the metabolic destruction of the molecule.

2) Phages have the ability to undergo mutations, this allows them to combat the resistance that bacteria could develop to their attack. On the other hand, antibiotics always have the same active principle, so when bacteria develop resistance to such active principles, antibiotics are useless

3) Phage therapy does not have side effects that may be harmful to patients.

4) The development of a new phage strain is a much faster and cheaper procedure than the discovery and development of a new antibiotic.

5) Antibiotics not only affect pathogenic bacteria, but also other potentially beneficial ones. Phages, on the other hand, can be species-specific, so the treatment against the bacteria responsible for the infection can be limited, without affecting other microorganisms.

6) Antibiotics do not kill all bacteria, therefore, the surviving bacteria can transmit the genetic information that confers resistance to the antibiotic to their offspring, thus creating resistant strains. Lysogenetic bacteriophages kill the bacteria they infect, reducing the possibility of the development of resistant bacterial strains.

References

1. L.-C. Fortier, O. Sekulovic (2013). Importance of prophages to evolution and virulence of bacterial pathogens. Virulence.
2. E. Kutter, D. De Vos, G. Gvasalia, Z. Alavidze, L. Gogokhia, S. Kuhl, ST Abedon (2010). Phage therapy in clinical practice: Treatment of human infections. Current Pharmaceutical Biotechnology.
3. Lysogenic cycle. On Wikipedia. Recovered from en.wikipedia.org.
4. R. Miller, M. Day (2008). Contribution of lysogeny, pseudolysogeny, and starvation to phage ecology. In: Stephen T Abedon (eds) Bacteriophage ecology: population growth, evolution, and impact of bacterial viruses. University Press, Cambridge.
5. C. Prada-Peñaranda, AV Holguín-Moreno, AF González-Barrios, MJ Vives-Flórez (2015). Phage therapy, alternative for the control of bacterial infections. Prospects in Colombia. Universitas Scientiarum.
6. M. Skurnik, E. Strauch (2006). Phage therapy: Facts and fiction. International Journal of Medical Microbiology.